Java Text Blocks

Text Blocks are a JDK Enhancement Proposal (JEP 355) available as preview language feature in JDK 13 and 14. It is planned to become a permanent feature in JDK 15. A Text Block is a String literal that spans over multiple lines and avoids the need for most escape sequences.

Motivation

Embedding formats like XML, JSON or SQL in standard Java Strings can become quite annoying. For example, a simple snippet of JSON with just two keys is barely readable in Java because of required escaping:

String json =
        "{\n" +
            "\"name\": \"john\",\n" +
            "\"age\": 42\n" +
        "}";

Text Blocks for the rescue

Using the new text blocks feature, we can rewrite our code to this:

String text = """
        {
            "name": "john",
            "age": "42"
        }
        """;

Text blocks are opened (and closed) using triple-quotes (“””). The text begins at the next line. After opening a text block, the rest of the line needs to stay empty.

If we print this string to the console we see:

{
    "name": "john",
    "age": "42"
}

As you might have been noticed, the indentation on the left side has been stripped away. That’s because a text block is processed in three steps:

◉ Line terminators are normalized to the LF character. This avoids problems between different platforms (like windows and unix).

◉ Incidental leading white spaces and all trailing white spaces are removed. Incidental leading white spaces are determined by finding the common number of leading white spaces for all lines.

◉ Escape sequences are interpreted. Text blocks can contain the same escape sequences as standard strings (e.g. \t or \n). Note that two new escape sequences have been added: \s for an explicit space and \<eol> as continuation indicator (more on \<eol> later).

In case we explicitly need leading white spaces we can use the indent() method:

String text = """

        {

            "name": "john",

            "age": "42"

        }

        """.indent(4);

This adds 4 additional leading spaces to our JSON snippet. So it looks like this:

  {

        "name": "john",

        "age": "42"

    }

Alternatively we can remove 4 leading spaces from the closing triple-quotes to produce the same result:

String text = """

        {

            "name": "john",

            "age": "42"

        }

    """; // <-- moving this 4 spaces to the left produces 4 additional leading spaces

The new \<eol> escape sequence

With the new \<eol> escape sequence we can split the content of a single line into multiple lines without creating an actual line terminator.

String text = """

        1

        2 \

        3 \

        4

        5

        """;

Results in:

1

2 3 4

5

Escaping triple-quotes

In case we need to write triple-quotes into a text block, only the first quote need to be escaped:

String text = """

        Java text blocks start with \"""

        """;

This produces:

Java text blocks start with """

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Java - Packages

Packages are used in Java in order to prevent naming conflicts, to control access, to make searching/locating and usage of classes, interfaces, enumerations and annotations easier, etc.

A Package can be defined as a grouping of related types (classes, interfaces, enumerations and annotations ) providing access protection and namespace management.

Some of the existing packages in Java are −

◉ java.lang − bundles the fundamental classes

◉ java.io − classes for input , output functions are bundled in this package

Programmers can define their own packages to bundle group of classes/interfaces, etc. It is a good practice to group related classes implemented by you so that a programmer can easily determine that the classes, interfaces, enumerations, and annotations are related.

Since the package creates a new namespace there won't be any name conflicts with names in other packages. Using packages, it is easier to provide access control and it is also easier to locate the related classes.

Creating a Package

While creating a package, you should choose a name for the package and include a package statement along with that name at the top of every source file that contains the classes, interfaces, enumerations, and annotation types that you want to include in the package.

The package statement should be the first line in the source file. There can be only one package statement in each source file, and it applies to all types in the file.

If a package statement is not used then the class, interfaces, enumerations, and annotation types will be placed in the current default package.

To compile the Java programs with package statements, you have to use -d option as shown below.

javac -d Destination_folder file_name.java

Then a folder with the given package name is created in the specified destination, and the compiled class files will be placed in that folder.

Example

Let us look at an example that creates a package called animals. It is a good practice to use names of packages with lower case letters to avoid any conflicts with the names of classes and interfaces.

Following package example contains interface named animals −

/* File name : Animal.java */
package animals;

interface Animal {
   public void eat();
   public void travel();
}

Now, let us implement the above interface in the same package animals −

package animals;
/* File name : MammalInt.java */

public class MammalInt implements Animal {

   public void eat() {
      System.out.println("Mammal eats");
   }

   public void travel() {
      System.out.println("Mammal travels");
   }

   public int noOfLegs() {
      return 0;
   }

   public static void main(String args[]) {
      MammalInt m = new MammalInt();
      m.eat();
      m.travel();
   }
}

Now compile the java files as shown below −

$ javac -d . Animal.java
$ javac -d . MammalInt.java

Now a package/folder with the name animals will be created in the current directory and these class files will be placed in it as shown below.

You can execute the class file within the package and get the result as shown below.

Mammal eats
Mammal travels

The import Keyword

If a class wants to use another class in the same package, the package name need not be used. Classes in the same package find each other without any special syntax.

Example

Here, a class named Boss is added to the payroll package that already contains Employee. The Boss can then refer to the Employee class without using the payroll prefix, as demonstrated by the following Boss class.

package payroll;
public class Boss {
   public void payEmployee(Employee e) {
      e.mailCheck();
   }
}

What happens if the Employee class is not in the payroll package? The Boss class must then use one of the following techniques for referring to a class in a different package.

