Methods: How to Write Them

By the end of this lesson, you will be able to write and understand methods, including how to define and use void and non-void methods, accessor and mutator methods, and how to pass and return values through methods.

Returns

A return is a value sent back to the calling method. Methods with a return type specify it in the method signature. If a method does not return a value, it uses the keyword void. For example, in the Triangle class, the perimeter method returns a double value representing the perimeter.

public double perimeter() {
      return side1 + side2 + side3;
   }

When a method returns a value, it uses the return keyword. The method ends once the return statement is reached, even if there is additional code after the return.

A non-void method returns a single value, specified in the method header. This is known as "return by value." If the return expression is a reference to an object, a copy of the reference is returned, not the object itself.

public boolean isEquilateral() {
      return (side1 == side2) && (side2 == side3);
   }

A void method does not return a value and uses the keyword void in its header.

public void displayPerimeter() {
      System.out.println("Perimeter: " + perimeter());
   }

Accessor Methods

Instance variables in a class are private and not accessible outside the class. Accessor methods allow other programs to obtain the values of these variables. Accessor methods are also known as “getter” methods since they get the values of the instance variables.

   public double getSide1() {
      return side1;
   }

   public double getSide2() {
      return side2;
   }

   public double getSide3() {
      return side3;
   }

These methods provide access to the private instance variables side1, side2, and side3.

Mutator Methods

A mutator (modifier) method is often a void method that changes the values of instance variables.

public void setSide1(double sideA) {
      side1 = sideA;
   }

   public void setSide2(double sideB) {
      side2 = sideB;
   }

   public void setSide3(double sideC) {
      side3 = sideC;
   }

Mutator methods have parameters to set new values for the instance variables, which is necessary since the variables are private. These are often known as the “setter” methods.

Methods Review

FRQ Practice

Look at the free response question #1 from the 2025 AP CS exam.

2025FRQs PDF File

Read through the question and write your own solution to part A. Then watch the video to see a possible solution.

2025FRQ1a Video

Next, write your own solution to Part B. Then watch the video to see one possible solution.

2025FRQ1b Video

CSAwesome Etext: Read Methods: How to Write Them and complete all practice activities.

AP Classroom: Complete all assigned work.

Reflection and Wrap-up

In this lesson, we learned how to write and understand methods. Specifically, we covered:

Void Methods: Methods that do not return a value and use the keyword void.
Non-Void Methods: Methods that return a single value, specified by the return type in the method header.
Return by Value: How non-void methods return a value compatible with the return type.
Accessor Methods: Methods that allow other classes to obtain the values of private instance variables.
Mutator Methods: Methods that change the values of instance variables, often using parameters.
Parameters and Arguments: How methods receive values through parameters and use them to accomplish tasks.

[CC BY-NC-SA 4.0] UNLESS OTHERWISE NOTED | IMAGES: LICENSED AND USED ACCORDING TO TERMS OF SUBSCRIPTION - INTENDED ONLY FOR USE WITHIN THE LESSON.