Java Methods Introduction
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modifier returnType methodName(parameters) {
// body
}
methodName(arguments);Creating and Calling a Method
A method bundles a sequence of statements under one name, and that code only actually runs when something explicitly calls the method by name -- writing the method doesn't execute it.
Example: Creating and Calling a Method
// Define the class Main
public class Main {
static void greet() {
// Print a line to the console
System.out.println("Hello!");
}
// Program entry point: the JVM starts running here
public static void main(String[] args) {
greet();
}
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Method Declaration Structure
A method's declaration specifies four things up front: who can call it (access modifier), what type of value it hands back (return type), its name, and what data it accepts (parameters in parentheses).
Example: Method Declaration Structure
// Define the class Main
public class Main {
public static int square(int number) {
// Send number * number back to the caller
return number * number;
}
// Program entry point: the JVM starts running here
public static void main(String[] args) {
// Print a line to the console
System.out.println(square(4));
}
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Flow of Execution
Execution always begins in main; whenever the code reaches a call to another method, control jumps to that method's body, runs it to completion, and then returns to the exact line right after the original call.
Example: Flow of Execution
// Define the class Main
public class Main {
static void step2() {
// Print a line to the console
System.out.println("Inside step2");
}
// Program entry point: the JVM starts running here
public static void main(String[] args) {
// Print a line to the console
System.out.println("Before call");
step2();
System.out.println("After call");
}
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Variable Scope
A variable declared inside a method exists only for that method's execution and is invisible to every other method, even ones in the same class -- this isolation prevents methods from accidentally interfering with each other's local state.
Example: Variable Scope
public class Main {
static void methodA() {
int x = 5;
System.out.println(x);
}
public static void main(String[] args) {
methodA();
// x is not visible here
}
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Why Use Methods?
Extracting repeated logic into a method means you write and test it once, then call it from anywhere it's needed -- if you later need to fix a bug or change behavior, you only edit it in that one place.
Example: Why Use Methods?
// Define the class Main
public class Main {
static int addTax(int price) {
// Send price + (price / 10) back to the caller
return price + (price / 10);
}
// Program entry point: the JVM starts running here
public static void main(String[] args) {
// Print a line to the console
System.out.println(addTax(100));
// Print a line to the console
System.out.println(addTax(200));
}
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- Forgetting
staticwhen calling a method frommainwithout an object, which gives a compile error. - Declaring a method inside another method, which is not allowed.
- Defining a method but never calling it, so it does not run.
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