Practice›Java Fundamentals›Module 7. Inheritance & Polymorphism›7.3 Object and equals

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7.3 Object and equals — writing it correctly

Lesson

Три части правильного `equals`

Overriding equals has a catch: the inherited signature takes an Object, not your class. Anything at all can be passed in, so the method must defend itself.

Образец

@Override
public boolean equals(Object other) {
    if (!(other instanceof Point)) {   // 1. is it even a Point?
        return false;
    }
    Point p = (Point) other;           // 2. now it is safe to convert
    return x == p.x && y == p.y;       // 3. compare the contents
}

Miss the first step and passing a String crashes the program. Write public boolean equals(Point other) instead, and you have not overridden anything — you have added a second, unrelated method, and the library will keep calling the old one.

Once equals is right, list.contains(...) and list.indexOf(...) start working with your objects, because they use equals internally.

▶ Try it: run this, then change something and run it again.

import java.util.ArrayList;

class Point {
    private int x;
    private int y;

    public Point(int x, int y) {
        this.x = x;
        this.y = y;
    }

    @Override
    public boolean equals(Object other) {
        if (!(other instanceof Point)) {
            return false;
        }
        Point p = (Point) other;
        return x == p.x && y == p.y;
    }
}

public class Main {
    public static void main(String[] args) {
        Point a = new Point(3, 4);
        Point b = new Point(3, 4);
        System.out.println(a.equals(b));
        System.out.println(a.equals("not a point"));
        ArrayList<Point> list = new ArrayList<Point>();
        list.add(a);
        System.out.println(list.contains(b));
    }
}

The parameter of an overriding equals must be of type...

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What happens if you skip the instanceof check?

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Type exactly what this program prints.

class Tag {
    private String text;

    public Tag(String text) {
        this.text = text;
    }

    @Override
    public boolean equals(Object other) {
        if (!(other instanceof Tag)) {
            return false;
        }
        Tag t = (Tag) other;
        return text.equals(t.text);
    }
}

public class Main { public static void main(String[] a){
    Tag x = new Tag("hi");
    Tag y = new Tag("hi");
    System.out.println(x.equals(y));
    System.out.println(x.equals(42));
}}

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Writing public boolean equals(Point other) instead...

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True or false?

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7.3 Object and equals — writing it correctly

Три части правильного equals

Образец

Три части правильного `equals`