To copy all the key-value pairs from one Map into another: public void putAll(Map map) : HashMap « Collections « Java Tutorial

1. Java Tutorial
2. Collections
3. HashMap

import java.util.HashMap;
import java.util.Map;

public class MainClass {
  public static void main(String[] a) {
    Map map = new HashMap();
    map.put("key1", "value1");
    map.put("key2", "value2");
    map.put("key3", "value3");
    map.put(null, null);

    Map map2 = new HashMap();
    map2.put("key4", "value4");
    map2.put("key5", "value5");
    map2.put("key6", "value6");
    map.putAll(map2);

    System.out.println(map);
  }
}

{key1=value1, key3=value3, key6=value6, null=null, key5=value5, key2=value2, key4=value4}

The value is replaced for every key in the passed-in map that already exists in the HashMap.