◉ The fully qualified name of the class can be used. For example −

payroll.Employee

◉ The package can be imported using the import keyword and the wild card (*). For example −

import payroll.*;

The class itself can be imported using the import keyword. For example −

import payroll.Employee;

Note − A class file can contain any number of import statements. The import statements must appear after the package statement and before the class declaration.

The Directory Structure of Packages

Two major results occur when a class is placed in a package −

◉ The name of the package becomes a part of the name of the class, as we just discussed in the previous section.

◉ The name of the package must match the directory structure where the corresponding bytecode resides.

Here is simple way of managing your files in Java −

Put the source code for a class, interface, enumeration, or annotation type in a text file whose name is the simple name of the type and whose extension is .java.

For example −

// File Name :  Car.java
package vehicle;

public class Car {
   // Class implementation. 
}

Now, put the source file in a directory whose name reflects the name of the package to which the class belongs −

....\vehicle\Car.java

Now, the qualified class name and pathname would be as follows −

◉ Class name → vehicle.Car
◉ Path name → vehicle\Car.java (in windows)

In general, a company uses its reversed Internet domain name for its package names.

Example − A company's Internet domain name is apple.com, then all its package names would start with com.apple. Each component of the package name corresponds to a subdirectory.

Example − The company had a com.apple.computers package that contained a Dell.java source file, it would be contained in a series of subdirectories like this −

....\com\apple\computers\Dell.java

At the time of compilation, the compiler creates a different output file for each class, interface and enumeration defined in it. The base name of the output file is the name of the type, and its extension is .class.

For example −

// File Name: Dell.java
package com.apple.computers;

public class Dell {
}

class Ups {
}

Now, compile this file as follows using -d option −

$javac -d . Dell.java

The files will be compiled as follows −

.\com\apple\computers\Dell.class
.\com\apple\computers\Ups.class

You can import all the classes or interfaces defined in \com\apple\computers\ as follows −

import com.apple.computers.*;

Like the .java source files, the compiled .class files should be in a series of directories that reflect the package name. However, the path to the .class files does not have to be the same as the path to the .java source files. You can arrange your source and class directories separately, as −

<path-one>\sources\com\apple\computers\Dell.java

<path-two>\classes\com\apple\computers\Dell.class

By doing this, it is possible to give access to the classes directory to other programmers without revealing your sources. You also need to manage source and class files in this manner so that the compiler and the Java Virtual Machine (JVM) can find all the types your program uses.

The full path to the classes directory, <path-two>\classes, is called the class path, and is set with the CLASSPATH system variable. Both the compiler and the JVM construct the path to your .class files by adding the package name to the class path.

Say <path-two>\classes is the class path, and the package name is com.apple.computers, then the compiler and JVM will look for .class files in <path-two>\classes\com\apple\computers.

A class path may include several paths. Multiple paths should be separated by a semicolon (Windows) or colon (Unix). By default, the compiler and the JVM search the current directory and the JAR file containing the Java platform classes so that these directories are automatically in the class path.

Set CLASSPATH System Variable

To display the current CLASSPATH variable, use the following commands in Windows and UNIX (Bourne shell) −

◉ In Windows → C:\> set CLASSPATH
◉ In UNIX → % echo $CLASSPATH

To delete the current contents of the CLASSPATH variable, use −

◉ In Windows → C:\> set CLASSPATH =
◉ In UNIX → % unset CLASSPATH; export CLASSPATH

To set the CLASSPATH variable −

◉ In Windows → set CLASSPATH = C:\users\jack\java\classes
◉ In UNIX → % CLASSPATH = /home/jack/java/classes; export CLASSPATH

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Java - Applet Basics

An applet is a Java program that runs in a Web browser. An applet can be a fully functional Java application because it has the entire Java API at its disposal.

There are some important differences between an applet and a standalone Java application, including the following −

◉ An applet is a Java class that extends the java.applet.Applet class.

◉ A main() method is not invoked on an applet, and an applet class will not define main().

◉ Applets are designed to be embedded within an HTML page.

◉ When a user views an HTML page that contains an applet, the code for the applet is downloaded to the user's machine.

◉ A JVM is required to view an applet. The JVM can be either a plug-in of the Web browser or a separate runtime environment.

◉ The JVM on the user's machine creates an instance of the applet class and invokes various methods during the applet's lifetime.

◉ Applets have strict security rules that are enforced by the Web browser. The security of an applet is often referred to as sandbox security, comparing the applet to a child playing in a sandbox with various rules that must be followed.

◉ Other classes that the applet needs can be downloaded in a single Java Archive (JAR) file.

Life Cycle of an Applet

Four methods in the Applet class gives you the framework on which you build any serious applet −

◉ init − This method is intended for whatever initialization is needed for your applet. It is called after the param tags inside the applet tag have been processed.

◉ start − This method is automatically called after the browser calls the init method. It is also called whenever the user returns to the page containing the applet after having gone off to other pages.

◉ stop − This method is automatically called when the user moves off the page on which the applet sits. It can, therefore, be called repeatedly in the same applet.

