Comparison with the Method equal()

Question

When i compare two instances in java of the same type class, with the same values of their Attributes, why i get false? I'm new with OOP and don't Understand some OOP logic.

 public static void main(String[] args) {
    Object o1= new Object(24,"Omar");
    Object o2= new Object(24,"Omar");
    System.out.println(o1.equals(o2));
 }

A) java.lang.Object does NOT have a constructor that takes 2 arguments ... so you decided to write your own class and call that Object: very bad idea. NEVER re-use such basic names of the java language B) So, you created your own class, but it seems that you missed to (properly) overwrite the inherited equals() implementation. That is all there is to this. — GhostCat, May 06 '22 at 13:41

score 2 · Answer 1 · answered May 06 '22 at 13:35

The equals method returns true if and only if o1 and o2 refer to the same object. And if We want to compare with members variable, we need to override the equals method in our custom class like below:

@Override
public boolean equals(Object o) {
   if (this == o) return true;
   if (o == null || getClass() != o.getClass()) return false;
   CustomObjClass obj = (CustomObjClass) o;
   return age == obj.age && Objects.equals(name, obj.name);
}

And then it will return true for the below code:

 CustomObjClass o1= new CustomObjClass(24,"Omar");
 CustomObjClass o2= new CustomObjClass(24,"Omar");
 System.out.println(o1.equals(o2)); // return true

score 1 · Answer 2 · answered May 06 '22 at 13:29

By default, the equals operator verifies if the two objects are the same one. So, for example, if we perform these operations:

Object o1= new Object(24,"Omar");
Object o2= o1
System.out.println(o1.equals(o2));

you would get true, as o2 is the same object as o1. In your case, you have two different objects (since you instantiate them with the "new") that have the same content. Therefore, the equal operator returns false. If you want it to return true, you have to override the equals operator and implement your own logic.

It could also be that the OP made a mistake when overriding equals(). — GhostCat, May 06 '22 at 13:42

score 1 · Answer 3 · answered May 06 '22 at 13:31

The default implementation of equals method compares the reference of both the objects using == operator. You can find the details here. If you want to compare the value of your Class override the equals method.

Example:

public class TestClass {
    private int age;
    private String name;

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (!(o instanceof TestClass)) return false;
        TestClass testClass = (TestClass) o;
        return getAge() == testClass.getAge() && Objects.equals(getName(), testClass.getName());
    }

    @Override
    public int hashCode() {
        return Objects.hash(getAge(), getName());
    }

    public TestClass(int age, String name) {
        this.age = age;
        this.name = name;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }
}

Comparison with the Method equal()

3 Answers3