Instance methods in Java - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to call the instance method greet on the object person.

Java

Person person = new Person();
person.[1]();

Drag options to blanks, or click blank then click option'

AsayHello

BGreet

Cgreet

DgreetPerson

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect method names or wrong capitalization.

Trying to call the method without the object.

The instance method is named greet, so calling person.greet() is correct.

2fill in blank

medium

Complete the code to define an instance method named sayAge that prints the age.

Java

public class Person {
    int age;
    public void [1]() {
        System.out.println("Age: " + age);
    }
}

Drag options to blanks, or click blank then click option'

AsayAge

BshowAge

CprintAge

DgetAge

Attempts:

3 left

💡 Hint

Common Mistakes

Using a different method name than requested.

Forgetting to include parentheses after the method name.

The method name must be sayAge as requested to match the task.

3fill in blank

hard

Fix the error in the instance method call to correctly print the name.

Java

public class Person {
    String name;
    public void printName() {
        System.out.println(name);
    }
}

Person p = new Person();
p.name = "Alice";
p.[1]();

Drag options to blanks, or click blank then click option'

Aprint_Name

BPrintName

Cprintname

DprintName

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong capitalization in method names.

Adding underscores or other characters not in the method name.

Method names are case-sensitive in Java. The correct method name is printName.

4fill in blank

hard

Fill both blanks to create a method that returns the full name by combining first and last names.

Java

public class Person {
    String firstName;
    String lastName;
    public String [1]() {
        return firstName + [2] + lastName;
    }
}

Drag options to blanks, or click blank then click option'

AgetFullName

B-

C " "

D+

Attempts:

3 left

💡 Hint

Common Mistakes

Using a minus sign instead of a space string.

Forgetting to add a space between names.

The method name should be getFullName. To combine names with a space, use " " between them.

5fill in blank

hard

Fill both blanks to define an instance method isAdult that returns true if age is 18 or more.

Java

public class Person {
    int age;
    public boolean [1]() {
        return age [2] 18;
    }
}

Drag options to blanks, or click blank then click option'

AisAdult

B>=

C;

D>

Attempts:

3 left

💡 Hint

Common Mistakes

Using > instead of >=, which excludes age 18.

Forgetting the semicolon at the end of the return statement.

The method name is isAdult. The condition uses >= to check if age is 18 or more. The statement ends with a semicolon.

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