This keyword usage in Java - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of this keyword in instance method

What is the output of this Java program?

Java

public class Test {
    int x = 10;
    void printX() {
        int x = 20;
        System.out.println(this.x);
    }
    public static void main(String[] args) {
        Test obj = new Test();
        obj.printX();
    }
}

A Runtime error

B 20

C Compilation error

D 10

💡 Hint

Remember that 'this' refers to the current object instance's field, not local variables.

The local variable x shadows the instance variable x. Using 'this.x' accesses the instance variable, which is 10.

❓ Predict Output

intermediate

Using this() to call constructor

What is the output of this Java program?

Java

public class Demo {
    Demo() {
        System.out.println("No-arg constructor");
    }
    Demo(int x) {
        this();
        System.out.println("Constructor with int: " + x);
    }
    public static void main(String[] args) {
        Demo d = new Demo(5);
    }
}

A Compilation error due to recursive constructor call

B

Constructor with int: 5
No-arg constructor

C

No-arg constructor
Constructor with int: 5

D Runtime error

💡 Hint

The this() call invokes another constructor in the same class.

The constructor with int calls the no-arg constructor first, so its message prints first, then the int constructor message.

❓ Predict Output

advanced

this keyword in static context

What error does this Java code produce?

Java

public class Sample {
    static void show() {
        System.out.println(this);
    }
    public static void main(String[] args) {
        show();
    }
}

A Compilation error: Cannot use 'this' in a static context

B Prints the class name Sample

C Prints null

D Runtime NullPointerException

💡 Hint

'this' refers to an instance, but static methods have no instance.

'this' cannot be used inside static methods because there is no current object instance.

❓ Predict Output

advanced

this keyword with inner class

What is the output of this Java program?

Java

public class Outer {
    int x = 100;
    class Inner {
        int x = 200;
        void print() {
            int x = 300;
            System.out.println(x);
            System.out.println(this.x);
            System.out.println(Outer.this.x);
        }
    }
    public static void main(String[] args) {
        Outer outer = new Outer();
        Outer.Inner inner = outer.new Inner();
        inner.print();
    }
}

A

100
200
300

B

300
200
100

C

300
100
200

D Compilation error due to ambiguous 'this'

💡 Hint

Inner class can refer to outer class instance using Outer.this.

Local x is 300, inner class field x is 200, outer class field x is 100. 'this.x' in inner class refers to inner's x.

🧠 Conceptual

expert

this keyword and method reference

Consider the following Java code snippet. What will be the output when main runs?

Java

import java.util.function.Supplier;
public class RefTest {
    int value = 42;
    Supplier<Integer> getValueSupplier() {
        return this::getValue;
    }
    int getValue() {
        return value;
    }
    public static void main(String[] args) {
        RefTest obj = new RefTest();
        Supplier<Integer> supplier = obj.getValueSupplier();
        System.out.println(supplier.get());
    }
}

A 42

B Compilation error: Cannot use 'this' in method reference

C Runtime error: NullPointerException

D 0

💡 Hint

Method references can use 'this' to refer to instance methods.

The method reference this::getValue refers to the instance method getValue of the current object, returning 42.