Quiz yourself: String manipulation and local and instance variables

Watch out for unexpected and undesirable side effects.

Imagine that your colleague from the IT department is working on a new chatbot application implementing—among some others—the Command design pattern. The bot’s command objects are created once upon startup and reused through the life of the application. Below is the partial code of the application, in particular, the AboutCommand interface, which must return the text Copyright (c) 2023, http://mycompany.com.

public interface Command {

  String execute(Context c);

}

public class AboutCommand implements Command {

  private String url;

  public AboutCommand(String s) {

    url = s;

  }

  public String execute(Context c) {

    url = url.substring(0, url.lastIndexOf("/")); // line 2

    url = url.substring(0, url.lastIndexOf("/")); // line 3

    return "Copyright (c) 2023, " + url; // line 4

  }

}

public class ChatBot {

  public static final String HOME_PAGE = "http://mycompany.com/botapp/index.html";

  private final Command about;

  ChatBot() {

    about = new AboutCommand(HOME_PAGE);

    … // create some more commands

  }

}

Which statement is correct? Choose one.

A. The code is compilable and works as expected.

B. The code is not compilable. To fix the code, the final modifier on the variable HOME_PAGE must be removed.

C. The code is incorrect. To fix it, modify lines 2 and 3 like this:

url.substring(0, url.lastIndexOf("/")); // line 2

url.substring(0, url.lastIndexOf("/")); // line 3

D. The code is incorrect. To fix it, modify lines 2 and 3 like this:

var url = this.url.substring(0, this.url.lastIndexOf("/")); // line 2

url = this.url.substring(0, this.url.lastIndexOf("/")); // line 3

E. The code is incorrect. To fix it, modify lines 2 and 3 like this:

var url = this.url.substring(0, this.url.lastIndexOf("/")); // line 2

url = url.substring(0, url.lastIndexOf("/")); // line 3

Answer. This question investigates aspects of string manipulation, local and instance variables, and logical thinking.

To begin with, option A is incorrect. While the first call of the execute() method will return the required text, the method has side effects. Specifically, it modifies the instance variable. Therefore, a second call will return only Copyright (c) 2023, http:. A third call will throw java.lang.StringIndexOutOfBoundsException.

To correct this, you could either recalculate the message each time without changing the value of the field named url, or you could calculate the message once and store the result for future use. The latter would be more efficient. Let’s investigate the other options to see if any achieve this result.

Option B is incorrect and entirely irrelevant to the problem you have. At no point does any of the code shown in the question attempt to reassign the HOME_PAGE constant, so the final modifier cannot be the cause of any problems.

Option C is also incorrect. It avoids reassigning the url field, which seems like a step forward. However, strings are immutable, so line 4 will return the initial unchanged value. Line 2 will create a new string containing the text http://mycompany.com/botapp, but that string is immediately abandoned because the reference to it (which is carried as the return value of the lastIndexOf method) is not stored anywhere. Line 3 simply repeats the process, and the new string is abandoned again. Therefore, the result will be Copyright (c) 2023, http://mycompany.com/botapp/index.html.

In addition, option D is incorrect. It introduces a new local variable, String url. Line 2 creates a new string (as in option C) by removing the /index.html part from the original text. This new string is stored in the local variable called url. However, line 3 simply repeats the behavior of line 2, because it reads its starting value from the field called url, not the value created by line 2. Consequently, this version produces the message Copyright (c) 2023, http://mycompany.com/botapp.

Option E is correct. Lines 2 and 3 both store the result of their operations, and line 3 uses the result of line 2 as its starting point. As a result, the code cuts off two elements from the url: /index.html first and then /botapp. Critically, this code does not modify the instance variable, so each subsequent run of the execute() method will return the desired text: Copyright (c) 2023, http://mycompany.com.

To be clear, this is not an efficient approach to coding this problem. It would be better to calculate the result once and store it for reuse. The following code, used as a constructor for the AboutCommand class, would achieve that:

public AboutCommand(String s) {

  url = s.substring(0, s.lastIndexOf("/"));

  url = url.substring(0, url.lastIndexOf("/"));

  url = "Copyright (c) 2023, " + url;

}

Given this initialization, the execute method would simply look like the following:

public String execute(Context c) {

  return url;

}

There’s a final note to mention here. Were you wondering about that constructor argument Context c? In quiz questions, you should assume that there’s enough code surrounding what’s shown, or enough context, to allow the code to work if it can work. If there’s a problem you are expected to recognize, it’ll be in the code you are shown, not in something that’s not shown.

The Java 17 exam objectives include some notes about the assumptions of this kind that you are expected to make. The notes are present for the Java 8 exam too, but for some reason, possibly simple oversight, they weren’t included for the Java 11 version.

Conclusion. The correct answer is option E.

Source: oracle.com