Object Class

1// Every class extends Object
2public class MyClass {
3    // Implicitly: extends Object
4}
5
6// All these are valid
7Object obj1 = new MyClass();
8Object obj2 = new String("Hello");
9Object obj3 = new Integer(42);
10Object obj4 = new int[]{1, 2, 3};
11
12// Object methods available on any object
13MyClass mc = new MyClass();
14mc.toString();      // Inherited from Object
15mc.equals(obj1);    // Inherited from Object
16mc.hashCode();      // Inherited from Object
17mc.getClass();      // Inherited from Object
18
19// getClass() returns runtime type
20Object mystery = "Hello";
21System.out.println(mystery.getClass().getName());  // java.lang.String
22System.out.println(mystery.getClass().getSimpleName());  // String

1// Default Object methods behavior
2public class Person {
3    private String name;
4    private int age;
5    
6    public Person(String name, int age) {
7        this.name = name;
8        this.age = age;
9    }
10}
11
12Person p1 = new Person("Alice", 25);
13Person p2 = new Person("Alice", 25);
14
15// Default toString() - class name + @ + hash code
16System.out.println(p1.toString());  // Person@15db9742
17
18// Default equals() - reference comparison (==)
19System.out.println(p1.equals(p2));  // false! Different objects
20
21// Default hashCode() - typically based on memory address
22System.out.println(p1.hashCode());  // Some integer
23System.out.println(p2.hashCode());  // Different integer
24
25// That's why we override these methods!

1// Properly overriding Object methods
2public class Person {
3    private String name;
4    private int age;
5    
6    public Person(String name, int age) {
7        this.name = name;
8        this.age = age;
9    }
10    
11    @Override
12    public String toString() {
13        return "Person{name='" + name + "', age=" + age + "}";
14    }
15    
16    @Override
17    public boolean equals(Object obj) {
18        if (this == obj) return true;
19        if (obj == null || getClass() != obj.getClass()) return false;
20        Person person = (Person) obj;
21        return age == person.age && Objects.equals(name, person.name);
22    }
23    
24    @Override
25    public int hashCode() {
26        return Objects.hash(name, age);
27    }
28}
29
30Person p1 = new Person("Alice", 25);
31Person p2 = new Person("Alice", 25);
32
33System.out.println(p1.toString());  // Person{name='Alice', age=25}
34System.out.println(p1.equals(p2)); // true! Same values
35System.out.println(p1.hashCode() == p2.hashCode()); // true!

1// Other Object methods
2public class Demo {
3    public static void main(String[] args) {
4        Object obj = new String("Hello");
5        
6        // getClass() - runtime type info
7        Class<?> clazz = obj.getClass();
8        System.out.println(clazz.getName());       // java.lang.String
9        System.out.println(clazz.isArray());       // false
10        System.out.println(clazz.getSuperclass()); // class java.lang.Object
11        
12        // instanceof vs getClass()
13        Object str = "Hello";
14        System.out.println(str instanceof String);  // true
15        System.out.println(str instanceof Object);  // true
16        System.out.println(str.getClass() == String.class); // true
17        
18        // clone() - needs Cloneable interface
19        // finalize() - deprecated, don't use
20        
21        // Thread methods (wait, notify, notifyAll)
22        // Used for inter-thread communication
23        // Requires synchronized block
24    }
25}