This, Super and other Keywords

Let’s now focus on important Java keywords that give you fine-grained control over object behavior and class design. These are subtle, powerful, and interview-favorite topics too.

We’ll cover:

• this
• super
• instanceof
• return
• break / continue (control flow)
• new
• null

1️⃣ this Keyword — Refers to the Current Object#

✅ Purpose:#

• To distinguish between instance variables and parameters when they have the same name.
• To call another constructor from a constructor.
• To pass current object as a reference.

📦 Example:#

class Student {
    String name;

    Student(String name) {
        this.name = name; // "this" refers to current object
    }

    void display() {
        System.out.println("Name: " + this.name);
    }
}

🔁 Constructor Chaining:#

class Student {
    String name;
    int age;

    Student() {
        this("Unknown", 0); // Calls another constructor
    }

    Student(String name, int age) {
        this.name = name;
        this.age = age;
    }
}

2️⃣ super Keyword — Refers to Parent Class#

✅ Purpose:#

• To call superclass constructor.
• To access superclass methods or variables that are overridden.

📦 Example:#

class Animal {
    void sound() {
        System.out.println("Animal sound");
    }
}

class Dog extends Animal {
    void sound() {
        super.sound(); // Calls Animal's sound()
        System.out.println("Dog barks");
    }
}

🏗 Super Constructor Call:#

class Animal {
    Animal(String type) {
        System.out.println("Animal: " + type);
    }
}

class Dog extends Animal {
    Dog() {
        super("Mammal"); // Calls Animal's constructor
        System.out.println("Dog created");
    }
}

3️⃣ instanceof — Type Check#

✅ Purpose:#

Used to check object type at runtime.

class Animal {}
class Dog extends Animal {}

public class Main {
    public static void main(String[] args) {
        Animal a = new Dog();
        System.out.println(a instanceof Dog); // true
        System.out.println(a instanceof Animal); // true
    }
}

4️⃣ return — Exits from Method#

✅ Purpose:#

Returns a value from a method or exits early.

int square(int x) {
    return x * x;
}

void printEven(int x) {
    if (x % 2 != 0) return; // Exit if not even
    System.out.println(x + " is even");
}

5️⃣ break and continue — Control Loop Execution#

🔄 break: Exit the loop immediately#

🔄 continue: Skip the current iteration#

for (int i = 0; i < 5; i++) {
    if (i == 3) break;
    System.out.println(i);  // Output: 0 1 2
}

for (int i = 0; i < 5; i++) {
    if (i == 3) continue;
    System.out.println(i);  // Output: 0 1 2 4
}

6️⃣ new Keyword — Object Creation#

✅ Purpose:#

Creates a new instance (object) of a class.

Student s1 = new Student("Alex");

This allocates memory and calls the constructor.

7️⃣ null Keyword — Represents Absence of Object#

✅ Purpose:#

null means a reference variable doesn’t point to any object.

String name = null;

if (name == null) {
    System.out.println("Name is not assigned");
}

🛑 Be careful: calling methods on a null reference leads to a NullPointerException.

🎓 Summary Table#

🏠 Homework for Students#

📌 Tasks:#

1. Create a class Employee with:

   • String name, int id
   • Use this to resolve name conflict in constructor.
   • Create another constructor that chains using this(...)

2. Create class Vehicle and subclass Bike:

   • Override method run(), but call parent version using super.

3. Create a class Shape and subclass Circle.

   • Check with instanceof whether a Shape reference points to a Circle.

4. Write a loop from 1 to 10:

   • Use break when the number is 5.
   • Use continue to skip even numbers.

Would you like to now dive into Exception Handling, or explore Object Class Methods like equals(), hashCode(), toString() next?