Instance methods in Java - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of instance method call

What is the output of this Java program when run?

Java

public class Car {
    private String model;
    public Car(String model) {
        this.model = model;
    }
    public String getModel() {
        return model;
    }
    public static void main(String[] args) {
        Car car = new Car("Tesla");
        System.out.println(car.getModel());
    }
}

A Tesla

B car

C null

D Compilation error

💡 Hint

Look at what the getModel() method returns.

The instance method getModel() returns the model field, which was set to "Tesla" in the constructor. So the output is "Tesla".

❓ Predict Output

intermediate

Instance method modifying object state

What will be printed after running this Java code?

Java

public class Counter {
    private int count = 0;
    public void increment() {
        count++;
    }
    public int getCount() {
        return count;
    }
    public static void main(String[] args) {
        Counter c = new Counter();
        c.increment();
        c.increment();
        System.out.println(c.getCount());
    }
}

A 1

B 0

C Compilation error

D 2

💡 Hint

Each call to increment increases count by one.

The increment method increases the count field by 1 each time it is called. It is called twice, so count becomes 2.

🔧 Debug

advanced

Identify the error in instance method usage

What error will this Java code produce when compiled or run?

Java

public class Person {
    private String name;
    public Person(String name) {
        this.name = name;
    }
    public void printName() {
        System.out.println(name);
    }
    public static void main(String[] args) {
        printName();
    }
}

A No error, prints the name

B Cannot make a static reference to the non-static method printName()

C Compilation error: missing semicolon

D NullPointerException at runtime

💡 Hint

Static methods cannot call instance methods without an object.

The main method is static and tries to call printName() which is an instance method without an object. This causes a compilation error.

❓ Predict Output

advanced

Instance method with parameter and return value

What is the output of this Java program?

Java

public class Calculator {
    public int multiply(int a, int b) {
        return a * b;
    }
    public static void main(String[] args) {
        Calculator calc = new Calculator();
        int result = calc.multiply(3, 4);
        System.out.println(result);
    }
}

A 12

B 7

C 34

D Compilation error

💡 Hint

Multiply 3 by 4.

The multiply method returns the product of the two parameters. 3 * 4 = 12.

🧠 Conceptual

expert

Understanding instance method behavior with multiple objects

Consider this Java code. What will be printed when it runs?

Java

public class Light {
    private boolean isOn = false;
    public void switchOn() {
        isOn = true;
    }
    public void switchOff() {
        isOn = false;
    }
    public String getState() {
        return isOn ? "On" : "Off";
    }
    public static void main(String[] args) {
        Light kitchenLight = new Light();
        Light bedroomLight = new Light();
        kitchenLight.switchOn();
        System.out.println(kitchenLight.getState() + " " + bedroomLight.getState());
    }
}

A On On

B Off Off

C On Off

D Compilation error

💡 Hint

Each object has its own isOn state.

kitchenLight switches on, so its isOn is true. bedroomLight remains off, so its isOn is false. Output is "On Off".

Practice

1. What is true about instance methods in Java?

easy

A. They belong to objects and can access instance variables.

B. They can be called without creating an object.

C. They must be declared as static.

D. They cannot use the this keyword.

Solution

1. Step 1: Understand instance methods

   Instance methods belong to objects and can access the object's data (instance variables).

2. Step 2: Check other options

   Instance methods require an object to be called, are not static, and can use this to refer to the current object.

3. Final Answer:

   They belong to objects and can access instance variables. -> Option A

4. Quick Check:

   Instance methods = object-specific behavior [OK]

Hint: Instance methods need an object to work with [OK]

Common Mistakes:

• Thinking instance methods are static
• Calling instance methods without an object
• Believing instance methods can't use 'this'

2. Which of the following is the correct way to call an instance method display() of an object obj?

easy

A. display();

B. static display();

C. ClassName.display();

D. obj.display();

Solution

1. Step 1: Recall instance method call syntax

   Instance methods are called using the object name followed by dot and method name, like obj.display();.

