Java different ways to iterate over java map

April 17, 2020 | No comments

Table of Content

Iterating Java Map

Java Map is an object that maps keys to values. A map cannot contain duplicate keys; each key can map to at most one value. If we want to get the object stored at a particular key we can do so using the the get method.
But if we want to traverse all the objects, then we have different ways of doing the same.

Different Options

In Java we have the following four options to iterate over java map.
Map Iterator
Classic way of using the Iterator of the keySet.
//Iterator
Map map = new TreeMap();
System.out.println("Print using Iterator");
Iterator it = map.keySet().iterator();

while (it.hasNext()) {
  Integer k = it.next();
  String  v = map.get(k);
  System.out.print( "Key=" + k + " Value=" + v + "  ");
}
Java 8 forEach
We can use the forEach introduced in Java 8 to iterate over key and value.
//forFeach
Map map = new TreeMap();
System.out.println("\nPrint using Java 8 forEach");
map.forEach((k,v) -> System.out.print("Key=" + k + " Value=" + v + "  "));
For loop over the keyset
Using for loop over the key set, and then getting the respective value from the map.
//For loop over the keySet
Map map = new TreeMap();
System.out.println("\nPrint using for loop");
for (Integer k : map.keySet()) {
  String  v = map.get(k);
  System.out.print("Key=" + k + " Value=" + v + "  ");
}

Full Example

Full Java Class with all the above options to iterate over map.
MapIterationExample
package bootng.com.learn.collections;

public class MapIterationExample {
  public static void main(String[] args) {
    Map<Integer, String> map = new TreeMap<Integer, String>();
    map.put(1, "A");
    map.put(3, "C");
    map.put(2, "B");
    map.put(4, "D");
    map.put(5, "E");

    // Iterator
    System.out.println("Print using Iterator");
    Iterator<Integer> it = map.keySet().iterator();

    while (it.hasNext()) {
      Integer k = it.next();
      String v = map.get(k);
      System.out.print("Key=" + k + " Value=" + v + "  ");
    }

    // forFeach
    System.out.println("\nPrint using Java 8 forEach");
    map.forEach((k, v) -> System.out.print("Key=" + k + " Value=" + v + "  "));

    // For loop over the keySet
    System.out.println("\nPrint using for loop");
    for (Integer k : map.keySet()) {
      String v = map.get(k);
      System.out.print("Key=" + k + " Value=" + v + "  ");
    }

    // Stream
    System.out.println("\nPrint using stream");
    map.keySet().stream().forEach(key -> {
      System.out.print("Key=" + key + " Value=" + map.get(key) + "  ");
    });    
  }
}
Output
Print using Iterator
Key=1 Value=A  Key=2 Value=B  Key=3 Value=C  Key=4 Value=D  Key=5 Value=E  
Print using Java 8 forEach
Key=1 Value=A  Key=2 Value=B  Key=3 Value=C  Key=4 Value=D  Key=5 Value=E  
Print using for loop
Key=1 Value=A  Key=2 Value=B  Key=3 Value=C  Key=4 Value=D  Key=5 Value=E  
Print using stream
Key=1 Value=A  Key=2 Value=B  Key=3 Value=C  Key=4 Value=D  Key=5 Value=E

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