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☕ Java Lesson – Java Objects, Predefined Methods, super, toString, import & package

Is lesson me hum seekhenge:

  • Java Objects aur Object class
  • Predefined Object Methods: getClass(), hashCode(), wait(), notify(), notifyAll(), clone(), equals(), finalize(), toString()
  • super keyword usage
  • toString() method override
  • Import & Package usage
  • Examples ke saath

1️⃣ Java Object Class

  • Java me har class Object class se inherit hoti hai
  • Object class me kuch predefined methods hote hain jo sabhi objects ke liye available hain

Common Object Methods

Method Description
getClass() Object ka class info return karta hai
hashCode() Object ka hash code return karta hai
toString() Object ka string representation return karta hai
equals(Object o) Object comparison (content based)
clone() Object ka shallow copy banata hai
finalize() Garbage collection se pehle call hota hai
wait() Thread ko wait state me dalta hai
notify() Waiting thread ko notify karta hai
notifyAll() Sab waiting threads ko notify karta hai

2️⃣ Predefined Object Methods Example

class Student implements Cloneable {
    String name;
    int age;

    Student(String n, int a){
        name = n;
        age = a;
    }

    @Override
    public String toString(){
        return "Name: " + name + ", Age: " + age;
    }

    @Override
    protected void finalize() throws Throwable {
        System.out.println(name + " object is destroyed by GC");
    }
}

public class ObjectMethodsExample {
    public static void main(String[] args) throws Exception {
        Student s1 = new Student("Sujit", 22);
        Student s2 = new Student("Rahul", 25);

        System.out.println(s1.toString());      // custom toString()
        System.out.println("s1 HashCode: " + s1.hashCode());
        System.out.println("s2 HashCode: " + s2.hashCode());
        System.out.println("s1 Class: " + s1.getClass());

        System.out.println("s1 equals s2? " + s1.equals(s2));

        Student s3 = (Student) s1.clone();     // clone()
        System.out.println("Cloned s3: " + s3);

        s1 = null;
        s2 = null;
        s3 = null;

        System.gc(); // finalize() call try
    }
}

Output (example, finalize may vary):

Name: Sujit, Age: 22
s1 HashCode: 366712642
s2 HashCode: 1829164700
s1 Class: class Student
s1 equals s2? false
Cloned s3: Name: Sujit, Age: 22
Sujit object is destroyed by GC
Rahul object is destroyed by GC

3️⃣ super Keyword Usage

  • super = parent class ka reference
  • Parent class variables, methods, aur constructor access karne ke liye use hota hai
class Animal {
    String color = "White";
    void eat(){
        System.out.println("Animal is eating");
    }
}

class Dog extends Animal {
    String color = "Black";

    void display(){
        System.out.println("Dog color: " + color);
        System.out.println("Animal color: " + super.color);
        eat();
        super.eat();
    }
}

public class SuperKeywordExample {
    public static void main(String[] args){
        Dog d = new Dog();
        d.display();
    }
}

Output:

Dog color: Black
Animal color: White
Dog is eating
Animal is eating

4️⃣ toString() Method Override

  • Object class ka toString() default representation return karta hai
  • Custom class me override karke meaningful string output create karte hain
class Employee {
    String name;
    int salary;

    Employee(String n, int s){
        name = n;
        salary = s;
    }

    @Override
    public String toString(){
        return "Employee: " + name + ", Salary: " + salary;
    }
}

public class ToStringOverrideExample {
    public static void main(String[] args){
        Employee e = new Employee("Sujit", 50000);
        System.out.println(e); // automatically calls toString()
    }
}

Output:

Employee: Sujit, Salary: 50000

5️⃣ Import & Package Usage

  • Package → classes organize karne ke liye
  • Import → dusre package ki classes use karne ke liye

Example – User-defined Package

File: mypackage/Hello.java

package mypackage;

public class Hello {
    public void show(){
        System.out.println("Hello from mypackage");
    }
}

File: Main.java

import mypackage.Hello;

public class Main {
    public static void main(String[] args){
        Hello h = new Hello();
        h.show();
    }
}

Output:

Hello from mypackage

⚡ Important Points

  • Object class ke predefined methods → getClass(), hashCode(), equals(), clone(), toString(), finalize(), wait(), notify(), notifyAll()
  • super keyword → parent class ka reference
  • toString() override → meaningful object representation
  • import & package → classes ka organization aur reuse

🎯 Interview Questions

1️⃣ Object class ke common methods kya hain?
2️⃣ super keyword ka use aur example batao
3️⃣ toString() method override kyun karte hain?
4️⃣ clone() aur equals() me difference kya hai?
5️⃣ finalize() method kab call hota hai?
6️⃣ wait(), notify(), notifyAll() ka basic use kya hai?
7️⃣ Package aur import ka purpose kya hai?

🧾 Lesson Summary

Is lesson me humne seekha:

✔ Java Object class aur predefined methods
✔ getClass(), hashCode(), equals(), clone(), finalize(), toString(), wait(), notify(), notifyAll()
✔ super keyword usage
✔ toString() method override
✔ Import & Package usage

Ye lesson Java me OOP fundamentals, object methods aur package organization ka complete reference hai.

🚀 Next Lesson

Next lesson me hum seekhenge:

  • Polymorphism
  • Compile-time & Runtime Polymorphism
  • Method Overloading & Overriding
  • Abstract Classes & Interface