Super keyword in Java - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the super keyword in Java?

The <code>super</code> keyword is used to refer to the parent class of the current object. It helps access parent class methods, constructors, and variables.

beginner

How do you call a parent class constructor using <code>super</code>?

You call a parent class constructor by writing <code>super(arguments);</code> as the first line inside the child class constructor.

intermediate

Can <code>super</code> be used to access parent class variables if they are hidden by child class variables?

Yes, <code>super.variableName</code> accesses the parent class variable when the child class has a variable with the same name.

intermediate

What happens if you don't explicitly call <code>super()</code> in a child class constructor?

Java automatically inserts a call to the no-argument parent constructor super() if you don't call it explicitly. If the parent has no no-arg constructor, it causes a compile-time error.

beginner

Can super be used to call overridden methods in the parent class?

Yes, <code>super.methodName()</code> calls the parent class version of a method that is overridden in the child class.

What does super() do inside a child class constructor?

A Calls the child class constructor

B Creates a new object

C Calls a method named super

D Calls the parent class constructor

super() calls the parent class constructor to initialize the parent part of the object.

How do you access a parent class variable hidden by a child class variable?

A Use <code>super.variableName</code>

B Use <code>this.variableName</code>

C Use <code>parent.variableName</code>

D Use <code>variableName()</code>

super.variableName accesses the parent class variable when hidden by the child class.

If a parent class has no no-argument constructor, what happens if the child constructor does not call super() explicitly?

A Code compiles fine

B Code throws a runtime exception

C Code fails to compile

D Parent constructor is called automatically with arguments

Java inserts super() automatically if not called explicitly, so if no no-arg constructor exists, compilation fails.

Which of these can super NOT be used for?

A Creating new objects

B Accessing parent class variables

C Calling parent class methods

D Calling parent class constructors

super is for accessing parent class members, not for creating new objects.

How do you call a parent class method that is overridden in the child class?

A <code>this.methodName()</code>

B <code>super.methodName()</code>

C <code>parent.methodName()</code>

D <code>methodName()</code>

super.methodName() calls the parent class version of an overridden method.

Explain how the super keyword helps in constructor chaining in Java inheritance.

Think about how child and parent constructors work together.

Describe how super can be used to access hidden variables and overridden methods in a child class.

Consider when child class has same names as parent.

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