meet it

Write and your objects print nicely.

Print an object and you get something like Main$Book@1b6d3586: its class name and a code. That comes from a plain toString method that every class gets for free. Java calls toString whenever it needs your object as text.

Write your own and you choose the text: . Now printing the book shows Matilda by Roald Dahl, and so does joining it to a string.

It must be public, open to code anywhere, because the free version is public too. Lists print each item with its own toString.

import java.util.*;
public class Main {
    static class Book {
        private String title;
        private String author;
        Book(String title, String author) {
            this.title = title;
            this.author = author;
        }
        public String toString() {
            return title + " by " + author;
        }
    }
    public static void main(String[] args) {
        Book book = new Book("Matilda", "Roald Dahl");
        System.out.println(book);
        System.out.println("Reading: " + book);
        System.out.println(List.of(book, new Book("Holes", "Louis Sachar")));
    }
}

The code after the @ in the free version can change from run to run, so never rely on it.

guess first

A list of planets. What will this show?

Guess before you run it. Wrong guesses are how you learn, and nothing is marked.

import java.util.*;
public class Main {
    static class Planet {
        private String name;
        private int moons;
        Planet(String name, int moons) {
            this.name = name;
            this.moons = moons;
        }
        public String toString() {
            return name + " (" + moons + ")";
        }
    }
    public static void main(String[] args) {
        ArrayList<Planet> planets = new ArrayList<>();
        planets.add(new Planet("Earth", 1));
        planets.add(new Planet("Mars", 2));
        System.out.println(planets);
    }
}

your turn

Your turn. Describe a pizza.

Finish the return on line 10 so a pizza describes itself using both of its fields. Then press run.

Your code

output

press run to see what happens

fix it

The dog will not describe itself. Fix it!

Broken code to fix

output

press run to see what happens

Java says toString() in Dog cannot override toString() in Object on line 7.
The free toString is open to code anywhere, so yours must be too. One word is missing.
Put public in front of String toString() on line 7, then press run.

nailed it

your objects introduce themselves. :D

Java calls toString whenever it needs an object as text.
Write public String toString() to choose that text.
Lists print each item using its own toString.

new words in your collection

toString
public

This lesson isn’t saved. A free account remembers every lesson you finish.

Write toString and your objects print nicely.

A list of planets. What will this show?

Your turn. Describe a pizza.

The dog will not describe itself. Fix it!

your objects introduce themselves. :D

Write and your objects print nicely.