Super keyword in Java - Step-by-Step Execution

Concept Flow - Super keyword

Create Subclass Object

↓

Call Subclass Constructor

↓

Inside Subclass Constructor

↓

Use super() to call Parent Constructor

↓

Parent Constructor runs

↓

Return to Subclass Constructor

↓

Complete Subclass Constructor

↓

Object Ready

Shows how creating a subclass object calls the subclass constructor, which uses super() to run the parent constructor first, then finishes.

Execution Sample

Java

class Parent {
  Parent() { System.out.println("Parent constructor"); }
}
class Child extends Parent {
  Child() { super(); System.out.println("Child constructor"); }
}
new Child();

Creates a Child object that calls Parent constructor first using super(), then runs Child constructor.

Execution Table

Step	Action	Output	Notes
1	Create Child object		Start creating Child instance
2	Call Child constructor		Child() starts
3	Call super() inside Child constructor	Parent constructor	Parent() runs first
4	Return to Child constructor	Child constructor	Child() continues
5	Child constructor ends		Object creation complete

💡 Child object fully created after Parent and Child constructors run

Variable Tracker

Variable	Start	After Step 3	After Step 5
this (Child object)	null	partially constructed (Parent part done)	fully constructed

Key Moments - 3 Insights

Why does the Parent constructor run before the Child constructor finishes?

Because super() in the Child constructor calls the Parent constructor first, as shown in step 3 of the execution_table.

What happens if we omit super() in the Child constructor?

Java inserts super() automatically if no other constructor call is present, so Parent constructor still runs first.

Can super() be called after some code in the Child constructor?

No, super() must be the first statement in the Child constructor, or the code will not compile.

Visual Quiz - 3 Questions

Look at the execution_table, what is printed at step 3?

A No output

B Child constructor

C Parent constructor

D Error

💡 Hint

Check the Output column for step 3 in the execution_table

At which step does the Child constructor print its message?

A Step 4

B Step 3

C Step 2

D Step 5

💡 Hint

Look at the Output and Notes columns for step 4 in the execution_table

If super() was missing in the Child constructor, what would happen?

A Parent constructor would not run

B Parent constructor runs automatically

C Compilation error

D Child constructor runs twice

💡 Hint

Recall the key_moments about implicit super() call

Concept Snapshot

Super keyword in Java:
- Used in subclass to call parent class constructor or methods
- super() must be first line in subclass constructor
- Ensures parent class is initialized before subclass
- Helps access overridden methods or hidden variables
- If omitted, Java inserts super() automatically in constructor

Full Transcript

This visual trace shows how the super keyword works in Java. When creating a Child object, the Child constructor runs. Inside it, super() calls the Parent constructor first, printing "Parent constructor". Then control returns to the Child constructor, which prints "Child constructor". This order ensures the parent part of the object is set up before the child part. The variable tracker shows the Child object is partially constructed after the Parent constructor runs, and fully constructed at the end. Key points include that super() must be the first statement in the constructor, and if omitted, Java adds it automatically. The quizzes test understanding of the print order and behavior if super() is missing.

Practice

1. What does the super keyword do in Java?

easy

A. It defines a static method in the class.

B. It creates a new object of the child class.

C. It accesses methods and variables from the parent class.

D. It terminates the program execution.

Solution

1. Step 1: Understand the role of super

   The super keyword is used to refer to the parent class's members (methods or variables) from a child class.

2. Step 2: Compare options with definition

   Only It accesses methods and variables from the parent class. correctly describes this behavior. Other options describe unrelated actions.

3. Final Answer:

   It accesses methods and variables from the parent class. -> Option C

4. Quick Check:

   super accesses parent members = A [OK]

Hint: Remember: super means parent class access [OK]

Common Mistakes:

• Thinking super creates new objects
• Confusing super with this keyword
• Assuming super ends program

2. Which of the following is the correct way to call a parent class constructor in Java?

easy

A. super();

B. this();

C. parent();

D. base();

Solution

1. Step 1: Recall syntax for parent constructor call

   In Java, super() is used inside a child constructor to call the parent class constructor.

2. Step 2: Evaluate options

   Only super(); uses the correct keyword super(). Others are invalid or refer to different concepts.

3. Final Answer:

   super(); -> Option A

4. Quick Check:

   Parent constructor call = super() [OK]

Hint: Use super() to call parent constructor [OK]

Common Mistakes:

• Using this() instead of super()
• Trying to call parent() which is invalid
• Confusing base() with super()

3. What will be the output of the following code?

class Parent {
  int x = 10;
}
class Child extends Parent {
  int x = 20;
  void printX() {
    System.out.println(super.x);
  }
}
public class Test {
  public static void main(String[] args) {
    Child c = new Child();
    c.printX();
  }
}

medium

A. 20

B. Compilation error

C. 0

D. 10

Solution

1. Step 1: Understand variable hiding and super usage

   The child class has its own x = 20, but super.x accesses the parent's x which is 10.

2. Step 2: Trace the print statement

   The method printX() prints super.x, so it prints 10.

3. Final Answer:

   10 -> Option D

4. Quick Check:

   super.x accesses parent variable = 10 [OK]

Hint: super.variable accesses parent class variable [OK]

Common Mistakes:

• Printing child variable instead of parent
• Confusing super.x with this.x
• Expecting compilation error

4. Identify the error in this code snippet:

class Parent {
  void show() {
    System.out.println("Parent show");
  }
}
class Child extends Parent {
  void show() {
    super.show();
    System.out.println("Child show");
  }
  void display() {
    super();
  }
}

medium

A. super.show() is invalid inside child class

B. super() cannot be called like a method in display()

C. Child class cannot override show() method

D. No error, code is correct

Solution

1. Step 1: Check usage of super in methods

   Calling super.show() inside overridden method is valid to call parent method.

2. Step 2: Analyze super() call in display()

   super() can only be used to call parent constructor inside child constructor, not as a method call elsewhere.

3. Final Answer:

   super() cannot be called like a method in display() -> Option B

4. Quick Check:

   super() only in constructor = D [OK]

Hint: super() only calls parent constructor inside child constructor [OK]

Common Mistakes:

• Using super() outside constructor
• Thinking super.show() is invalid
• Believing overriding is not allowed

5. Given these classes:

class Animal {
  String name;
  Animal(String name) {
    this.name = name;
  }
  void sound() {
    System.out.println("Animal sound");
  }
}
class Dog extends Animal {
  Dog() {
    super("Dog");
  }
  void sound() {
    super.sound();
    System.out.println("Bark");
  }
}
public class Test {
  public static void main(String[] args) {
    Dog d = new Dog();
    d.sound();
    System.out.println(d.name);
  }
}

What is the output when running Test.main()?

hard

A. Animal sound
Bark
Dog

B. Bark
Animal sound
Dog

C. Animal sound
Dog
Bark

D. Compilation error due to super()

Solution

1. Step 1: Understand constructor chaining

   The Dog constructor calls super("Dog"), setting name to "Dog" in Animal.

2. Step 2: Trace the sound() method call

   Dog.sound() calls super.sound() which prints "Animal sound", then prints "Bark".

3. Step 3: Print the name field

   Printing d.name outputs "Dog".

4. Final Answer:

   Animal sound Bark Dog -> Option A

5. Quick Check:

   super() sets name, super.sound() prints parent sound = A [OK]

Hint: super() sets parent state; super.method() calls parent method [OK]

Common Mistakes:

• Expecting Dog before Animal sound
• Confusing order of prints
• Thinking super() causes error