◉ destroy − This method is only called when the browser shuts down normally. Because applets are meant to live on an HTML page, you should not normally leave resources behind after a user leaves the page that contains the applet.

◉ paint − Invoked immediately after the start() method, and also any time the applet needs to repaint itself in the browser. The paint() method is actually inherited from the java.awt.

A "Hello, World" Applet

Following is a simple applet named HelloWorldApplet.java −

import java.applet.*;
import java.awt.*;

public class HelloWorldApplet extends Applet {
   public void paint (Graphics g) {
      g.drawString ("Hello World", 25, 50);
   }
}

These import statements bring the classes into the scope of our applet class −

◉ java.applet.Applet
◉ java.awt.Graphics

Without those import statements, the Java compiler would not recognize the classes Applet and Graphics, which the applet class refers to.

The Applet Class

Every applet is an extension of the java.applet.Applet class. The base Applet class provides methods that a derived Applet class may call to obtain information and services from the browser context.

These include methods that do the following −

◉ Get applet parameters
◉ Get the network location of the HTML file that contains the applet
◉ Get the network location of the applet class directory
◉ Print a status message in the browser
◉ Fetch an image
◉ Fetch an audio clip
◉ Play an audio clip
◉ Resize the applet

Additionally, the Applet class provides an interface by which the viewer or browser obtains information about the applet and controls the applet's execution. The viewer may −

◉ Request information about the author, version, and copyright of the applet
◉ Request a description of the parameters the applet recognizes
◉ Initialize the applet
◉ Destroy the applet
◉ Start the applet's execution
◉ Stop the applet's execution

The Applet class provides default implementations of each of these methods. Those implementations may be overridden as necessary.

The "Hello, World" applet is complete as it stands. The only method overridden is the paint method.

Invoking an Applet

An applet may be invoked by embedding directives in an HTML file and viewing the file through an applet viewer or Java-enabled browser.

The <applet> tag is the basis for embedding an applet in an HTML file. Following is an example that invokes the "Hello, World" applet −

<html>
   <title>The Hello, World Applet</title>
   <hr>
   <applet code = "HelloWorldApplet.class" width = "320" height = "120">
      If your browser was Java-enabled, a "Hello, World"
      message would appear here.
   </applet>
   <hr>
</html>

Note − You can refer to HTML Applet Tag to understand more about calling applet from HTML.

The code attribute of the <applet> tag is required. It specifies the Applet class to run. Width and height are also required to specify the initial size of the panel in which an applet runs. The applet directive must be closed with an </applet> tag.

If an applet takes parameters, values may be passed for the parameters by adding <param> tags between <applet> and </applet>. The browser ignores text and other tags between the applet tags.

Non-Java-enabled browsers do not process <applet> and </applet>. Therefore, anything that appears between the tags, not related to the applet, is visible in non-Java-enabled browsers.

The viewer or browser looks for the compiled Java code at the location of the document. To specify otherwise, use the codebase attribute of the <applet> tag as shown −

<applet codebase = "https://amrood.com/applets" code = "HelloWorldApplet.class"
   width = "320" height = "120">

If an applet resides in a package other than the default, the holding package must be specified in the code attribute using the period character (.) to separate package/class components. For example −

<applet  = "mypackage.subpackage.TestApplet.class"
   width = "320" height = "120">

Getting Applet Parameters

The following example demonstrates how to make an applet respond to setup parameters specified in the document. This applet displays a checkerboard pattern of black and a second color.

The second color and the size of each square may be specified as parameters to the applet within the document.

CheckerApplet gets its parameters in the init() method. It may also get its parameters in the paint() method. However, getting the values and saving the settings once at the start of the applet, instead of at every refresh, is convenient and efficient.

The applet viewer or browser calls the init() method of each applet it runs. The viewer calls init() once, immediately after loading the applet. (Applet.init() is implemented to do nothing.) Override the default implementation to insert custom initialization code.

The Applet.getParameter() method fetches a parameter given the parameter's name (the value of a parameter is always a string). If the value is numeric or other non-character data, the string must be parsed.

The following is a skeleton of CheckerApplet.java −

import java.applet.*;
import java.awt.*;

public class CheckerApplet extends Applet {
   int squareSize = 50;   // initialized to default size
   public void init() {}
   private void parseSquareSize (String param) {}
   private Color parseColor (String param) {}
   public void paint (Graphics g) {}
}

Here are CheckerApplet's init() and private parseSquareSize() methods −

public void init () {
   String squareSizeParam = getParameter ("squareSize");
   parseSquareSize (squareSizeParam);
 
   String colorParam = getParameter ("color");
   Color fg = parseColor (colorParam);
 
   setBackground (Color.black);
   setForeground (fg);
}

private void parseSquareSize (String param) {
   if (param == null) return;
   try {
      squareSize = Integer.parseInt (param);
   } catch (Exception e) {
      // Let default value remain
   }
}

The applet calls parseSquareSize() to parse the squareSize parameter. parseSquareSize() calls the library method Integer.parseInt(), which parses a string and returns an integer. Integer.parseInt() throws an exception whenever its argument is invalid.

Therefore, parseSquareSize() catches exceptions, rather than allowing the applet to fail on bad input.

