Java - return Examples - Dot Net Perls

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.

Info ComputeSize() receives 2 arguments, both of type int. In the return expression, the two numbers are multiplied.

And The evaluated result is returned to the calling location. The expression itself is not returned, just its result.

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); } }

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

And The cube method itself returns the result of Math.pow, a built-in mathematics method.

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); } }

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.

Here If the "password" argument has a length greater than or equal to 5, we return early. Otherwise we print a warning message.

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.

boolean

And The result of isValid is a boolean. Both logical conditions must be satisfied for isValid to return true.

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

Result This has the same effect as returning two arguments. The syntax is more verbose. The object can be reused between method calls.

Tip It is often a better design decision to have related methods on a class. Then those methods simply modify fields of the 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.

Also I found that expressions with temporary local variables were optimized well, to match single-expression return values.

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.

Dot Net Perls is a collection of tested code examples. Pages are continually updated to stay current, with code correctness a top priority.

Sam Allen is passionate about computer languages. In the past, his work has been recommended by Apple and Microsoft and he has studied computers at a selective university in the United States.

This page was last updated on Mar 1, 2023 (simplify).

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