Override equals and hashCode

If you want to customize the equals method of an object the general pattern for doing so is the following.

First, declare an equals method that matches the one that comes from Object. Don't forget @Override.

class Position {
    int x;
    int y;

    @Override
    public boolean equals(Object o) {

    }
}

This definition of equals accepts any Object as an argument. Therefore we first need to make sure that the class of that Object is the same as the class you are comparing it to.

class Position {
    int x;
    int y;

    @Override
    public boolean equals(Object o) {
        if (o instanceof Position otherPosition) {

        }
        else {
            return false;
        }
    }
}

Then you compare all the fields to make sure they are equal to each other as well.

class Position {
    int x;
    int y;

    @Override
    public boolean equals(Object o) {
        if (o instanceof Position otherPosition) {
            return this.x == otherPosition.x && this.y == otherPosition.y;
        }
        else {
            return false;
        }
    }
}

How you do those comparisons depends on what is in the fields. For int and such you can use ==. For fields like String you need to use .equals.

class Tree {
    String barkDescription;

    @Override
    public boolean equals(Object o) {
        if (o instanceof Tree otherTree) {
            // 
            return this.barkDescription.equals(otherTree.barkDescription);
        }
        else {
            return false;
        }
    }
}

If you anticipate, or don't take actions to prevent, a field potentially being null then instead of a.equals(b) use java.util.Objects.equals. All it does special is handle null without crashing.

import java.util.Objects;

class Tree {
    String barkDescription;

    @Override
    public boolean equals(Object o) {
        if (o instanceof Tree otherTree) {
            return Objects.equals(otherTree.barkDescription);
        }
        else {
            return false;
        }
    }
}

Whenever you override equals you are supposed to override hashCode as well. This is because - by default - every Object is going to give a hashCode consistent with the default equals method. If you change how equals works, then you could violate the contract of "if hashCode gives a different value, they definitely aren't equal."

Some parts of Java will rely on that, so its worth addressing.

If you define your equals method as above - essentially "they are equal if all their fields are equal" - then you can use java.util.Objects.hash to define your hashCode.¹

import java.util.Objects;

class Position {
    int x;
    int y;

    @Override
    public boolean equals(Object o) {
        if (o instanceof Position otherPosition) {
            return this.x == otherPosition.x && this.y == otherPosition.y;
        }
        else {
            return false;
        }
    }

    @Override
    public int hashCode() {
        // Just give it all the fields you have.
        return Objects.hash(this.x, this.y);
    }
}

1. If you defined a more exotic form of equals then how to properly make a hashCode is an exercise for you, the reader. If you give up then return 1; will always be "correct," if not ideal for the code that uses hashCode as a quick "might be equal" check. ↩