The applet calls parseColor() to parse the color parameter into a Color value. parseColor() does a series of string comparisons to match the parameter value to the name of a predefined color. You need to implement these methods to make this applet work.

Specifying Applet Parameters

The following is an example of an HTML file with a CheckerApplet embedded in it. The HTML file specifies both parameters to the applet by means of the <param> tag.

<html>
   <title>Checkerboard Applet</title>
   <hr>
   <applet code = "CheckerApplet.class" width = "480" height = "320">
      <param name = "color" value = "blue">
      <param name = "squaresize" value = "30">
   </applet>
   <hr>
</html>

Note − Parameter names are not case sensitive.

Application Conversion to Applets

It is easy to convert a graphical Java application (that is, an application that uses the AWT and that you can start with the Java program launcher) into an applet that you can embed in a web page.

Following are the specific steps for converting an application to an applet.

◉ Make an HTML page with the appropriate tag to load the applet code.

◉ Supply a subclass of the JApplet class. Make this class public. Otherwise, the applet cannot be loaded.

◉ Eliminate the main method in the application. Do not construct a frame window for the application. Your application will be displayed inside the browser.

◉ Move any initialization code from the frame window constructor to the init method of the applet. You don't need to explicitly construct the applet object. The browser instantiates it for you and calls the init method.

◉ Remove the call to setSize; for applets, sizing is done with the width and height parameters in the HTML file.

◉ Remove the call to setDefaultCloseOperation. An applet cannot be closed; it terminates when the browser exits.

◉ If the application calls setTitle, eliminate the call to the method. Applets cannot have title bars. (You can, of course, title the web page itself, using the HTML title tag.)

◉ Don't call setVisible(true). The applet is displayed automatically.

Event Handling

Applets inherit a group of event-handling methods from the Container class. The Container class defines several methods, such as processKeyEvent and processMouseEvent, for handling particular types of events, and then one catch-all method called processEvent.

In order to react to an event, an applet must override the appropriate event-specific method.

import java.awt.event.MouseListener;
import java.awt.event.MouseEvent;
import java.applet.Applet;
import java.awt.Graphics;

public class ExampleEventHandling extends Applet implements MouseListener {
   StringBuffer strBuffer;

   public void init() {
      addMouseListener(this);
      strBuffer = new StringBuffer();
      addItem("initializing the apple ");
   }

   public void start() {
      addItem("starting the applet ");
   }

   public void stop() {
      addItem("stopping the applet ");
   }

   public void destroy() {
      addItem("unloading the applet");
   }

   void addItem(String word) {
      System.out.println(word);
      strBuffer.append(word);
      repaint();
   }

   public void paint(Graphics g) {
      // Draw a Rectangle around the applet's display area.
      g.drawRect(0, 0,
      getWidth() - 1,
      getHeight() - 1);

      // display the string inside the rectangle.
      g.drawString(strBuffer.toString(), 10, 20);
   }
 
   public void mouseEntered(MouseEvent event) {
   }
   public void mouseExited(MouseEvent event) {
   }
   public void mousePressed(MouseEvent event) {
   }
   public void mouseReleased(MouseEvent event) {
   }
   public void mouseClicked(MouseEvent event) {
      addItem("mouse clicked! ");
   }
}

Now, let us call this applet as follows −

<html>
   <title>Event Handling</title>
   <hr>
   <applet code = "ExampleEventHandling.class"
      width = "300" height = "300">
   </applet>
   <hr>
</html>

Initially, the applet will display "initializing the applet. Starting the applet." Then once you click inside the rectangle, "mouse clicked" will be displayed as well.

Displaying Images

An applet can display images of the format GIF, JPEG, BMP, and others. To display an image within the applet, you use the drawImage() method found in the java.awt.Graphics class.

Following is an example illustrating all the steps to show images −

import java.applet.*;
import java.awt.*;
import java.net.*;

public class ImageDemo extends Applet {
   private Image image;
   private AppletContext context;
 
   public void init() {
      context = this.getAppletContext();
      String imageURL = this.getParameter("image");
      if(imageURL == null) {
         imageURL = "java.jpg";
      }
      try {
         URL url = new URL(this.getDocumentBase(), imageURL);
         image = context.getImage(url);
      } catch (MalformedURLException e) {
         e.printStackTrace();
         // Display in browser status bar
         context.showStatus("Could not load image!");
      }
   }
 
   public void paint(Graphics g) {
      context.showStatus("Displaying image");
      g.drawImage(image, 0, 0, 200, 84, null);
      g.drawString("www.javalicense.com", 35, 100);
   }
}

Now, let us call this applet as follows −

<html>
   <title>The ImageDemo applet</title>
   <hr>
   <applet code = "ImageDemo.class" width = "300" height = "200">
      <param name = "image" value = "java.jpg">
   </applet>
   <hr>
</html>

Playing Audio

An applet can play an audio file represented by the AudioClip interface in the java.applet package. The AudioClip interface has three methods, including −

◉ public void play() − Plays the audio clip one time, from the beginning.

◉ public void loop() − Causes the audio clip to replay continually.

◉ public void stop() − Stops playing the audio clip.

