Creating a list

Say a program reads the names of everyone coming to a party. With one variable per name, you'd need name1, name2, name3 and so on, and you'd have to know in advance how many guests there will be. A list solves both problems: it's one variable that holds any number of values, in order.

Java's list is called ArrayList:

import java.util.ArrayList;

public class Main {
    public static void main(String[] args) {
        ArrayList<String> planets = new ArrayList<>();
        planets.add("Mercury");
        planets.add("Venus");
        planets.add("Earth");
        System.out.println(planets);
    }
}

[Mercury, Venus, Earth]

• import java.util.ArrayList; goes at the very top, like the import for Scanner. Without it, Java doesn't know the name ArrayList.
• ArrayList<String> planets declares a variable for a list of Strings. The type between < and > says what kind of values the list holds.
• new ArrayList<>() creates a new, empty list, and = stores it in the variable.
• planets.add("Mercury") adds a value at the end of the list. The list grows by one each time; there's no size to set in advance.
• Printing a list prints all its values in square brackets, separated by commas, in the order they were added. An empty list prints [].

A list can hold the same value more than once. And + joins a list to text, just as it does a number: "Planets: " + planets gives Planets: [Mercury, Venus, Earth].