4 Example to Iterate over Map, HashMap, Hashtable or TreeMap in Java

There are multiple ways to iterate, traverse or loop through Map, HashMap, or TreeMap in Java and we are all familiar with 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, Hashtable, ConcurrentHashMap, or TreeMap in Java in at least 4 ways. Just like me he was also surprised by 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 there 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 over a Map, HashMap, or TreeMap in Java

In the 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 the following the HashMap for our example. You can see that it contains just two mappings, which means only two keys and two values to deal with. 

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

1. Iterating or looping map using Java 5 foreach loop

Here we will use the 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

2. Iterating Map in Java using KeySet Iterator

In this Example of looping over HashMap in Java we have used Java Iterator instead of for loop, rest are similar to earlier example of looping. One advantage of using Iterator is that it also allows you to remove keys and entries while iterating. I mean you can remove a particular key from the keySet() and entry from the entrySet() to actually delete that key or entry from the Map. 

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

3. 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

4. Iterating HashMap in Java using EntrySet and Java iterator

This is the fourth and last example of a looping Map and here we have used a combination of Iterator and entrySet() method to display all keys and values of a Java Map. This is my favorite and also the fastest way to iterate over HashMap in Java as you don't need to call get() method to get the value. The Map.Entry object already contains both key and value and you can use it.

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 about multiple ways of looping over a Map in Java. In this Java tutorial, we have seen exactly 4 examples to iterator over a Java Map like HashMap, TreeMap, Hashtable in a combination of KeySet and EntrySet by using for loop and Iterator. You can choose the way you want but if you want to access both keys and values then iterating using entrySet() is the best and fastest way as you don't need to call the get() method again to retrieve the value.  

While these code example uses HashMap, it will work for all kind of Map like TreeMap, LinkedHashMap, IdentityHashMap and so on because its using generic method of Map interface. The only difference could be Hashtable which uses Enumeration instead of Iterator so where ever I have used Iterator, it may not work for Hashtable but other should work just fine. 

Let me know if you are familiar with any other ways of iterating and getting each key-value from Map in Java.

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