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To copy all the key-value pairs from one Map into another

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.