4 example to Iterate over HashMap, Hashtable or any Map in Java

There are multiple ways to iterate, traverse or loop through Map, HashMap or TreeMap in Java and we all familiar of either all of those or some of those. But to my surprise, one of my friends was asked in his interview (he has more than 6 years of experience in Java programming) to write code for getting values from hashmap or TreeMap in Java with at least 4 ways. Just like me he also surprised on this question but written it. I don't know why exactly someone asks this kind of java interview question to a relatively senior java programmer. Though my closest guess is to verify that whether he is still hands on with coding in java. Anyway, that gives me an idea to write this Java tutorial and here are multiple ways to traverse, iterate or loop on a Map in Java, so remember this because you may also ask this question J.

How to traverse or loop Map, HashMap or TreeMap in Java

In next section of this Java tutorial, we will see four different ways of looping or iterating over Map in Java and will display each key and value from HashMap. We will use following hashmap for our example:

HashMap<String, String> loans = new HashMap<String, String>();
loans.put<"home loan", "Citibank");
loans.put<"personal loan", "Wells Fargo");

Iterating or looping map using Java 5 foreach loop

Here we will use new foreach loop introduced in JDK5 for iterating over any map in java and using KeySet of the map for getting keys. this will iterate through all values of Map and display key and value together.

HashMap<String, String> loans = new HashMap<String, String>();
loans.put("home loan", "citibank");
loans.put("personal loan", "Wells Fargo");

for (String key : loans.keySet()) {
   System.out.println("------------------------------------------------");
   System.out.println("Iterating or looping map using java5 foreach loop");
   System.out.println("key: " + key + " value: " + loans.get(key));
}

Output:
------------------------------------------------
Iterating or looping map using java5 foreach loop
key: home loan value: Citibank
------------------------------------------------
Iterating or looping map using java5 foreach loop
key: personal loan value: Wells Fargo

Iterating Map in Java using KeySet Iterator

In this Example of looping hashmap in Java we have used Java Iterator instead of for loop, rest are similar to earlier example of looping:

Set<String> keySet = loans.keySet();
Iterator<String> keySetIterator = keySet.iterator();
while (keySetIterator.hasNext()) {
   System.out.println("------------------------------------------------");
   System.out.println("Iterating Map in Java using KeySet Iterator");
   String key = keySetIterator.next();
   System.out.println("key: " + key + " value: " + loans.get(key));
}

Output:
------------------------------------------------
Iterating Map in Java using KeySet Iterator
key: home loan value: Citibank
------------------------------------------------
Iterating Map in Java using KeySet Iterator
key: personal loan value: Wells Fargo

Looping HashMap in Java using EntrySet and Java 5 for loop

In this Example of traversing Map in Java, we have used EntrySet instead of KeySet. EntrySet is a collection of all Map Entries and contains both Key and Value

Set<Map.Entry<String, String>> entrySet = loans.entrySet();

for (Entry entry : entrySet) {

   System.out.println("------------------------------------------------");

   System.out.println("looping HashMap in Java using EntrySet and java5 for loop");

   System.out.println("key: " + entry.getKey() + " value: " + entry.getValue());

}

Output:

------------------------------------------------

looping HashMap in Java using EntrySet and java5 for loop

key: home loan value: Citibank

------------------------------------------------

looping HashMap in Java using EntrySet and java5 for loop

key: personal loan value: Wells Fargo

Iterating HashMap in Java using EntrySet and Java iterator

This is the fourth and last example of looping Map and here we have used Combination of Iterator and EntrySet to display all keys and values of a Java Map.

Set<Map.Entry<String, String>> entrySet1 = loans.entrySet();

Iterator<Entry<String, String>> entrySetIterator = entrySet1.iterator();

while (entrySetIterator.hasNext()) {

   System.out.println("------------------------------------------------");

   System.out.println("Iterating HashMap in Java using EntrySet and Java iterator");

   Entry entry = entrySetIterator.next();

   System.out.println("key: " + entry.getKey() + " value: " + entry.getValue());

}

Output:

------------------------------------------------

Iterating HashMap in Java using EntrySet and Java iterator

key: home loan value: Citibank

------------------------------------------------

Iterating HashMap in Java using EntrySet and Java iterator

key: personal loan value: Wells Fargo

That’s all on multiple ways of looping Map in Java. We have seen exactly 4 examples to iterator on Java Map in a combination of KeySet and EntrySet by using for loop and Iterator. Let me know if you are familiar with any other ways of iterating and getting each key value from Map in Java.

Source: javarevisited.blogspot.com