super keyword

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Definition

• super is a keyword used to refer to members of the immediate superclass.
• super cannot be used in static context.

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Uses of super

1️⃣ Access Superclass Variables

Syntax:

super.variableName;

Example:

super.a;
super.b;

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2️⃣ Access Superclass Methods

Syntax:

super.methodName();

Example:

super.m1();
super.m2();

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3️⃣ Invoke Superclass Constructors

Syntax:

super(parameters);

Examples:

super();// Calls default constructor
super(99);// Calls 1-arg constructor
super(99, 88);// Calls 2-arg constructor

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Lab440.java

class Lab440 {

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

class Hai {

  int a = 10;
}

class Hello extends Hai {

  int a = 20;

  void show() {
    int a = 30;

    System.out.println(a); // Local variable
    System.out.println(this.a); // Current class variable
    System.out.println(super.a); // Parent class variable
  }
}

---

Output:

30
20
10

---

Lab441.java

class Lab441 {

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

class Hai {}

class Hello extends Hai {

  int a = 20;

  void show() {
    int a = 30;

    System.out.println(a); // Local variable
    System.out.println(this.a); // Current class variable
    System.out.println(super.a); //  Compile-time error
  }
}

---

Error:

• super.a → Not allowed
• Because superclass (Hai) does not have variable a

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🧠 Key Concepts

🔹 Variable Priority

Local variable  >  this.variable  >  super.variable

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🔹 Important Rules

• super always refers to immediate parent class
• Works only inside non-static methods / constructors
• If parent member does not exist → Compilation error

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⚡ Quick Summary

• super.a → access parent variable
• this.a → access current class variable
• a → local variable (highest priority)
• Cannot use super in static context
• Cannot access non-existing parent members

Lab442.java

class Lab442 {

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

class Hai {

  static int a = 10;
}

class Hello extends Hai {

  static int a = 20;

  void show() {
    int a = 30;

    System.out.println(a); // local
    System.out.println(this.a); // current class static
    System.out.println(super.a); // parent static
  }
}

Output:

30
20
10

---

Lab443.java

class Lab443 {

  public static void main(String[] args) {
    Hello.show();
  }
}

class Hai {

  static int a = 10;
}

class Hello extends Hai {

  static int a = 20;

  static void show() {
    int a = 30;

    System.out.println(a); // ✔ allowed
    System.out.println(this.a); //  ERROR (no this in static)
    System.out.println(super.a); //  ERROR (no super in static)
  }
}

Key Point:

• this and super cannot be used in static context

---

Lab444.java

class Lab444 {

  public static void main(String[] args) {
    Hello.show();
  }
}

class Hai {

  static int a = 10;
}

class Hello extends Hai {

  static int a = 20;

  static void show() {
    int a = 30;

    System.out.println(a); // local
    System.out.println(Hello.a); // current class static
    System.out.println(Hai.a); // parent static
  }
}

Output:

30
20
10

---

Lab445.java

class Lab445 {

  public static void main(String[] args) {
    new B();
  }
}

class A {

  A(int a) {
    System.out.println("A (int) Cons");
  }
}

class B extends A {

  B() {
    super(); //  ERROR
    System.out.println("B -> D.C.");
  }
}

Error:

• No default constructor in A
• super() fails because A(int a) exists only

---

Lab446.java

class Lab446 {

  public static void main(String[] args) {
    new B();
  }
}

class A {

  A(int a) {
    System.out.println("A (int) Cons");
  }
}

class B extends A {

  B() {
    super(10); // ✔ Correct
    System.out.println("B -> D.C.");
  }
}

Output:

A (int) Cons
B -> D.C.

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Key Concepts from this Page

1. Static Variables with super

• Can be accessed using:

super.a// from parent
this.a// current class

---

2. Static Context Rules

Not allowed:

this.a
super.a

✔ Use class name instead:

ClassName.variable

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3. Constructor Rules (VERY IMPORTANT)

Rule:

👉 If parent has only parameterized constructor,

child must call it explicitly using super(args)

---

4. Constructor Execution Flow

Child constructor → calls super(...) → Parent constructor executes first

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⚡ Final Summary

• this & super not allowed in static methods
• Use ClassName.variable in static context
• If parent constructor has args → must call super(args)
• Constructor always executes Parent → Child

Lab447.java

class Lab447 {

  public static void main(String[] args) {
    new B();
  }
}

class A {

  A(int a) {
    System.out.println("A (int) Cons");
  }
}

class B extends A {

  B() {
    System.out.println("B -> D.C.");
    super(10); //  ERROR
  }
}

Error:

• super() must be the first statement in constructor
• Cannot place it after any statement

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Lab448.java

class Lab448 {

  public static void main(String[] args) {
    new B(10);
  }
}

class A {

  A() {
    System.out.println("A -> D.C.");
  }
}

class B extends A {

  B() {
    System.out.println("B -> D.C.");
  }

  B(int a) {
    this(); // calls B()
    System.out.println("B (int)");
  }
}

Output:

A ->D.C.
B ->D.C.
B (int)

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Lab449.java

class Lab449 {

  public static void main(String[] args) {
    B bobj = new B(10, "JavaWorld");
    bobj.show();
  }
}

class A {

  int a;

  A(int a) {
    System.out.println("A (int) Cons");
    this.a = a;
  }
}

class B extends A {

  String a;

  B(int a1, String a2) {
    super(a1); // calls parent constructor
    this.a = a2; // assigns child variable
    System.out.println("B (int) Cons");
  }

  void show() {
    System.out.println(this.a); // child variable
    System.out.println(super.a); // parent variable
  }
}

---

Output:

A (int)Cons
B (int)Cons
JavaWorld
10

---

Key Concepts from this Page

1. super() Rule (VERY IMPORTANT)

👉 Must be the first statement in constructor

Invalid:

System.out.println("test");
super();//  not allowed here

✔ Valid:

super();// must be first
System.out.println("test");

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2. this() vs super()

Feature	this()	super()
Calls	Same class constructor	Parent class constructor
Position	First line only	First line only
Together?	Cannot use both in same constructor

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3. Constructor Chaining

Flow (Lab448):

B(int) → this() → B() → super() → A()

Execution order:

A → B() → B(int)

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4. Variable Shadowing

Lab449 Insight:

• Same variable name in parent & child → shadowing
• Access using:

this.a// child
super.a// parent

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⚡ Final Summary

• super() must be first statement
• this() calls same class constructor
• Cannot use this() and super() together
• Constructor execution: Parent → Child
• Use this & super to resolve variable conflicts