How to Override equals() and hashcode() methods in Java?
In this tutorial we will see how to Override equals() and hashcode() method in our code. First up all lets see what is equals and hashcode method.
We all knows that HashMap won't allow duplicate key. In that case first example we will see about storing Object in key without Overriding equals() and hashcode() methods and second example by Overriding equals() and hashcode() methods.
Without overriding equals() and hashcode() methods
OUTPUT:
3 instance we have created and stored in HashMap.
OUTPUT:
- equals() method indicates whether other Object is equal to same class Object. It will return boolean value as true when both Objects are equal and false when both Objects are not equals.
- hashcode() method returns the hashcode of the Object passed and will be called automatically first whenever we use equals() method to compare 2 Objects.
We all knows that HashMap won't allow duplicate key. In that case first example we will see about storing Object in key without Overriding equals() and hashcode() methods and second example by Overriding equals() and hashcode() methods.
Without overriding equals() and hashcode() methods
class MyObject{
String val;
public MyObject(String val) {
this.val = val;
}
}
public class MyTest {
public static void main(String[] args) {
MyObject obj1 = new MyObject("sunday"); // 1st
MyObject obj2 = new MyObject("monday"); // 2nd
MyObject obj3 = new MyObject("sunday"); // 3rd
HashMap<MyObject, String> hm = new HashMap<MyObject, String>();
hm.put(obj1, "1");
hm.put(obj2, "2");
hm.put(obj3, "3");
hm.put(obj1, "4");
hm.put(obj1, "5");
System.out.println("HASHMAP SIZE : "+hm.size());
}
}
OUTPUT:
HASHMAP SIZE : 3
3 instance we have created and stored in HashMap.
With overriding equals() and hashcode() methods
class MyObject{
String val;
public MyObject(String val) {
this.val = val;
}
@Override
public boolean equals(Object obj) {
return ((MyObject)obj).val.equals(this.val);
}
@Override
public int hashCode() {
/*We will return same hashcode as 0 for
all Object since its same class Object
*/
return 0;
}
}
OUTPUT:
HASHMAP SIZE : 2
Same MyTest class and MyObject class Overrides equals() and hashcode() methods. In equals method we have compared 2 Objects of "MyObject" class member variable "val" and returned true when 2 objects have same value "sunday". Hence we have only 2 elements in HashMap.
