Methods and Return Values

Similar to how methods can take in values as parameters, methods can also return values.

Keeping Results

Recall the addTen method from the previous chapter:

public void addTen(int x)
{
  int xPlusTen = x + 10;
  System.out.println(xPlusTen);
}

This method takes in a parameter, x, and adds 10 to it. Lastly, the program prints the value to the console.

But what if we wanted to store the value of x + 10? In the method above, x + 10 is stored into a local variable called xPlusTen. However, this variable is lost when the method is finished -- it cannot be accessed outside of the method.

Using a return statement will allow us to pass a value back out of the method. That return value can be stored into a variable for later use.

Here's how to rewrite addTen to return a value instead of printing:

public int addTen(int x)
{
  int xPlusTen = x + 10;
  return xPlusTen;
}

There are a few differences here. First, take a look at the first line of the method: public int addTen(int x). Instead of void, we now use int. This tells the program that the addTen method will return a value that is an int. This is called the return type of the method. Up to this point, we've only used the void return type, which indicates that the method does not return anything. Because the addTen method will return an integer, we set the return type to int.

Note also that the return keyword does not require parentheses. Also, returning a value does not print that value to the console, similar to how passing in a value as a parameter does not print the value to the console.

Calling a Method With a Return Value

A return value by itself would not be very useful, given that it does not print to the console. Fortunately, we can store a method's return value into a variable. Take a look at the following code:

int num = 7;
int tenAdded = addTen(7);

First, we create a variable named num and initialize it to 7. Then, we create another variable called tenAdded. Notice that tenAdded is not given a normal value. Instead, we are setting it equal to addTen(7). This means that the tenAdded variable will hold the result of whatever the function call addTen(7) returns. We know that addTen(7) will return 17, so tenAdded will be 17.

Multiple Parameters With a Return Value

Return values work in many situations. For example, we can rewrite the add method from the previous section to return the sum instead of print it to the screen:

public int add(int x, int y)
{
  int sum = x + y;
  return sum;
}

We can now call the method and store its return values

int sum = add(10, 90);
System.out.println(sum);

which would print 100 to the console.