Modify existing Key of HashMap In Java

Question

I'm working with HashMap since few days, and facing below weird situation.

Case 1:Changed Key which is already existing in HashMap, and print HashMap Case 2: Changed key which is already existing in HashMap and Put that key again into the HashMap. Print HashMap.

Please find below code as well as two different output of two case.

Could you please anyone let me know, whats going on in below code.

import java.util.HashMap; import java.util.Set; class Emp{ private String id ; public String getId() { return id; } public void setId(String id) { this.id = id; } public Emp(String id) { super(); this.id = id; } @Override public int hashCode() { final int prime = 31; int result = 1; result = prime * result + ((id == null) ? 0 : id.hashCode()); return result; } @Override public boolean equals(Object obj) { if (this == obj) return true; if (obj == null) return false; if (getClass() != obj.getClass()) return false; Emp other = (Emp) obj; if (id == null) { if (other.id != null) return false; } else if (!id.equals(other.id)) return false; return true; } @Override public String toString() { return "Emp [id=" + id + "]"; } } public class HashMapChanges { public static void main(String[] args) { // TODO Auto-generated method stub Emp e1 = new Emp("1"); Emp e2 = new Emp("2"); Emp e3 = new Emp("3"); Emp e4 = new Emp("4"); HashMap<Emp, String> hm = new HashMap<Emp,String>(); hm.put(e1,"One"); hm.put(e2,"Two"); hm.put(e3,"Three"); hm.put(e4,"Four"); e1.setId("5"); /** Uncomment below line to get different output**/ //hm.put(e1,"Five-5"); Set<Emp> setEmpValue = hm.keySet(); for(Emp e : setEmpValue){ System.out.println("Key"+ e +" Value "+ hm.get(e)); } } }

Output of above code :

KeyEmp [id=2] Value Two KeyEmp [id=5] Value null KeyEmp [id=4] Value Four KeyEmp [id=3] Value Three

Output After uncommenting line

KeyEmp [id=5] Value Five-5 KeyEmp [id=2] Value Two KeyEmp [id=5] Value Five-5 KeyEmp [id=4] Value Four KeyEmp [id=3] Value Three

Jim Garrison · Accepted Answer · 2018-02-23 06:00:58Z

Using mutable objects as keys in a Map is not permitted when the key used to determine its location in the Map is mutable.

From the Javadoc for java.util.Map<K,V>:

Note: great care must be exercised if mutable objects are used as map keys. The behavior of a map is not specified if the value of an object is changed in a manner that affects equals comparisons while the object is a key in the map.

You are violating the contract required of map keys because your Emp object is mutable, and the change modifies the attribute used to determine where in the map the key resides.

The result is undefined behavior.

I suspect you have misunderstood the Map concept, based on your code, but without understanding what you're actually trying to achieve we really cannot help further. I suggest you ask a new question explaining your actual goals.

Abhishek kumar · Accepted Answer · 2018-02-23 06:01:07Z

You overwrite the hashCode() and equals() method , then ,the Map's key is the hashCode result. then id=1 and id=5 are two different items.

You can comment the two methods and try again.

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