Return

In Java methods have "return" statements. A "void" method does not need to have a return, but it can include one to exit early. Other methods must return the correct type.

In a return statement, we evaluate expressions—and as part of this evaluation, other methods may run. Types must match, or an error occurs. Code after a return may become unreachable.

Expression

We can return a single value. But often we want a more complex return statement—we use an expression after the return keyword. Here an expression that multiplies two values.

public class Program {

    static int computeSize(int height, int width) {
        // Return an expression based on two arguments (variables).
        return height * width;
    }

    public static void main(String[] args) {

        // Assign to the result of computeSize.
        int result = computeSize(10, 3);
        System.out.println(result);
    }
}
30

Return method result

In a return statement, we can invoke another method. In this example, we return the result of cube() when getVolume() returns.

public class Program {

    static int cube(int value) {
        // Return number to the power of 3.
        return (int) Math.pow(value, 3);
    }

    static int getVolume(int size) {
        // Return cubed number.
        return cube(size);
    }

    public static void main(String[] args) {

        // Assign to the return value of getVolume.
        int volume = getVolume(2);
        System.out.println(volume);
    }
}
8

Return, void method

In a void method, an implicit (hidden) return is always at the end of the method. But we can specify a return statement (with no argument) to have an early exit.

public class Program {

    static void displayPassword(String password) {
        // Write the password to the console.
        System.out.println("Password: " + password);
        // Return if our password is long enough.
        if (password.length() >= 5) {
            return;
        }
        System.out.println("Password too short!");
        // An implicit return is here.
    }

    public static void main(String[] args) {
        displayPassword("furball");
        displayPassword("cat");
    }
}
Password: furball
Password: cat
Password too short!

Boolean

We can use an expression to compose a boolean return value. This is a powerful technique—we combine several branches of logic into a single statement.

public class Program {

    static boolean isValid(String name, boolean exists) {
        // Return a boolean based on the two arguments.
        return name.length() >= 3 && exists;
    }

    public static void main(String[] args) {

        // Test the results of the isValid method.
        System.out.println(isValid("green", true));
        System.out.println(isValid("", true));
        System.out.println(isValid("orchard", false));
    }
}
true
false
false

Compilation error

A method that is supposed to return a value (like an int) must return that value. Otherwise a helpful compilation error occurs.

public class Program {

    static int getResult(String id) {
        // This method does not compile.
        // ... It must return an int.
        if (id.length() <= 4) {
            System.out.println("Short");
        }
    }

    public static void main(String[] args) {

        int result = getResult("cat");
        System.out.println(result);
    }
}
Exception in thread "main" java.lang.Error:
    Unresolved compilation problem:
    This method must return a result of type int

    at Program.getResult(Program.java:3)
    at Program.main(Program.java:13)

Multiple return values

This is a common programming problem. To return two values, we can use a class argument—then we set values within that class.

class Data {
    public String name;
    public int size;
}

public class Program {

    static void getTwoValues(Data data) {
        // This method returns two values.
        // ... It sets values in a class.
        data.name = "Java";
        data.size = 100;
    }

    public static void main(String[] args) {

        // Create our data object and call getTwoValues.
        Data data = new Data();
        getTwoValues(data);
        System.out.println(data.name);
        System.out.println(data.size);
    }
}
Java
100

Unreachable code

This is a fatal error in Java. If code cannot be reached, we get a java.lang.Error. To fix this program, we must remove the "return" or the final, unreachable statement.

public class Program {

    public static void main(String[] args) {

        System.out.println("Hello");
        return;
        System.out.println("World");
    }
}
Exception in thread "main" java.lang.Error:
    Unresolved compilation problem:
    Unreachable code

    at Program.main(Program.java:9)

Type mismatch error

We cannot assign a variable to void method. This returns in a "type mismatch" error. Void means "no value." To fix, remove the variable assignment.

public class Program {

    static void test() {
        System.out.println(123);
    }

    public static void main(String[] args) {

        // This does not compile.
        // ... We cannot assign to a void method.
        int result = test();
    }
}
Exception in thread "main" java.lang.Error:
    Unresolved compilation problem:
    Type mismatch: cannot convert from void to int

    at Program.main(Program.java:11)

Performance

In my tests, I found that the JVM does a good job of inlining methods. So if we return a statement that evaluates into another method call, this causes no apparent slowdown.

A review

The return keyword signals an end to a method. It accepts an argument of a type that matches the method's signature. In a void method, we use a no-argument return.