Objects

// String is a class. greeting is an object (instance of String)
String greeting = "Hello, AP CS!";

// greeting has state (the characters) and behavior (methods like length(), substring())
System.out.println(greeting.length());     // 13
System.out.println(greeting.toUpperCase()); // HELLO, AP CS!

String name1 = "Alice";
String name2 = "Bob";
String name3 = "Charlie";
// Three different String objects, all from the same String class

// Primitive — holds the value directly
int score = 95;

// Reference — holds a reference to an object in memory
String name = "Alice";

Primitive:     score  ->  [ 95 ]

Reference:     name   ->  [ address ] ---->  String object: "Alice"

String s = null;  // s doesn't reference any object
System.out.println(s.length());  // NullPointerException!

String a = "hello";
String b = a;  // b now points to the same object as a

System.out.println(a == b);  // true — same reference

a  -->  [ address ] --\\
                       \\---->  "hello"
b  -->  [ address ] --/

String word = "computer";

// Calling methods with the dot operator
int len = word.length();           // 8
char first = word.charAt(0);       // 'c'
String upper = word.toUpperCase(); // "COMPUTER"
boolean hasC = word.contains("com"); // true

public class ObjectDemo {
    public static void main(String[] args) {
        // Creating String objects
        String course = "AP Computer Science A";
        String empty = "";
        String nothing = null;
        
        // Using methods (behavior)
        System.out.println(course.length());        // 22
        System.out.println(course.substring(0, 2)); // "AP"
        System.out.println(empty.length());          // 0
        
        // Checking for null before using
        if (nothing != null) {
            System.out.println(nothing.length());
        } else {
            System.out.println("nothing is null!");
        }
    }
}