Instance variables in Java - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an instance variable in Java?

An instance variable is a variable declared inside a class but outside any method, constructor, or block. Each object of the class has its own copy of the instance variable.

intermediate

Where are instance variables stored in memory?

Instance variables are stored in the heap memory as part of the object they belong to.

beginner

How do instance variables differ from static variables?

Instance variables belong to objects and each object has its own copy. Static variables belong to the class itself and are shared among all objects.

beginner

Can instance variables have default values if not initialized?

Yes, instance variables have default values based on their type (e.g., 0 for int, null for objects) if not explicitly initialized.

beginner

Example: Identify the instance variable in this code snippet:<br><pre>class Car {<br>  String color;<br>  void setColor(String c) {<br>    color = c;<br>  }<br>}</pre>

The instance variable is <code>color</code>. It is declared inside the class but outside any method, so each Car object has its own color.

What does each object of a class have regarding instance variables?

A Its own copy of instance variables

B Shares instance variables with other objects

C No instance variables

D Only static variables

Each object has its own copy of instance variables, so changes in one object do not affect others.

Where are instance variables declared in Java?

A Inside methods

B Inside the class but outside methods

C Outside the class

D Inside constructors only

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

What is the default value of an uninitialized instance variable of type int?

A undefined

B null

C 0

D 1

Instance variables of type int default to 0 if not initialized.

Which memory area stores instance variables?

A Heap

B Stack

C Method area

D Register

Instance variables are stored in the heap as part of the object.

How do instance variables differ from static variables?

A Instance variables are shared; static are unique

B Both are the same

C Static variables are stored in heap; instance in stack

D Instance variables belong to objects; static belong to class

Instance variables belong to each object separately; static variables belong to the class and are shared.

Explain what instance variables are and how they behave in Java objects.

Think about how each object keeps its own data separate.

Describe the difference between instance variables and static variables in Java.

Consider who owns the variable: the object or the class.

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