Methods and Arrays

Introduction

A method is a named block of code that you can run whenever you need it. You have already used one: main. Methods let you break a big problem into small, understandable pieces, and avoid writing the same code twice. An array holds a fixed-size list of values.

Writing a method

For now, all our methods are static, which means they belong to the class itself and can be called directly from main.

Java

public class Main {
    static void sayHello() {
        System.out.println("Hello!");
    }

    public static void main(String[] args) {
        sayHello();
        sayHello();
    }
}

Hello!
Hello!

The word void means the method returns nothing.

Parameters and return values

A method's header lists the type of each parameter, and the return type goes before the name.

Java

public class Main {
    static int add(int a, int b) {
        return a + b;
    }

    static double average(int a, int b) {
        return (a + b) / 2.0;
    }

    static boolean isEven(int n) {
        return n % 2 == 0;
    }

    public static void main(String[] args) {
        int total = add(3, 4);
        System.out.println(total);
        System.out.println(average(3, 4));
        System.out.println(isEven(10) + " " + isEven(7));
    }
}

7
3.5
true false

return sends a value back to the caller. The compiler checks that you return the right type, and that every path returns something.

Quick quiz

What does the void in static void sayHello() mean?

Overloading

You may have several methods with the same name but different parameters. Java picks the one that matches the call. This is called overloading.

Java

public class Main {
    static int max(int a, int b) {
        return a > b ? a : b;
    }

    static double max(double a, double b) {
        return a > b ? a : b;
    }

    static int max(int a, int b, int c) {
        return max(max(a, b), c);
    }

    public static void main(String[] args) {
        System.out.println(max(3, 9));
        System.out.println(max(2.5, 1.5));
        System.out.println(max(4, 8, 6));
    }
}

9
2.5
8

Arrays

An array stores several values of the same type, with a fixed size. Index numbers start at 0.

Java

public class Main {
    public static void main(String[] args) {
        int[] scores = {90, 72, 85, 64};

        System.out.println(scores[0]);
        System.out.println(scores.length);

        scores[1] = 75;
        System.out.println(scores[1]);

        int total = 0;
        for (int i = 0; i < scores.length; i++) {
            total += scores[i];
        }
        System.out.println("Total: " + total);
    }
}

90
4
75
Total: 314

scores.length is the number of items (no brackets after length for arrays). Asking for an index that does not exist, like scores[4] here, throws an ArrayIndexOutOfBoundsException at runtime.

You can also create an empty array of a given size. Numbers start as 0:

Java

public class Main {
    public static void main(String[] args) {
        int[] counts = new int[3];
        counts[2] = 5;
        System.out.println(counts[0] + " " + counts[1] + " " + counts[2]);

        String[] names = new String[2];
        System.out.println(names[0]);
    }
}

0 0 5
null

Array items of object types, like String, start as null, meaning "no object".

The enhanced for loop

When you only need the items, not the index, use the shorter for-each loop.

Java

import java.util.Arrays;

public class Main {
    public static void main(String[] args) {
        String[] fruits = {"banana", "apple", "cherry"};

        for (String fruit : fruits) {
            System.out.println(fruit);
        }

        Arrays.sort(fruits);
        System.out.println(Arrays.toString(fruits));
    }
}

banana
apple
cherry
[apple, banana, cherry]

Arrays is a helper class with useful methods: Arrays.toString(arr) prints an array readably (printing an array directly gives a meaningless code), Arrays.sort(arr) sorts it, and Arrays.fill, Arrays.copyOf and Arrays.equals do what their names say. You need import java.util.Arrays; at the top to use it.

Quick quiz

What is the index of the last item in an array of length 5?

Passing arrays to methods

Java

public class Main {
    static int sum(int[] numbers) {
        int total = 0;
        for (int n : numbers) {
            total += n;
        }
        return total;
    }

    static int largest(int[] numbers) {
        int best = numbers[0];
        for (int n : numbers) {
            if (n > best) {
                best = n;
            }
        }
        return best;
    }

    public static void main(String[] args) {
        int[] data = {4, 9, 2, 7};
        System.out.println(sum(data));
        System.out.println(largest(data));
    }
}

22
9

Two-dimensional arrays

An array of arrays works like a grid.

Java

public class Main {
    public static void main(String[] args) {
        int[][] grid = {
            {1, 2, 3},
            {4, 5, 6}
        };

        System.out.println(grid[1][2]);

        for (int[] row : grid) {
            for (int value : row) {
                System.out.print(value + " ");
            }
            System.out.println();
        }
    }
}

6
1 2 3 
4 5 6

Try it yourself

On your computer, write a method static double average(int[] numbers) that returns the average of an array, and call it from main with {70, 85, 90}. It should print 81.66666666666667. Then use String.format("%.1f", value) to print 81.7.

Summary

A method has a return type, a name and typed parameters. void means no return value.
Methods with the same name and different parameters are overloaded.
An array has a fixed size, starts at index 0 and has a length field. Use Arrays.toString and Arrays.sort.
For-each loops for (Type x : array) visit every item. A 2D array is an array of arrays.

Methods and Arrays

In this lesson you will

Introduction

Writing a method

Parameters and return values

Overloading

Arrays

The enhanced for loop

Passing arrays to methods

Two-dimensional arrays

Try it yourself

Summary

Lesson complete