What is a method?
java
String greeting = "Hello, World!";
int length = greeting.length(); // calling the length() method → 13
Calling methods — the dot operator
java
objectName.methodName(arguments)
java
String name = "Alice";
System.out.println(name.length()); // 5
System.out.println(name.toUpperCase()); // "ALICE"
System.out.println(name.substring(0, 3)); // "Ali"
Two types of methods
1. Void methods — do something, return nothing
java
System.out.println("Hello"); // prints to console, returns nothing
java
// WRONG — println is void, it doesn't return anything
String result = System.out.println("Hello"); // COMPILE ERROR
2. Non-void methods — do something AND return a value
java
String name = "Alice";
int len = name.length(); // returns 5
String upper = name.toUpperCase(); // returns "ALICE"
boolean match = name.equals("Bob"); // returns false
Parameters and arguments
java
// argument
// ↓
name.substring(3);
// In the String class, substring is defined as:
// public String substring(int beginIndex) ← parameter
Methods with multiple parameters
java
String text = "Hello, World!";
// substring(int beginIndex, int endIndex)
String sub = text.substring(0, 5); // "Hello"
// ↑ ↑
// arg 1 arg 2
java
// Math.pow(base, exponent)
Math.pow(2, 10); // 2^10 = 1024.0
Math.pow(10, 2); // 10^2 = 100.0 (order matters!)
Method signatures
java
// Two different methods — different signatures:
substring(int) // one int parameter
substring(int, int) // two int parameters
Overloaded methods
java
// Both are named substring, but have different parameters:
"Hello".substring(2); // "llo" — signature: substring(int)
"Hello".substring(1, 4); // "ell" — signature: substring(int, int)
Chaining method calls
java
String result = " Hello ".trim().toUpperCase();
// Step 1: " Hello ".trim() → "Hello"
// Step 2: "Hello".toUpperCase() → "HELLO"
// result = "HELLO"
Static methods vs. instance methods
java
// Instance method — called on an object
String name = "Alice";
name.length(); // called on the name object
// Static method — called on the class
Math.sqrt(25); // called on the Math CLASS, not an object
A complete trace example
java
String word = "Computer";
int len = word.length();
String upper = word.toUpperCase();
String part = word.substring(0, 4);
boolean same = word.equals("computer");
AP Exam Tips
- •
- •
- •
- •
- •
Common Mistakes
- •
- •
- •
- •