Instance methods in Java - Mini Project: Build & Apply

Instance Methods in Java

📖 Scenario: You are creating a simple program to manage a car's details and actions. You will use instance methods to show how each car object can perform actions on its own data.

🎯 Goal: Build a Java class called Car with instance variables and instance methods. Then create an object of this class and call its methods to display information and perform actions.

📋 What You'll Learn

Create a class named Car with instance variables make and year.

Add an instance method displayInfo that prints the car's make and year.

Add an instance method startEngine that prints "Engine started".

Create a Car object in the main method and call both instance methods.

💡 Why This Matters

🌍 Real World

Instance methods let objects perform actions using their own data, just like a real car can start its own engine or show its own details.

💼 Career

Understanding instance methods is essential for object-oriented programming, which is widely used in software development jobs to model real-world entities.

1

Create the Car class with instance variables

Create a class called Car with two instance variables: String make and int year.

Java

public class Car {
    // Your code here
}

Hint

Instance variables are declared inside the class but outside any method.

2

Add instance methods to the Car class

Inside the Car class, add an instance method displayInfo that prints the car's make and year, and another instance method startEngine that prints "Engine started".

Java

public class Car {
    String make;
    int year;

    // Your code here
}

Hint

Instance methods use instance variables to perform actions related to the object.

3

Create a Car object and set its variables

In a Main class with a main method, create a Car object named myCar. Set myCar.make to "Toyota" and myCar.year to 2020.

Java

class Car {
    String make;
    int year;

    void displayInfo() {
        System.out.println("Make: " + make + ", Year: " + year);
    }

    void startEngine() {
        System.out.println("Engine started");
    }
}

public class Main {
    public static void main(String[] args) {
        // Your code here
    }
}

Hint

Use the new keyword to create an object and dot notation to set variables.

4

Call instance methods and print output

In the main method, call myCar.displayInfo() and myCar.startEngine() to print the car's details and start the engine.

Java

class Car {
    String make;
    int year;

    void displayInfo() {
        System.out.println("Make: " + make + ", Year: " + year);
    }

    void startEngine() {
        System.out.println("Engine started");
    }
}

public class Main {
    public static void main(String[] args) {
        Car myCar = new Car();
        myCar.make = "Toyota";
        myCar.year = 2020;

        // Your code here
    }
}

Hint

Call instance methods using the object name followed by dot and method name with parentheses.

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