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

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.
Further Learning
Data Structures and Algorithms: Deep Dive Using Java
Algorithms and Data Structures - Part 1 and 2
Data Structures in Java 9 by Heinz Kabutz
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.
Further Learning
Data Structures and Algorithms: Deep Dive Using Java
Algorithms and Data Structures - Part 1 and 2
Data Structures in Java 9 by Heinz Kabutz
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Thanks for readin this article so far, if you like this tutorial then please share with your friends and colleagues.
Thanks for readin this article so far, if you like this tutorial then please share with your friends and colleagues.
20 comments :
Good Examples of iterating hashmap. I think java 5 way of looping hashmap using new for loop is best in terms of cleanliness and readability but Iterator provides in own advantage. as you can remove elements while looping or iterating over hashmap.
How to loop through map - use forloop or enhanced for loop.
How to iterate through map - use Iterator
How to loop map in JSP - use JSTL foreach tag
In order to loop HashMap in JSTL you need to include JSTL core library (tag library) and taglib on JSP page.
Performance wise best approach:
If you're only interested in the keys, you can iterate through the keySet() of the map:
Map map = ...;
for (String key : map.keySet()) {
// ...
}
If you only need the values, use values():
for (Object value : map.values()) {
// ...
}
Finally, if you want both the key and value, use entrySet():
for (Map.Entry entry : map.entrySet()) {
String key = entry.getKey();
Object value = entry.getValue();
// ...
}
If you want to remove items mid-iteration, you'll need to do so via an Iterator.
I think using Entry Set along with For loop for traversing Map is the fastest way to iterate. Anyone has, any data to proof this, I just heard and wondering if that is correct or not.
Another way is :
Map map = new HashMap();
map.put("1", "2");
for (Iterator itr=map.keySet().iterator();itr.hasNext();){
String next = itr.next();
System.out.println(map.get(next));
}
Object[] keyObjectArray = (Object[])map.keySet().toArray();
for(Object key : keyObjectArray){
System.out.println( "Key :"+key+" Value : "+map.get(key));
}
It is useful to beginners.Plz provide explanation about -"What type of searching algorithm is using while getting key and values from HashMap."
hanks for very detailed explanation.
Hello Sir, Which one is the best way to iterate over HashMap? I personally think foreach loop using with entry set, what is your opinion?
it's not really 4 ways, just 2. the for loops are using iterators, too.
hey Javin
I have already mentioned your blog there and this is for just reference in future.
Hope you will not mind
keep writing :)
Hi Deepak, I understand, but please remove that article, as Google doesn't like duplicate copies. You can keep summary and link to original post.
Hello Javin, What is the best way to iterate over Map in Java? Best in terms of performance, robustness and elegant coding?
Map map=new HashMap();
map.put("E1", "Ram");
map.put("E2", "Ravi");
Enumeration keys = Collections.enumeration(map.keySet());
while( keys.hasMoreElements() ) {
Object key = keys.nextElement();
Object value = map.get(key);
System.out.println(value);
}
I think entryset() with iterator is the better way to iterate any kind of abj with respect to key as well as value
Wrong line of code :
for (Entry entry : entrySet) {
Using streams (Java 8) it might look like this (with a simpler print):
HashMap loans = new HashMap();
loans.put("home loan", "citibank");
loans.put("personal loan", "Wells Fargo");
loans.keySet().stream().forEach(key ->
System.out.println("key: " + key + " value: " + loans.get(key)));
Indeed @Knut, Java 8 makes iterating over Map or List really a one liner, no boiler plate due to imperative nature of Java language any more.
Using foreach on Map.Entry can be written shortly as:
for (Map.Entry entry : entrySet) { /* perform something with the entry*/ }
Using this notation the typing for entry (in this case ) is still intact. Which may not bother for the sake of this tutorial, but turns out to be important, when more complex types are about to be processed.
for(Entry ent : hasmapObject.enterySet())
{
sop(ent.getKey() +" ===>" +ent.getValue)
}
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