3.05 Method Writing
Read method headers, write void and returning methods, and call them to check their results.
Unit 3 · Lesson 3.5 · 5 minutes
A Java method defines a behavior inside a class. In five minutes, you will read a method header one part at a time, write methods with and without return values, and call them with the correct arguments.
Learning Targets
void method and a value-returning methodMinute 1 — Read a Method Header
Method headers put several pieces of information on one line. If that feels dense, begin with one question: does the calling code need a value returned? That determines whether the return type is a data type such as int or the keyword void. Then read the rest from left to right.
public int calculateArea(int length, int width)| Part | Purpose | Example |
|---|---|---|
| Access modifier | Controls where the method can be called | public |
| Return type | Type of value sent back; use void for no value |
int |
| Method name | Describes the action | calculateArea |
| Parameters | Typed inputs available inside the method | int length, int width |
The first line is the method header, and the braces contain its body. In Java, the method signature specifically means the method name and parameter types. If the declared return type is not void, every possible path through the method must execute a return statement with a compatible value.
Minutes 2–3 — Write Two Methods
Run the code cell. Read the two methods before reading main: printDimensions() prints but does not return a value, so it uses void. calculateArea() computes an integer, so it declares int and returns one.
Code Runner Challenge
Run the example, then locate each method's return type, name, parameters, and return statement.
View IPYNB Source
// CODE_RUNNER: Run the example, then locate each method's return type, name, parameters, and return statement.
public class Shape {
// void means this method does not return a value.
public void printDimensions(int length, int width) {
System.out.println("Dimensions: " + length + " by " + width);
}
// The two int parameters make this method reusable with different inputs.
public int calculateArea(int shapeLength, int shapeWidth) {
return shapeLength * shapeWidth;
}
public static void main(String[] args) {
Shape rectangle = new Shape();
rectangle.printDimensions(8, 5);
int area = rectangle.calculateArea(8, 5);
System.out.println("Area: " + area);
}
}
Shape.main(null);
Minutes 4–5 — Call and Check
The call rectangle.calculateArea(8, 5) uses two arguments. Java assigns them to shapeLength and shapeWidth in order. The returned 40 is stored in area.
Quick check: why would this method fail to compile?
public int doubleNumber(int number) {
System.out.println(number * 2);
}Its declared return type is int, but the method never returns an integer. Fix it with return number * 2;, or change the return type to void if the method should only print.
Five-minute recap: choose a return type, give the method a name that describes its behavior, declare typed parameters, write one focused behavior, and use return when the declared type requires a value.
Homework Hacks
Three hacks · 5 minutes each
Complete each starter code cell. Keep the given class and method names so the test calls continue to work.
Homework Hack 1 — Complete a Returning Method (5 minutes)
header practice Complete perimeter. It must accept length and width, then return 2 * length + 2 * width. The expected output is 26.
Code Runner Challenge
Complete the perimeter method, then run the provided test.
View IPYNB Source
// CODE_RUNNER: Complete the perimeter method, then run the provided test.
public class PerimeterHack {
// TODO: add the return type, parameters, and method body.
public _____ perimeter(_____) {
// your code here
}
public static void main(String[] args) {
PerimeterHack hack = new PerimeterHack();
System.out.println(hack.perimeter(8, 5)); // expected: 26
}
}
PerimeterHack.main(null);
Homework Hack 2 — Write a void Method (5 minutes)
parameter practice Write printRectangle with two int parameters. It should print Rectangle: 8 by 5. Call it from main.
Code Runner Challenge
Write and call a void method that prints the required output.
View IPYNB Source
// CODE_RUNNER: Write and call a void method that prints the required output.
public class DescriptionHack {
// TODO: write printRectangle here.
public static void main(String[] args) {
DescriptionHack hack = new DescriptionHack();
// TODO: call printRectangle on hack with 8 and 5.
}
}
DescriptionHack.main(null);
Homework Hack 3 — Return a Decision (5 minutes)
boolean practice Write isSquare. It accepts length and width and returns true when they are equal. Both provided checks should print true.
Code Runner Challenge
Write a boolean method, then run both provided checks.
View IPYNB Source
// CODE_RUNNER: Write a boolean method, then run both provided checks.
public class SquareHack {
// TODO: write isSquare here.
public static void main(String[] args) {
SquareHack hack = new SquareHack();
System.out.println(hack.isSquare(4, 4)); // expected: true
System.out.println(!hack.isSquare(8, 5)); // expected: true
}
}
SquareHack.main(null);
Submission Checklist and Grading
Each homework hack is worth 1 point.
- Hack 1: correct
intreturn type, twointparameters, and perimeter formula - Hack 2: correct
voidmethod, twointparameters, exact output, and a working method call - Hack 3: correct
booleanreturn type, twointparameters, and equality check
Before submitting, run every code cell and confirm its output matches the comments. Do not rename the supplied classes or methods.
References
- College Board. (2025). AP Computer Science A course and exam description.
- Oracle. (n.d.). Defining methods. The Java Tutorials. https://docs.oracle.com/javase/tutorial/java/javaOO/methods.html
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