Every class inherits from Object

Since module 17 you've written classes of your own, and some things worked without your writing them. Module 18 noted that println can print an object whose class has no toString, module 23 called equals on objects whose class had no equals yet, and module 32 said that every object has a hashCode. Where do those methods come from?

Java has a class called Object. It's in java.lang, like String, so it needs no import. Every class inherits from Object: it gets Object's methods, as if they were written in the class itself. This module is about inheritance, and a class of yours can inherit from another class of yours too, not only from Object.

What Object gives every class

A museum keeps a record of each exhibit: its name and the year it was made. This Exhibit class has a constructor and nothing else, yet the program calls four methods on it:

public class Main {
    public static void main(String[] args) {
        Exhibit vase = new Exhibit("Blue vase", 1720);
        Exhibit copy = new Exhibit("Blue vase", 1720);
        System.out.println(vase.getClass().getName());
        System.out.println(vase.equals(copy));
        System.out.println(vase.equals(vase));
        System.out.println(vase.hashCode() == vase.hashCode());
        String text = vase.toString();
        System.out.println(text.startsWith("Exhibit@"));
    }
}

public class Exhibit {
    private String name;
    private int year;

    public Exhibit(String name, int year) {
        this.name = name;
        this.year = year;
    }
}

Exhibit
false
true
true
true

All four methods come from Object:

• getClass().getName() gives the name of the object's class. It's the same on every run.
• Object's equals does the same as ==: it's true only for the very same object (module 23). vase and copy have the same values, but they're two objects.
• Object's hashCode gives a number for the object. It stays the same while the program runs, but it can change from one run to the next (module 32).
• Object's toString gives the class name, an @ and the hash code written in hexadecimal (base 16, with the digits 0 to 9 and the letters a to f), something like Exhibit@1b6d3586. The hash code changes from run to run, so this course never prints that text: the program above only checks how it starts.

Your own methods replace Object's

When a class writes a method with the same header as one of Object's, the class's own method is used instead of Object's. That's what your toString (module 18), equals (module 23) and hashCode (module 32) have done all along: each one replaced Object's version. Step 4 gives this a name, overriding.

import java.util.ArrayList;

public class Main {
    public static void main(String[] args) {
        Exhibit vase = new Exhibit("Blue vase", 1720);
        Exhibit copy = new Exhibit("Blue vase", 1720);
        System.out.println(vase);
        System.out.println(vase.equals(copy));

        ArrayList<Exhibit> hall = new ArrayList<>();
        hall.add(vase);
        hall.add(new Exhibit("Iron helmet", 1450));
        System.out.println(hall);
        System.out.println("Next up: " + hall.get(1));
        System.out.println(hall.contains(new Exhibit("Iron helmet", 1450)));
    }
}

import java.util.Objects;

public class Exhibit {
    private String name;
    private int year;

    public Exhibit(String name, int year) {
        this.name = name;
        this.year = year;
    }

    public boolean equals(Object compared) {
        if (this == compared) {
            return true;
        }
        if (!(compared instanceof Exhibit)) {
            return false;
        }
        Exhibit other = (Exhibit) compared;
        return this.name.equals(other.name) && this.year == other.year;
    }

    public int hashCode() {
        return Objects.hash(this.name, this.year);
    }

    public String toString() {
        return this.name + " (" + this.year + ")";
    }
}

Blue vase (1720)
true
[Blue vase (1720), Iron helmet (1450)]
Next up: Iron helmet (1450)
true

• println(vase), the way a list prints itself, and + with a String all call the object's toString, and now that's Exhibit's own.
• The list's contains calls equals, and now that's Exhibit's own too.
• The name, the parameter types and the return type must be Object's, and the method must stay public: public String toString(), public boolean equals(Object compared) and public int hashCode(). (The parameter's name can be anything, as module 23 said; Object's own is obj.) That's why equals takes an Object: Object's equals takes one, since any object may be compared with any other. A method equals(Exhibit compared) would be a second method next to Object's (overloading, module 21), and lists and maps would go on calling Object's.
• Object's other methods, such as getClass, stay as they are.

Inheriting, in one word

A class that doesn't say which class it inherits from inherits from Object. So these two headers mean the same:

public class Exhibit {

public class Exhibit extends Object {

You'll hardly ever see the second one written out, but it shows the word that the next step is about: extends.