To obtain an AudioClip object, you must invoke the getAudioClip() method of the Applet class. The getAudioClip() method returns immediately, whether or not the URL resolves to an actual audio file. The audio file is not downloaded until an attempt is made to play the audio clip.

Following is an example illustrating all the steps to play an audio −

import java.applet.*;
import java.awt.*;
import java.net.*;

public class AudioDemo extends Applet {
   private AudioClip clip;
   private AppletContext context;
 
   public void init() {
      context = this.getAppletContext();
      String audioURL = this.getParameter("audio");
      if(audioURL == null) {
         audioURL = "default.au";
      }
      try {
         URL url = new URL(this.getDocumentBase(), audioURL);
         clip = context.getAudioClip(url);
      } catch (MalformedURLException e) {
         e.printStackTrace();
         context.showStatus("Could not load audio file!");
      }
   }
 
   public void start() {
      if(clip != null) {
         clip.loop();
      }
   }
 
   public void stop() {
      if(clip != null) {
         clip.stop();
      }
   }
}

Now, let us call this applet as follows −

<html>
   <title>The ImageDemo applet</title>
   <hr>
   <applet code = "ImageDemo.class" width = "0" height = "0">
      <param name = "audio" value = "test.wav">
   </applet>
   <hr>
</html>

You can use test.wav on your PC to test the above example.

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Java - Multithreading

Java is a multi-threaded programming language which means we can develop multi-threaded program using Java. A multi-threaded program contains two or more parts that can run concurrently and each part can handle a different task at the same time making optimal use of the available resources specially when your computer has multiple CPUs.

By definition, multitasking is when multiple processes share common processing resources such as a CPU. Multi-threading extends the idea of multitasking into applications where you can subdivide specific operations within a single application into individual threads. Each of the threads can run in parallel. The OS divides processing time not only among different applications, but also among each thread within an application.

Multi-threading enables you to write in a way where multiple activities can proceed concurrently in the same program.

Life Cycle of a Thread

A thread goes through various stages in its life cycle. For example, a thread is born, started, runs, and then dies. The following diagram shows the complete life cycle of a thread.

Following are the stages of the life cycle −

◉ New − A new thread begins its life cycle in the new state. It remains in this state until the program starts the thread. It is also referred to as a born thread.

◉ Runnable − After a newly born thread is started, the thread becomes runnable. A thread in this state is considered to be executing its task.

◉ Waiting − Sometimes, a thread transitions to the waiting state while the thread waits for another thread to perform a task. A thread transitions back to the runnable state only when another thread signals the waiting thread to continue executing.

◉ Timed Waiting − A runnable thread can enter the timed waiting state for a specified interval of time. A thread in this state transitions back to the runnable state when that time interval expires or when the event it is waiting for occurs.

◉ Terminated (Dead) − A runnable thread enters the terminated state when it completes its task or otherwise terminates.

Thread Priorities

Every Java thread has a priority that helps the operating system determine the order in which threads are scheduled.

Java thread priorities are in the range between MIN_PRIORITY (a constant of 1) and MAX_PRIORITY (a constant of 10). By default, every thread is given priority NORM_PRIORITY (a constant of 5).

Threads with higher priority are more important to a program and should be allocated processor time before lower-priority threads. However, thread priorities cannot guarantee the order in which threads execute and are very much platform dependent.

Create a Thread by Implementing a Runnable Interface

If your class is intended to be executed as a thread then you can achieve this by implementing a Runnable interface. You will need to follow three basic steps −

Step 1

As a first step, you need to implement a run() method provided by a Runnable interface. This method provides an entry point for the thread and you will put your complete business logic inside this method. Following is a simple syntax of the run() method −

public void run( )

Step 2

As a second step, you will instantiate a Thread object using the following constructor −

Thread(Runnable threadObj, String threadName);

Where, threadObj is an instance of a class that implements the Runnable interface and threadName is the name given to the new thread.

Step 3

Once a Thread object is created, you can start it by calling start() method, which executes a call to run( ) method. Following is a simple syntax of start() method −

void start();

Example

Here is an example that creates a new thread and starts running it −

Live Demo
class RunnableDemo implements Runnable {
   private Thread t;
   private String threadName;
 
   RunnableDemo( String name) {
      threadName = name;
      System.out.println("Creating " +  threadName );
   }
 
   public void run() {
      System.out.println("Running " +  threadName );
      try {
         for(int i = 4; i > 0; i--) {
            System.out.println("Thread: " + threadName + ", " + i);
            // Let the thread sleep for a while.
            Thread.sleep(50);
         }
      } catch (InterruptedException e) {
         System.out.println("Thread " +  threadName + " interrupted.");
      }
      System.out.println("Thread " +  threadName + " exiting.");
   }
 
   public void start () {
      System.out.println("Starting " +  threadName );
      if (t == null) {
         t = new Thread (this, threadName);
         t.start ();
      }
   }
}

public class TestThread {

   public static void main(String args[]) {
      RunnableDemo R1 = new RunnableDemo( "Thread-1");
      R1.start();
   
      RunnableDemo R2 = new RunnableDemo( "Thread-2");
      R2.start();
   } 
}

This will produce the following result −

Output

Creating Thread-1
Starting Thread-1
Creating Thread-2
Starting Thread-2
Running Thread-1
Thread: Thread-1, 4
Running Thread-2
Thread: Thread-2, 4
Thread: Thread-1, 3
Thread: Thread-2, 3
Thread: Thread-1, 2
Thread: Thread-2, 2
Thread: Thread-1, 1
Thread: Thread-2, 1
Thread Thread-1 exiting.
Thread Thread-2 exiting.

