Parent and child classes in Java - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a parent class in Java?

A parent class, also called a superclass, is a class that other classes (child classes) inherit from. It provides common properties and methods that child classes can use or override.

beginner

What does it mean when a child class extends a parent class?

When a child class extends a parent class, it means the child inherits all the fields and methods of the parent. The child can use or change these inherited features and add new ones.

intermediate

How can a child class override a method from its parent class?

A child class can provide its own version of a method by defining a method with the same name and parameters. This is called method overriding and allows the child to change behavior.

intermediate

What keyword is used in Java to call a parent class constructor from a child class?

The keyword <code>super</code> is used inside a child class constructor to call the parent class constructor. This helps initialize the parent part of the object.

intermediate

Can a child class access private members of its parent class directly?

No, private members of a parent class are not accessible directly by the child class. They can only be accessed through public or protected methods provided by the parent.

Which keyword is used to create a child class from a parent class in Java?

A inherits

B implements

C extends

D super

The extends keyword is used to create a child class that inherits from a parent class.

What happens when a child class overrides a method from its parent?

A The child class provides its own version of the method.

B The parent method is deleted.

C The method becomes private.

D The child class cannot use the method.

Overriding means the child class gives its own version of the method, replacing the parent's behavior when called on the child.

How can a child class call the constructor of its parent class?

A Using the <code>extends()</code> keyword.

B Using the <code>this()</code> keyword.

C Using the <code>parent()</code> keyword.

D Using the <code>super()</code> keyword.

The super() keyword calls the parent class constructor from the child class.

Can a child class access private fields of its parent class directly?

A Yes, always.

B No, private fields are not accessible directly.

C Only if the child class is in the same package.

D Only if the child class overrides them.

Private fields are hidden from child classes and can only be accessed through public or protected methods.

What is the main benefit of using parent and child classes?

A Code reuse and organization.

B Slower program execution.

C Making code harder to read.

D Preventing any changes to code.

Inheritance allows child classes to reuse code from parent classes, making programs easier to organize and maintain.

Explain how inheritance works between parent and child classes in Java.

Think about how a child can get features from a parent and change them.

Describe how to use the super keyword in a child class constructor.

Remember how to connect child and parent constructors.

Practice

1.

What keyword is used in Java to create a child class from a parent class?

easy

A. extends

B. implements

C. inherits

D. super

Solution

1. Step 1: Understand class inheritance in Java

   Java uses a specific keyword to link a child class to a parent class, allowing reuse of code.

2. Step 2: Identify the correct keyword

   The keyword extends is used to create a child class that inherits from a parent class.

3. Final Answer:

   extends -> Option A

4. Quick Check:

   Inheritance keyword = extends [OK]

Hint: Remember: child class extends parent class [OK]

Common Mistakes:

• Using 'implements' which is for interfaces
• Using 'inherits' which is not a Java keyword
• Confusing 'super' keyword with inheritance declaration

2.

Which of the following is the correct syntax to declare a child class Dog that inherits from a parent class Animal?

?

easy

A. class Dog implements Animal {}

B. class Dog inherits Animal {}

C. class Dog extends Animal {}

D. class Dog Animal {}

Solution

1. Step 1: Recall Java class inheritance syntax

   In Java, the child class uses the keyword extends followed by the parent class name.

2. Step 2: Match the correct syntax

   Only class Dog extends Animal {} is valid syntax for inheritance.

3. Final Answer:

   class Dog extends Animal {} -> Option C

4. Quick Check:

   Syntax for inheritance = extends [OK]

Hint: Use 'extends' keyword to link child and parent classes [OK]

Common Mistakes:

• Using 'inherits' which is not a Java keyword
• Using 'implements' which is for interfaces
• Omitting the keyword between class names

3.

What will be the output of the following Java code?

class Parent {
    void show() {
        System.out.println("Parent class");
    }
}
class Child extends Parent {
    void show() {
        System.out.println("Child class");
    }
}
public class Test {
    public static void main(String[] args) {
        Parent obj = new Child();
        obj.show();
    }
}

medium

A. Parent class

B. Runtime error

C. Compilation error

D. Child class

Solution

1. Step 1: Understand method overriding and polymorphism

   The child class overrides the show() method of the parent class. The object is declared as parent type but created as child type.

2. Step 2: Determine which method runs at runtime

   Java uses runtime polymorphism, so the child class's show() method is called.

3. Final Answer:

   Child class -> Option D

4. Quick Check:

   Overridden method runs from child class [OK]

Hint: Object type decides method at runtime, not reference type [OK]

Common Mistakes:

• Thinking parent method runs because of reference type
• Expecting compilation or runtime errors
• Ignoring method overriding rules

4.

Find the error in the following Java code snippet:

class Parent {
    void greet() {
        System.out.println("Hello from Parent");
    }
}
class Child extends Parent {
    void greet() {
        System.out.println("Hello from Child");
    }
}
public class Test {
    public static void main(String[] args) {
        Child obj = new Parent();
        obj.greet();
    }
}

medium

A. Cannot assign Parent object to Child reference

B. Method greet() is missing return type

C. Child cannot extend Parent

D. No error, code runs fine

Solution

1. Step 1: Analyze object assignment compatibility

   In Java, a parent class object cannot be assigned to a child class reference because the parent may lack child-specific features.

2. Step 2: Identify the error in the code

   The line Child obj = new Parent(); causes a compile-time error due to incompatible types.

3. Final Answer:

   Cannot assign Parent object to Child reference -> Option A

4. Quick Check:

   Parent object cannot be assigned to child variable [OK]

Hint: Child reference needs child or subclass object [OK]

Common Mistakes:

• Thinking parent object can be assigned to child reference
• Confusing method return types with assignment errors
• Ignoring Java type compatibility rules

5.

Given the classes below, what will be the output when running new Child().display();?

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

hard

A. Child display

B. Parent display
Child display

C. Parent display

D. Compilation error

Solution

1. Step 1: Understand use of super in child method

   The child class's display() method calls super.display(), which runs the parent class's display() method first.

2. Step 2: Determine output sequence

   First, "Parent display" is printed, then "Child display" is printed on the next line.

3. Final Answer:

   Parent display Child display -> Option B

4. Quick Check:

   super calls parent method before child output [OK]

Hint: super.method() runs parent method inside child method [OK]

Common Mistakes:

• Ignoring the call to super.display()
• Expecting only child output
• Thinking super causes error without constructor