Encapsulation best practices in Java - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a private field in a class.

Java

public class Person {
    private String [1];
}

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

Apublic

BgetName

CPerson

Dname

Attempts:

3 left

💡 Hint

Common Mistakes

Using public instead of private for fields.

Using method names instead of field names.

The field name is declared private to follow encapsulation best practices.

2fill in blank

medium

Complete the code to create a public getter method for the private field.

Java

public class Person {
    private String name;

    public String [1]() {
        return name;
    }
}

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

AgetName

BName

CsetName

Dname

Attempts:

3 left

💡 Hint

Common Mistakes

Naming the getter method incorrectly.

Using the field name directly as method name.

The getter method is named getName to provide controlled access to the private field name.

3fill in blank

hard

Fix the error in the setter method to properly set the private field.

Java

public class Person {
    private String name;

    public void setName(String [1]) {
        name = [1];
    }
}

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

AnewName

Bthis.name

Cname

DName

Attempts:

3 left

💡 Hint

Common Mistakes

Using the same name for parameter and field without this keyword.

Not assigning the parameter value to the field.

The parameter should have a different name like newName to avoid confusion and correctly assign the value to the field.

4fill in blank

hard

Fill both blanks to complete the setter method using the this keyword.

Java

public class Person {
    private String name;

    public void setName(String [1]) {
        [2].name = name;
    }
}

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

Aname

BnewName

Cthis

DPerson

Attempts:

3 left

💡 Hint

Common Mistakes

Not using this when parameter and field names are the same.

Using class name instead of this.

The parameter is named name and the this keyword is used to refer to the field to avoid confusion.

5fill in blank

hard

Fill all three blanks to create a class with a private field and a getter following encapsulation best practices.

Java

public class Car {
    private String [1];

    public String [2]() {
        return [3];
    }
}

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

Amodel

BgetModel

DsetModel

Attempts:

3 left

💡 Hint

Common Mistakes

Using public fields instead of private.

Getter method not returning the correct field.

The private field is model. The getter method is getModel which returns the field model.

Practice

1.

What is the main purpose of encapsulation in Java?

easy

A. To hide the internal data of a class and control access to it

B. To make all variables public so they can be accessed anywhere

C. To allow direct access to class variables without methods

D. To write code faster by skipping method definitions

Solution

1. Step 1: Understand encapsulation concept

   Encapsulation means hiding data inside a class to protect it from outside access.

2. Step 2: Identify the purpose of encapsulation

   It controls how data is accessed or changed using getter and setter methods.

3. Final Answer:

   To hide the internal data of a class and control access to it -> Option A

4. Quick Check:

   Encapsulation = Data hiding and controlled access [OK]

Hint: Encapsulation means hiding data and controlling access [OK]

Common Mistakes:

• Thinking encapsulation means making variables public
• Confusing encapsulation with inheritance
• Believing encapsulation allows direct variable access

2.

Which of the following is the correct way to declare a private variable in a Java class?

class Person {
    ? String name;
}

easy

A. private

B. public

C. protected

D. static

Solution

1. Step 1: Recall Java access modifiers

   Private variables are declared with the keyword private to hide them inside the class.

2. Step 2: Check the options

   Only private hides the variable from outside access, others allow wider access.

3. Final Answer:

   private -> Option A

4. Quick Check:

   Private keyword hides variables [OK]

Hint: Use 'private' to hide variables inside class [OK]

Common Mistakes:

• Using public instead of private for encapsulation
• Confusing protected with private
• Using static which controls memory, not access

3.

What will be the output of the following code?

class Car {
    private String model = "Tesla";
    public String getModel() {
        return model;
    }
}

public class Test {
    public static void main(String[] args) {
        Car car = new Car();
        System.out.println(car.getModel());
    }
}

medium

A. Runtime error

B. Tesla

C. Compilation error

D. null

Solution

1. Step 1: Understand private variable access

   The variable model is private but accessed via the public getter getModel().

2. Step 2: Check the output of getModel()

   The getter returns the string "Tesla", so printing it outputs "Tesla".

3. Final Answer:

   Tesla -> Option B

4. Quick Check:

   Getter returns private variable value [OK]

Hint: Private data accessed via public getter returns value [OK]

Common Mistakes:

• Expecting direct access to private variable
• Thinking code causes compilation error
• Confusing output with null or error

4.

Identify the error in the following code related to encapsulation:

class BankAccount {
    private double balance;
    public void setBalance(double balance) {
        balance = balance;
    }
    public double getBalance() {
        return balance;
    }
}

medium

A. The balance variable should be public

B. The getter method should be private

C. The setter method does not update the class variable correctly

D. The setter method should return a value

Solution

1. Step 1: Analyze the setter method

   The setter uses balance = balance; which assigns the parameter to itself, not the class variable.

2. Step 2: Understand correct assignment

   To update the class variable, use this.balance = balance; to refer to the instance variable.

3. Final Answer:

   The setter method does not update the class variable correctly -> Option C

4. Quick Check:

   Use 'this' to assign parameter to instance variable [OK]

Hint: Use 'this' to assign setter parameter to class variable [OK]

Common Mistakes:

• Forgetting 'this' keyword in setter
• Making getter private which breaks access
• Expecting setter to return a value

5.

You want to create a class Student with a private variable grade that can only be set if the value is between 0 and 100. Which is the best way to implement this using encapsulation?

hard

A. Make grade public and check the value before assigning

B. Make grade static and assign directly

C. Use a protected grade variable and no setter

D. Use a private grade variable with a setter that validates the value

Solution

1. Step 1: Understand encapsulation for validation

   Encapsulation allows controlling how variables are set by using private variables and setters with checks.

2. Step 2: Choose the best practice

   Using a private variable with a setter that validates the input ensures grade stays between 0 and 100.

3. Final Answer:

   Use a private grade variable with a setter that validates the value -> Option D

4. Quick Check:

   Setters with validation keep data safe [OK]

Hint: Validate data inside setter to protect private variables [OK]

Common Mistakes:

• Making variables public and trusting external code
• Skipping validation in setter
• Using static which shares data across all instances