Comparing objects

In Java, you can compare objects using different approaches. One thing you should never do is use ==.

Let’s review an example: We have the class

public class Animal {
    private String name;

    public Animal(String name) {
        this.name = name;
    }

    public String getName() {
        return name;
    }
}

And then we do

public void testAnimalEquals() {
    Animal schatzi = new Animal("Schatzi");
    Animal schatzi2 = new Animal("Schatzi");
    assertTrue(schatzi == schatzi2);
}

Both animals have the same exact name and nothing else. Using schatzi == schatzi2, what’s the result?

This will return false. This is not how you compare objects in java. Because these are memory pointers, using == compares the memory pointer, not the actual value. This is also valid for native Java objects like String

If you do

public void testAnimalEquals() {
    Animal schatzi = new Animal("Schatzi");
    Animal schatzi2 = schatzi;
    assertTrue(schatzi == schatzi2);
}

This will return true, as both variables point to the same memory address.

So, how to compare object values correctly?

Using equals() Method

By default, equals() (from Object class) checks for reference equality. To compare object content, we need to override it.

class Animal {
    private String name;
    private int age;

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

    @Override
    public boolean equals(Object obj) {
        if (this == obj) return true;
        if (obj instanceof Animal) {
            Animal animal = (Animal) obj;
            return name.equals(animal.name);
        }
        return false
    }
}

So now, if you do

public void testAnimalEquals() {
    Animal schatzi = new Animal("Schatzi");
    Animal schatzi2 = new Animal("Schatzi");
    assertTrue(schatzi.equals(schatzi2));
}

This will succeed!

Using hashCode() (Recommended with equals())

If you override equals(), you should also override hashCode() for correct behavior in collections like HashMap.

@Override
public int hashCode() {
    return Objects.hash(name, age);
}

Using compareTo() (for Sorting)

Implement Comparable to compare objects.

class Animal implements Comparable<Animal> {
    private String name;
    private int age;

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

    @Override
    public int compareTo(Animal other) {
        return Integer.compare(this.age, other.age);
    }
}

The compareTo method is called to understand how to sort objects with the different algorithms. This returns -1 if the object is smaller, 0 if they are equal or 1 if it’s larger than the object it’s being compared to.
Usage:

Animal kali = new Animal("Kali", 8);
Animal schatzi = new Animal("Schatzi", 9);
assertEquals(-1, a1.compareTo(a2)); // -1 (kali is younger than schatzi)