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.