Instance variables in Java - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare an instance variable named age of type int.

Java

public class Person {
    [1] age;
}

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

Astatic

Bint

Cvoid

Dfinal

Attempts:

3 left

💡 Hint

Common Mistakes

Using static makes the variable shared by all objects, not an instance variable.

Using void is only for methods, not variables.

The keyword int declares the variable age as an integer instance variable.

2fill in blank

medium

Complete the code to assign the value 25 to the instance variable age inside the constructor.

Java

public class Person {
    int age;
    public Person() {
        [1] = 25;
    }
}

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

Athis.age

Bage()

CPerson.age

Dstatic age

Attempts:

3 left

💡 Hint

Common Mistakes

Using age() which looks like a method call.

Using Person.age which is for static variables.

Use this.age to refer to the instance variable age inside the constructor.

3fill in blank

hard

Fix the error in the code by completing the declaration of the instance variable name of type String.

Java

public class Person {
    [1] name;
}

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

Afinal int

Bstatic String

Cvoid String

DString

Attempts:

3 left

💡 Hint

Common Mistakes

Using static String makes it a class variable, not an instance variable.

Using void String is invalid syntax.

The correct way to declare an instance variable of type String is simply String name;.

4fill in blank

hard

Fill both blanks to complete the constructor that sets the instance variables name and age.

Java

public class Person {
    String name;
    int age;

    public Person(String name, int age) {
        this.name = [1];
        this.age = [2];
    }
}

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

Aname

Bage

C"name"

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Assigning string literals like "name" instead of the parameter name.

Assigning zero 0 instead of the parameter age.

The constructor parameters name and age are assigned to the instance variables using this.name = name; and this.age = age;.

5fill in blank

hard

Fill all three blanks to complete the method that returns a description using instance variables name and age.

Java

public class Person {
    String name;
    int age;

    public String getDescription() {
        return [1] + " is " + [2] + [3];
    }
}

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

Aname

Bage

C " years old"

D"Hello"

Attempts:

3 left

💡 Hint

Common Mistakes

Using string literals instead of variables for name or age.

Forgetting to add the descriptive text like " years old".

The method returns a string combining the instance variable name, the integer age, and the string literal " years old".

Practice

1. Which statement best describes instance variables in Java?

easy

A. They store data unique to each object of a class.

B. They are shared by all objects of a class.

C. They are declared inside methods only.

D. They must be static to be used.

Solution

1. Step 1: Understand instance variable location

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

2. Step 2: Understand instance variable behavior

   Each object has its own copy, so data is unique per object.

3. Final Answer:

   They store data unique to each object of a class. -> Option A

4. Quick Check:

   Instance variables = unique per object [OK]

Hint: Instance variables belong to objects, not the class itself [OK]

Common Mistakes:

• Confusing instance variables with static variables
• Thinking instance variables are declared inside methods
• Assuming instance variables are shared across all objects

2. Which of the following is the correct way to declare an instance variable in Java?

easy

A. static int count;

B. int count() { }

C. public int count;

D. void count;

Solution

1. Step 1: Identify valid instance variable syntax

   Instance variables are declared like normal variables inside a class but outside methods, e.g., public int count;.

2. Step 2: Eliminate invalid options

   int count() { } is a method, C is static (not instance), D is invalid syntax.

3. Final Answer:

   public int count; -> Option C

4. Quick Check:

   Instance variable declaration = variable with type and name [OK]

Hint: Instance variables look like normal variable declarations outside methods [OK]

Common Mistakes:

• Using parentheses which define methods, not variables
• Adding static keyword which makes variable class-level
• Missing type or using invalid syntax

3. What will be the output of this Java code?

class Car {
  String color = "Red";
}

public class Test {
  public static void main(String[] args) {
    Car c1 = new Car();
    Car c2 = new Car();
    c2.color = "Blue";
    System.out.println(c1.color);
  }
}

medium

A. Compilation error

B. Blue

C. null

D. Red

Solution

1. Step 1: Understand instance variable values per object

   Each Car object has its own color. c1.color is "Red" initially.

2. Step 2: Check changes to c2.color

   Changing c2.color to "Blue" does not affect c1.color.

3. Final Answer:

   Red -> Option D

4. Quick Check:

   Instance variables are unique per object [OK]

Hint: Changing one object's instance variable doesn't affect others [OK]

Common Mistakes:

• Assuming changing c2.color changes c1.color
• Confusing instance variables with static variables
• Expecting null because of misunderstanding initialization

4. Find the error in this Java class related to instance variables:

public class Person {
  String name;
  int age;

  public void setName(String name) {
    name = name;
  }
}

medium

A. Instance variable 'name' is not assigned correctly in setName method.

B. Missing return type for setName method.

C. Instance variables must be static.

D. Class Person must have a constructor.

Solution

1. Step 1: Analyze setName method parameter and assignment

   The method parameter name shadows the instance variable name.

2. Step 2: Understand assignment effect

   Assignment name = name; assigns parameter to itself, not instance variable.

3. Final Answer:

   Instance variable 'name' is not assigned correctly in setName method. -> Option A

4. Quick Check:

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

Hint: Use 'this.' to refer to instance variables inside methods [OK]

Common Mistakes:

• Not using 'this' to distinguish instance variables
• Assuming parameter assignment updates instance variable
• Thinking constructor is mandatory for instance variables

5. You want to create a class Book where each book has a unique title and author. Which code correctly uses instance variables to achieve this?

public class Book {
  // Choose the correct instance variable declarations and constructor

  A) String title, author;
     public Book(String t, String a) {
       title = t;
       author = a;
     }

  B) static String title, author;
     public Book(String t, String a) {
       title = t;
       author = a;
     }

  C) String title, author;
     public Book() {
       title = "";
       author = "";
     }

  D) static String title, author;
     public Book() {
       title = "";
       author = "";
     }

hard

A. Static variables with constructor assigning values (shared by all).

B. Instance variables with constructor assigning unique values.

C. Instance variables with default constructor (no unique values).

D. Static variables with default constructor (shared and default).

Solution

1. Step 1: Identify instance vs static variables

   Instance variables allow each object to have unique data; static variables share data across all objects.

2. Step 2: Check constructor usage

   Instance variables with constructor assigning unique values. uses instance variables with a constructor that assigns unique values from parameters, matching the requirement.

3. Final Answer:

   Instance variables with constructor assigning unique values. -> Option B

4. Quick Check:

   Use instance variables + constructor for unique object data [OK]

Hint: Use instance variables with constructor to set unique object data [OK]

Common Mistakes:

• Using static variables which share data across all objects
• Not initializing instance variables with constructor parameters
• Assuming default constructor sets unique values