Create a Thread by Extending a Thread Class

The second way to create a thread is to create a new class that extends Thread class using the following two simple steps. This approach provides more flexibility in handling multiple threads created using available methods in Thread class.

Step 1

You will need to override run( ) method available in Thread class. This method provides an entry point for the thread and you will put your complete business logic inside this method. Following is a simple syntax of run() method −

public void run( )

Step 2

Once Thread object is created, you can start it by calling start() method, which executes a call to run( ) method. Following is a simple syntax of start() method −

void start( );

Example

Here is the preceding program rewritten to extend the Thread −

class ThreadDemo extends Thread {
   private Thread t;
   private String threadName;
 
   ThreadDemo( String name) {
      threadName = name;
      System.out.println("Creating " +  threadName );
   }
 
   public void run() {
      System.out.println("Running " +  threadName );
      try {
         for(int i = 4; i > 0; i--) {
            System.out.println("Thread: " + threadName + ", " + i);
            // Let the thread sleep for a while.
            Thread.sleep(50);
         }
      } catch (InterruptedException e) {
         System.out.println("Thread " +  threadName + " interrupted.");
      }
      System.out.println("Thread " +  threadName + " exiting.");
   }
 
   public void start () {
      System.out.println("Starting " +  threadName );
      if (t == null) {
         t = new Thread (this, threadName);
         t.start ();
      }
   }
}

public class TestThread {

   public static void main(String args[]) {
      ThreadDemo T1 = new ThreadDemo( "Thread-1");
      T1.start();
   
      ThreadDemo T2 = new ThreadDemo( "Thread-2");
      T2.start();
   } 
}

This will produce the following result −

Output

Creating Thread-1
Starting Thread-1
Creating Thread-2
Starting Thread-2
Running Thread-1
Thread: Thread-1, 4
Running Thread-2
Thread: Thread-2, 4
Thread: Thread-1, 3
Thread: Thread-2, 3
Thread: Thread-1, 2
Thread: Thread-2, 2
Thread: Thread-1, 1
Thread: Thread-2, 1
Thread Thread-1 exiting.
Thread Thread-2 exiting.

Thread Methods

Following is the list of important methods available in the Thread class.

Sr.No.	Method & Description
1	public void start() Starts the thread in a separate path of execution, then invokes the run() method on this Thread object.
2	public void run() If this Thread object was instantiated using a separate Runnable target, the run() method is invoked on that Runnable object.
3	public final void setName(String name) Changes the name of the Thread object. There is also a getName() method for retrieving the name.
4	public final void setPriority(int priority) Sets the priority of this Thread object. The possible values are between 1 and 10.
5	public final void setDaemon(boolean on) A parameter of true denotes this Thread as a daemon thread.
6	public final void join(long millisec) The current thread invokes this method on a second thread, causing the current thread to block until the second thread terminates or the specified number of milliseconds passes.
7	public void interrupt() Interrupts this thread, causing it to continue execution if it was blocked for any reason.
8	public final boolean isAlive() Returns true if the thread is alive, which is any time after the thread has been started but before it runs to completion.

The previous methods are invoked on a particular Thread object. The following methods in the Thread class are static. Invoking one of the static methods performs the operation on the currently running thread.

Sr.No.	Method & Description
1	public static void yield() Causes the currently running thread to yield to any other threads of the same priority that are waiting to be scheduled.
2	public static void sleep(long millisec) Causes the currently running thread to block for at least the specified number of milliseconds.
3	public static boolean holdsLock(Object x) Returns true if the current thread holds the lock on the given Object.
4	public static Thread currentThread() Returns a reference to the currently running thread, which is the thread that invokes this method.
5	public static void dumpStack() Prints the stack trace for the currently running thread, which is useful when debugging a multithreaded application.

Example

The following ThreadClassDemo program demonstrates some of these methods of the Thread class. Consider a class DisplayMessage which implements Runnable −

// File Name : DisplayMessage.java

// Create a thread to implement Runnable

public class DisplayMessage implements Runnable {

   private String message;

   

   public DisplayMessage(String message) {

      this.message = message;

   }

   

   public void run() {

      while(true) {

         System.out.println(message);

      }

   }

}

Following is another class which extends the Thread class −

// File Name : GuessANumber.java

// Create a thread to extentd Thread

public class GuessANumber extends Thread {

   private int number;

   public GuessANumber(int number) {

      this.number = number;

   }

   

   public void run() {

      int counter = 0;

      int guess = 0;

      do {

         guess = (int) (Math.random() * 100 + 1);

         System.out.println(this.getName() + " guesses " + guess);

         counter++;

      } while(guess != number);

      System.out.println("** Correct!" + this.getName() + "in" + counter + "guesses.**");

   }

}

Following is the main program, which makes use of the above-defined classes −

// File Name : ThreadClassDemo.java

public class ThreadClassDemo {

   public static void main(String [] args) {

      Runnable hello = new DisplayMessage("Hello");

