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.

Practice

1. What does the this keyword refer to in a Java class?

easy

A. A static method

B. The current object instance

C. A superclass object

D. A local variable

Solution

1. Step 1: Understand the role of this

   The this keyword always points to the current object instance inside a class.

2. Step 2: Differentiate from other options

   It does not refer to static methods, superclass objects, or local variables.

3. Final Answer:

   The current object instance -> Option B

4. Quick Check:

   this = current object instance [OK]

Hint: Remember: this means "this object" [OK]

Common Mistakes:

• Confusing this with static context
• Thinking this refers to superclass
• Assuming this is a local variable

2. Which of the following is the correct way to use this to call another constructor in the same class?

easy

A. call(this);

B. super();

C. this.call();

D. this();

Solution

1. Step 1: Identify constructor call syntax

   In Java, calling another constructor in the same class uses this(); as the first statement.

2. Step 2: Eliminate incorrect options

   super(); calls superclass constructor, this.call(); is invalid syntax, and call(this); is not a constructor call.

3. Final Answer:

   this(); -> Option D

4. Quick Check:

   Constructor chaining uses this(); [OK]

Hint: Use this(); to call another constructor [OK]

Common Mistakes:

• Using super(); instead of this();
• Trying to call constructor like a method
• Placing this(); not as first statement

3. What will be the output of the following code?

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

medium

A. 10\n20

B. 20\n20

C. 20\n10

D. 10\n10

Solution

1. Step 1: Identify local and instance variables

   Inside printX(), local x is 20, instance x is 10.

2. Step 2: Understand this.x usage

   System.out.println(x); prints local 20, System.out.println(this.x); prints instance 10.

3. Final Answer:

   20 10 -> Option C

4. Quick Check:

   Local x = 20, this.x = 10 [OK]

Hint: Local variable hides instance; use this for instance [OK]

Common Mistakes:

• Confusing local and instance variables
• Ignoring this keyword
• Expecting both prints to be same

4. Identify the error in the following code snippet:

class Sample {
  int value;
  Sample(int value) {
    value = value;
  }
}

medium

A. The constructor does not assign parameter to instance variable

B. Syntax error: missing this keyword

C. Cannot have parameter and instance variable with same name

D. No error, code works fine

Solution

1. Step 1: Analyze assignment in constructor

   The statement value = value; assigns the parameter to itself, not to the instance variable.

2. Step 2: Correct usage with this

   To assign parameter to instance variable, use this.value = value;.

3. Final Answer:

   The constructor does not assign parameter to instance variable -> Option A

4. Quick Check:

   Missing this. causes no instance update [OK]

Hint: Use this.variable = variable; to assign correctly [OK]

Common Mistakes:

• Assuming parameter assigns instance variable automatically
• Thinking same names cause syntax error
• Ignoring need for this keyword

5. Consider the following class:

class Point {
  int x, y;
  Point() {
    this(0, 0);
  }
  Point(int x, int y) {
    this.x = x;
    this.y = y;
  }
  void move(int x, int y) {
    x = x;
    this.y = y;
  }
  String display() {
    return "(" + this.x + ", " + this.y + ")";
  }
  public static void main(String[] args) {
    Point p = new Point();
    p.move(5, 10);
    System.out.println(p.display());
  }
}

What will be the output when running the main method?

hard

A. (0, 10)

B. (5, 10)

C. (0, 0)

D. Compilation error

Solution

1. Step 1: Understand constructor chaining

   The no-arg constructor calls this(0, 0); setting x=0 and y=0.

2. Step 2: Analyze move method

   Inside move, x = x; assigns parameter to itself, so instance x remains 0. this.y = y; updates instance y to 10.

3. Step 3: Check display output

   Returns string with instance variables: (0, 10).

4. Final Answer:

   (0, 10) -> Option A

5. Quick Check:

   this.x unchanged, this.y updated [OK]

Hint: Assign instance vars with this.var = var; inside methods [OK]

Common Mistakes:

• Assuming x = x; updates instance variable
• Ignoring constructor chaining effect
• Expecting both coordinates to update