What does equals(Object obj) do?

Question

I´ve often found an equals method in different places. What does it actually do? Is it important that we have to have this in every class?

   public boolean          


        

Answer 1

By default, the Object class equals method invokes when we do not provide the implementation for our custom class. The Object class equals method compares the object using reference.

i.e. a.equals(a); always returns true.

If we are going to provide our own implementation then we will use certain steps for object equality.

    Reflexive:  a.equals(a) always returns true;

    Symmetric: if a.equals(b) is true then b.equals(a) should also be true.

    Transitive: If a.equals(b), b.equals(c) then a.equals(c) should be true/false according to previous 2 result.

    Consistent: a.equals(b) should be the same result without modifying the values of a and b.

Note: default equals method check the reference i.e. == operator.

Note: For any non-null reference value a, a.equals(null) should return false.

public class ObjectEqualExample{

     public static void main(String []args){
        Employee e1 = new Employee(1, "A");
        Employee e2 = new Employee(1, "A");

        // if we are using equals method then It should follow the some properties such as Reflexive, Symmetric, Transitive, and constistent
        /* 
        Reflexive:  a.equals(a) always returns true;
        Symmetric: if a.equals(b) is true then b.equals(a) should also be true.
        Transitive: If a.equals(b), b.equals(c) then a.equals(c) should be true/false according to previous 2 result.

        Consistent: a.equals(b) should be the same result without modifying the values of a and b.

        Note: default equals method check the reference i.e. == operator.
        Note: For any non-null reference value a, a.equals(null) should return false
        */
        System.out.println(e1.equals(e1));
        System.out.println(e1.equals(e2));
     }
}

class Employee {
    private int id;
    private String name;

    @Override
    public String toString() {
        return "{id ="+id+", name = "+name+"} ";
    }

    @Override
    public boolean equals(Object o) {
        // now check the referenc of both object
        if(this == o) return true;

        // check the type of class
        if(o == null || o.getClass() != this.getClass()) return false;

        // now compare the value
        Employee employee = (Employee)o;
        if(employee.id == this.id && employee.name.equals(this.name)) {
            return true; 
        } else return false;
    }

    public int hashCode() {
         // here we are using id. We can also use other logic such as prime number addition or memory address.
        return id;
    }

    public Employee(int id, String name) {
        this.id = id;
        this.name = name;
    }

    public int getId() {
        return id;
    }

    public String getName() {
        return name;
    }
}