      Thread thread1 = new Thread(hello);

      thread1.setDaemon(true);

      thread1.setName("hello");

      System.out.println("Starting hello thread...");

      thread1.start();

      

      Runnable bye = new DisplayMessage("Goodbye");

      Thread thread2 = new Thread(bye);

      thread2.setPriority(Thread.MIN_PRIORITY);

      thread2.setDaemon(true);

      System.out.println("Starting goodbye thread...");

      thread2.start();

      System.out.println("Starting thread3...");

      Thread thread3 = new GuessANumber(27);

      thread3.start();

      try {

         thread3.join();

      } catch (InterruptedException e) {

         System.out.println("Thread interrupted.");

      }

      System.out.println("Starting thread4...");

      Thread thread4 = new GuessANumber(75);

      

      thread4.start();

      System.out.println("main() is ending...");

   }

}

This will produce the following result. You can try this example again and again and you will get a different result every time.

Output

Starting hello thread...

Starting goodbye thread...

Hello

Hello

Hello

Hello

Hello

Hello

Goodbye

Goodbye

Goodbye

Goodbye

Goodbye

.......

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Java - Serialization

Java provides a mechanism, called object serialization where an object can be represented as a sequence of bytes that includes the object's data as well as information about the object's type and the types of data stored in the object.

After a serialized object has been written into a file, it can be read from the file and deserialized that is, the type information and bytes that represent the object and its data can be used to recreate the object in memory.

Most impressive is that the entire process is JVM independent, meaning an object can be serialized on one platform and deserialized on an entirely different platform.

Classes ObjectInputStream and ObjectOutputStream are high-level streams that contain the methods for serializing and deserializing an object.

The ObjectOutputStream class contains many write methods for writing various data types, but one method in particular stands out −

public final void writeObject(Object x) throws IOException

The above method serializes an Object and sends it to the output stream. Similarly, the ObjectInputStream class contains the following method for deserializing an object −

public final Object readObject() throws IOException, ClassNotFoundException

This method retrieves the next Object out of the stream and deserializes it. The return value is Object, so you will need to cast it to its appropriate data type.

To demonstrate how serialization works in Java, I am going to use the Employee class that we discussed early on in the book. Suppose that we have the following Employee class, which implements the Serializable interface −

Example

public class Employee implements java.io.Serializable {
   public String name;
   public String address;
   public transient int SSN;
   public int number;
 
   public void mailCheck() {
      System.out.println("Mailing a check to " + name + " " + address);
   }
}

Notice that for a class to be serialized successfully, two conditions must be met −

◉ The class must implement the java.io.Serializable interface.

◉ All of the fields in the class must be serializable. If a field is not serializable, it must be marked transient.

If you are curious to know if a Java Standard Class is serializable or not, check the documentation for the class. The test is simple: If the class implements java.io.Serializable, then it is serializable; otherwise, it's not.

Serializing an Object

The ObjectOutputStream class is used to serialize an Object. The following SerializeDemo program instantiates an Employee object and serializes it to a file.

When the program is done executing, a file named employee.ser is created. The program does not generate any output, but study the code and try to determine what the program is doing.

Note − When serializing an object to a file, the standard convention in Java is to give the file a .ser extension.

Example

import java.io.*;
public class SerializeDemo {

   public static void main(String [] args) {
      Employee e = new Employee();
      e.name = "Reyan Ali";
      e.address = "Phokka Kuan, Ambehta Peer";
      e.SSN = 11122333;
      e.number = 101;
   
      try {
         FileOutputStream fileOut =
         new FileOutputStream("/tmp/employee.ser");
         ObjectOutputStream out = new ObjectOutputStream(fileOut);
         out.writeObject(e);
         out.close();
         fileOut.close();
         System.out.printf("Serialized data is saved in /tmp/employee.ser");
      } catch (IOException i) {
         i.printStackTrace();
      }
   }
}

Deserializing an Object

The following DeserializeDemo program deserializes the Employee object created in the SerializeDemo program. Study the program and try to determine its output −

Example

import java.io.*;
public class DeserializeDemo {

   public static void main(String [] args) {
      Employee e = null;
      try {
         FileInputStream fileIn = new FileInputStream("/tmp/employee.ser");
         ObjectInputStream in = new ObjectInputStream(fileIn);
         e = (Employee) in.readObject();
         in.close();
         fileIn.close();
      } catch (IOException i) {
         i.printStackTrace();
         return;
      } catch (ClassNotFoundException c) {
         System.out.println("Employee class not found");
         c.printStackTrace();
         return;
      }
   
      System.out.println("Deserialized Employee...");
      System.out.println("Name: " + e.name);
      System.out.println("Address: " + e.address);
      System.out.println("SSN: " + e.SSN);
      System.out.println("Number: " + e.number);
   }
}

This will produce the following result −

Output

Deserialized Employee...
Name: Reyan Ali
Address:Phokka Kuan, Ambehta Peer
SSN: 0
Number:101

Here are following important points to be noted −

◉ The try/catch block tries to catch a ClassNotFoundException, which is declared by the readObject() method. For a JVM to be able to deserialize an object, it must be able to find the bytecode for the class. If the JVM can't find a class during the deserialization of an object, it throws a ClassNotFoundException.

