Parent and child classes in Java - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of method overriding in parent and child classes

What is the output of the following Java code?

Java

class Parent {
    void show() {
        System.out.println("Parent show");
    }
}

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

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

A Parent show

B Runtime error

C Compilation error

D Child show

💡 Hint

Remember that method overriding uses the actual object's method, not the reference type's method.

The object obj is declared as Parent but created as Child. The show() method is overridden in Child. At runtime, the Child version is called, so it prints "Child show".

❓ Predict Output

intermediate

Output of accessing parent and child fields

What will be printed when running this Java code?

Java

class Parent {
    String name = "Parent";
}

class Child extends Parent {
    String name = "Child";
}

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

A Parent

B Child

C Compilation error

D Runtime error

💡 Hint

Field access depends on the reference type, not the object type.

The variable obj is of type Parent. Field access uses the reference type, so obj.name accesses Parent's name field, printing "Parent".

🔧 Debug

advanced

Identify the error in constructor chaining

What error will this Java code produce?

Java

class Parent {
    Parent() {
        System.out.println("Parent constructor");
    }
}

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

public class Test {
    public static void main(String[] args) {
        new Child();
    }
}

A StackOverflowError at runtime

B Compilation error: recursive constructor call

C Prints "Parent constructor" then "Child constructor"

D Prints "Child constructor" only

💡 Hint

Check how constructors call each other using this() and super().

The constructor Child() calls itself recursively with this(), causing a compile-time error for recursive constructor invocation.

❓ Predict Output

advanced

Output of method call with super keyword

What is the output of this Java program?

Java

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

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

public class Test {
    public static void main(String[] args) {
        Child c = new Child();
        c.show();
    }
}

A Child display\nChild display

B Child display\nParent display

C Parent display\nChild display

D Parent display\nParent display

💡 Hint

super.display() calls the parent method, display() calls the child method.

The show() method calls super.display() which prints "Parent display", then calls display() which is overridden in Child and prints "Child display".

🧠 Conceptual

expert

Number of objects created in inheritance

Consider this Java code:

class Parent {
    Parent() {
        System.out.println("Parent created");
    }
}

class Child extends Parent {
    Child() {
        System.out.println("Child created");
    }
}

public class Test {
    public static void main(String[] args) {
        Child c = new Child();
    }
}

How many objects are created when new Child() is executed?

A One object of type Child

B One object of type Parent

C Two objects: one Parent and one Child

D No objects, only constructors run

💡 Hint

Remember that a child object includes the parent part; only one object is created.

When new Child() runs, it creates one object of type Child. This object contains the parent part automatically. Constructors for both classes run, but only one object is created.

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