Methods

objectName.methodName(arguments)

String word = "computer";
int len = word.length();           // calling length() on a String object
String sub = word.substring(0, 4); // calling substring() with two arguments

// Method signature:  int length()
//   Name: length
//   Parameters: none
//   Returns: int

// Method signature:  String substring(int from, int to)
//   Name: substring
//   Parameters: two ints
//   Returns: String

// void method — just does something (prints)
System.out.println("Hello");  // cannot assign to a variable

// Return method — gives back a value you can store
int len = "Hello".length();  // stores 5 in len

// substring has parameters: (int beginIndex, int endIndex)
// We pass arguments: (0, 5)
String result = "computer".substring(0, 5);  // "compu"

public static void changeValue(int x) {
    x = 99;  // changes the local copy, not the original
}

int num = 5;
changeValue(num);
System.out.println(num);  // still 5!

String text = "  Hello World  ";
String result = text.trim().toLowerCase();
// Step 1: text.trim() returns "Hello World"
// Step 2: "Hello World".toLowerCase() returns "hello world"
System.out.println(result);  // "hello world"

String s = "AP Computer Science";

s.length()            // 19
s.substring(0, 2)     // "AP"
s.substring(3)        // "Computer Science"
s.indexOf("Computer") // 3
s.charAt(0)           // 'A'
s.equals("AP Computer Science")  // true
s.compareTo("B")     // negative ('A' < 'B')

String s = "mississippi";

// Two versions of indexOf:
s.indexOf("ss");      // 2 — indexOf(String)
s.indexOf("ss", 3);   // 5 — indexOf(String, int) — starts searching at index 3

// Two versions of substring:
s.substring(4);       // "issippi" — substring(int)
s.substring(4, 7);    // "iss" — substring(int, int)

public class MethodDemo {
    public static void main(String[] args) {
        String sentence = "The quick brown fox jumps";
        
        // Return methods — capture the result
        int wordCount = sentence.length();
        int foxPos = sentence.indexOf("fox");
        String animal = sentence.substring(foxPos, foxPos + 3);
        
        System.out.println("Length: " + wordCount);   // 25
        System.out.println("Fox at: " + foxPos);       // 16
        System.out.println("Animal: " + animal);       // "fox"
        
        // Method chaining
        String loud = sentence.substring(16, 19).toUpperCase();
        System.out.println(loud);  // "FOX"
        
        // Boolean method for comparison
        System.out.println(animal.equals("fox"));  // true
        System.out.println(animal.equals("Fox"));  // false (case-sensitive)
    }
}