2. Step 2: Check other options

   Calling without object or using class name is for static methods; static display(); is invalid syntax.

3. Final Answer:

   obj.display(); -> Option D

4. Quick Check:

   Call instance method with object name [OK]

Hint: Use objectName.methodName() to call instance methods [OK]

Common Mistakes:

• Calling instance method without object
• Using class name for instance method call
• Trying to call instance method as static

3. What will be the output of this code?

class Car {
  String model;
  void setModel(String m) {
    model = m;
  }
  void printModel() {
    System.out.println(model);
  }
}
public class Test {
  public static void main(String[] args) {
    Car c = new Car();
    c.setModel("Tesla");
    c.printModel();
  }
}

medium

A. Compilation error

B. Tesla

C. null

D. Runtime error

Solution

1. Step 1: Analyze method calls

   The object c calls setModel("Tesla"), which sets the instance variable model to "Tesla".

2. Step 2: Check printModel output

   printModel() prints the current value of model, which is "Tesla".

3. Final Answer:

   Tesla -> Option B

4. Quick Check:

   Instance variable set then printed = Tesla [OK]

Hint: Instance methods change and show object data [OK]

Common Mistakes:

• Expecting null because of forgetting setModel call
• Confusing instance and static variables
• Thinking printModel returns a value

4. Identify the error in this code snippet:

class Person {
  String name;
  void setName(String name) {
    name = name;
  }
  void printName() {
    System.out.println(name);
  }
}
public class Main {
  public static void main(String[] args) {
    Person p = new Person();
    p.setName("Alice");
    p.printName();
  }
}

medium

A. The class Person should be declared public.

B. The printName method is missing a return statement.

C. The method setName does not assign the parameter to the instance variable.

D. The main method should be static void.

Solution

1. Step 1: Check setName method assignment

   The line name = name; assigns the parameter to itself, not to the instance variable.

2. Step 2: Understand effect on output

   Because instance variable name is not set, printName() prints null.

3. Final Answer:

   The method setName does not assign the parameter to the instance variable. -> Option C

4. Quick Check:

   Parameter shadows instance variable, no assignment [OK]

Hint: Use this.name = name to assign instance variable [OK]

Common Mistakes:

• Not using 'this' to distinguish variables
• Expecting printName to throw error
• Thinking setName returns a value

5. You want to create a class BankAccount with an instance method deposit that adds money to the account balance, and another method getBalance that returns the current balance. Which code correctly implements these instance methods?

hard

A. class BankAccount {
private double balance;
void deposit(double amount) {
balance += amount;
}
double getBalance() {
return balance;
}
}

B. class BankAccount {
static double balance;
void deposit(double amount) {
balance += amount;
}
double getBalance() {
return balance;
}
}

C. class BankAccount {
private double balance;
static void deposit(double amount) {
balance += amount;
}
static double getBalance() {
return balance;
}
}

D. class BankAccount {
double balance;
void deposit() {
balance += amount;
}
double getBalance() {
return balance;
}
}

Solution

1. Step 1: Check instance variable and method signatures

   class BankAccount { private double balance; void deposit(double amount) { balance += amount; } double getBalance() { return balance; } } uses a private instance variable balance and instance methods that correctly update and return it.

2. Step 2: Verify other options

   class BankAccount { static double balance; void deposit(double amount) { balance += amount; } double getBalance() { return balance; } } uses static variable, which shares balance across all accounts. class BankAccount { private double balance; static void deposit(double amount) { balance += amount; } static double getBalance() { return balance; } } uses static methods incorrectly. class BankAccount { double balance; void deposit() { balance += amount; } double getBalance() { return balance; } }'s deposit method lacks parameter amount.

3. Final Answer:

   class BankAccount { private double balance; void deposit(double amount) { balance += amount; } double getBalance() { return balance; } } -> Option A

4. Quick Check:

   Instance methods update and return object-specific data [OK]

Hint: Instance methods use instance variables, not static, with parameters [OK]

Common Mistakes:

• Using static variables for per-object data
• Missing method parameters
• Making instance methods static incorrectly