◉ Notice that the return value of readObject() is cast to an Employee reference.

◉ The value of the SSN field was 11122333 when the object was serialized, but because the field is transient, this value was not sent to the output stream. The SSN field of the deserialized Employee object is 0.

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Java - Generics

It would be nice if we could write a single sort method that could sort the elements in an Integer array, a String array, or an array of any type that supports ordering.

Java Generic methods and generic classes enable programmers to specify, with a single method declaration, a set of related methods, or with a single class declaration, a set of related types, respectively.

Generics also provide compile-time type safety that allows programmers to catch invalid types at compile time.

Using Java Generic concept, we might write a generic method for sorting an array of objects, then invoke the generic method with Integer arrays, Double arrays, String arrays and so on, to sort the array elements.

Generic Methods

You can write a single generic method declaration that can be called with arguments of different types. Based on the types of the arguments passed to the generic method, the compiler handles each method call appropriately. Following are the rules to define Generic Methods −

◉ All generic method declarations have a type parameter section delimited by angle brackets (< and >) that precedes the method's return type ( < E > in the next example).

◉ Each type parameter section contains one or more type parameters separated by commas. A type parameter, also known as a type variable, is an identifier that specifies a generic type name.

◉ The type parameters can be used to declare the return type and act as placeholders for the types of the arguments passed to the generic method, which are known as actual type arguments.

◉ A generic method's body is declared like that of any other method. Note that type parameters can represent only reference types, not primitive types (like int, double and char).

Example

Following example illustrates how we can print an array of different type using a single Generic method −

Live Demo
public class GenericMethodTest {
   // generic method printArray
   public static < E > void printArray( E[] inputArray ) {
      // Display array elements
      for(E element : inputArray) {
         System.out.printf("%s ", element);
      }
      System.out.println();
   }

   public static void main(String args[]) {
      // Create arrays of Integer, Double and Character
      Integer[] intArray = { 1, 2, 3, 4, 5 };
      Double[] doubleArray = { 1.1, 2.2, 3.3, 4.4 };
      Character[] charArray = { 'H', 'E', 'L', 'L', 'O' };

      System.out.println("Array integerArray contains:");
      printArray(intArray);   // pass an Integer array

      System.out.println("\nArray doubleArray contains:");
      printArray(doubleArray);   // pass a Double array

      System.out.println("\nArray characterArray contains:");
      printArray(charArray);   // pass a Character array
   }
}

This will produce the following result −

Output

Array integerArray contains:
1 2 3 4 5

Array doubleArray contains:
1.1 2.2 3.3 4.4

Array characterArray contains:
H E L L O

Bounded Type Parameters

There may be times when you'll want to restrict the kinds of types that are allowed to be passed to a type parameter. For example, a method that operates on numbers might only want to accept instances of Number or its subclasses. This is what bounded type parameters are for.

To declare a bounded type parameter, list the type parameter's name, followed by the extends keyword, followed by its upper bound.

Example

Following example illustrates how extends is used in a general sense to mean either "extends" (as in classes) or "implements" (as in interfaces). This example is Generic method to return the largest of three Comparable objects −

public class MaximumTest {
   // determines the largest of three Comparable objects
 
   public static <T extends Comparable<T>> T maximum(T x, T y, T z) {
      T max = x;   // assume x is initially the largest
     
      if(y.compareTo(max) > 0) {
         max = y;   // y is the largest so far
      }
     
      if(z.compareTo(max) > 0) {
         max = z;   // z is the largest now               
      }
      return max;   // returns the largest object 
   }
 
   public static void main(String args[]) {
      System.out.printf("Max of %d, %d and %d is %d\n\n",
         3, 4, 5, maximum( 3, 4, 5 ));

      System.out.printf("Max of %.1f,%.1f and %.1f is %.1f\n\n",
         6.6, 8.8, 7.7, maximum( 6.6, 8.8, 7.7 ));

      System.out.printf("Max of %s, %s and %s is %s\n","pear",
         "apple", "orange", maximum("pear", "apple", "orange"));
   }
}

This will produce the following result −

Output

Max of 3, 4 and 5 is 5

Max of 6.6,8.8 and 7.7 is 8.8

Max of pear, apple and orange is pear

Generic Classes

A generic class declaration looks like a non-generic class declaration, except that the class name is followed by a type parameter section.

As with generic methods, the type parameter section of a generic class can have one or more type parameters separated by commas. These classes are known as parameterized classes or parameterized types because they accept one or more parameters.

Example

Following example illustrates how we can define a generic class −

public class Box<T> {
   private T t;

   public void add(T t) {
      this.t = t;
   }

   public T get() {
      return t;
   }

   public static void main(String[] args) {
      Box<Integer> integerBox = new Box<Integer>();
      Box<String> stringBox = new Box<String>();
   
      integerBox.add(new Integer(10));
      stringBox.add(new String("Hello World"));

      System.out.printf("Integer Value :%d\n\n", integerBox.get());
      System.out.printf("String Value :%s\n", stringBox.get());
   }
}

This will produce the following result −

Output

Integer Value :10
String Value :Hello World

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