Custom getters and setters in Kotlin - Mini Project: Build & Apply

Custom getters and setters

📖 Scenario: You are creating a simple Kotlin class to represent a bank account. You want to control how the balance is accessed and changed by using custom getters and setters.

🎯 Goal: Build a Kotlin class BankAccount with a balance property. Use custom getter to show the balance with a message, and a custom setter to prevent setting a negative balance.

📋 What You'll Learn

Create a class called BankAccount with a balance property of type Double initialized to 0.0

Add a custom getter for balance that returns the balance as a string with the message: "Current balance: $X" where X is the balance

Add a custom setter for balance that only updates the balance if the new value is not negative

Create an instance of BankAccount, set the balance to 100.0, then try to set it to -50.0, and finally print the balance

💡 Why This Matters

🌍 Real World

Bank accounts and financial apps often need to control how money values are accessed and changed to avoid errors or fraud.

💼 Career

Understanding custom getters and setters is important for writing safe and clear Kotlin code in real-world applications.

1

Create the BankAccount class with a private balance

Create a Kotlin class called BankAccount with a balance property of type Double initialized to 0.0.

Kotlin

class BankAccount {
    // Your code here
}

Hint

Use var balance: Double = 0.0 inside the class.

2

Add a custom getter for balance

Add a custom getter for the balance property that returns a String with the message: "Current balance: $X" where X is the current balance value.

Kotlin

class BankAccount {
    var balance: Double = 0.0

    // Your code here
}

Hint

Create a new val displayBalance with a custom getter that returns the message string.

3

Add a custom setter to prevent negative balance

Add a custom setter for the balance property that only updates the balance if the new value is not negative. If the new value is negative, do not change the balance.

Kotlin

class BankAccount {
    var balance: Double = 0.0
        set(value) {
            // Your code here
        }

    val displayBalance: String
        get() = "Current balance: \$${balance}"
}

Hint

Use set(value) { if (value >= 0) field = value } to protect the balance.

4

Create an instance, set balances, and print

Create an instance of BankAccount called account. Set the balance to 100.0, then try to set it to -50.0. Finally, print account.displayBalance.

Kotlin

class BankAccount {
    var balance: Double = 0.0
        set(value) {
            if (value >= 0) {
                field = value
            }
        }

    val displayBalance: String
        get() = "Current balance: \$${balance}"
}

// Your code here

Hint

Remember to create account, set balance twice, and print account.displayBalance.

Practice

1. What is the main purpose of custom getters and setters in Kotlin?

easy

A. To create new classes automatically

B. To define functions outside a class

C. To control how properties are read and written

D. To import external libraries

Solution

1. Step 1: Understand property access

   Custom getters and setters allow you to define how a property value is retrieved or changed.

2. Step 2: Identify their purpose

   They help control and protect the data by customizing read/write behavior.

3. Final Answer:

   To control how properties are read and written -> Option C

4. Quick Check:

   Custom getters/setters = control property access [OK]

Hint: Remember: getters/setters control property reading/writing [OK]

Common Mistakes:

• Confusing getters/setters with class creation
• Thinking they define functions outside classes
• Assuming they import libraries

2. Which of the following is the correct syntax to define a custom setter for a Kotlin property age?

easy

A. var age: Int
set(value) { field = value }

B. var age: Int
get(value) { field = value }

C. val age: Int
set(value) { field = value }

D. var age: Int
set { value = field }

Solution

1. Step 1: Check property mutability

   Setter can only be defined for var, not val.

2. Step 2: Verify setter syntax

   The setter uses set(value) { field = value } to assign the new value.

3. Final Answer:

   var age: Int set(value) { field = value } -> Option A

4. Quick Check:

   Setter syntax = set(value) { field = value } [OK]

Hint: Setter needs var and set(value) syntax [OK]

Common Mistakes:

• Using get instead of set for setter
• Defining setter on val property
• Reversing field and value in setter

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

class Person {
    var name: String = ""
        get() = field.uppercase()
        set(value) { field = value.trim() }
}

fun main() {
    val p = Person()
    p.name = "  alice  "
    println(p.name)
}

medium

A. ALICE

B. alice

C. ecila

D. Alice

Solution

1. Step 1: Analyze setter behavior

   Setter trims the input, so " alice " becomes "alice" stored in field.

2. Step 2: Analyze getter behavior

   Getter returns the stored value in uppercase, so "alice" becomes "ALICE".

3. Final Answer:

   ALICE -> Option A

4. Quick Check:

   Setter trims, getter uppercases = ALICE [OK]

Hint: Setter trims, getter uppercases output [OK]

Common Mistakes:

• Ignoring trimming in setter
• Not converting to uppercase in getter
• Assuming original input prints

4. Identify the error in this Kotlin class with custom getter and setter:

class Box {
    var size: Int = 0
        get() = field
        set(value) {
            if (value > 0) field = value
            else field = 0
        }

    fun printSize() {
        println(size)
    }
}

medium

A. Setter does not assign value correctly

B. No error, code works fine

C. Getter should not use field

D. Property size should be val, not var

Solution

1. Step 1: Check setter logic

   Setter assigns field = value if positive, else 0. This is valid.

2. Step 2: Check getter and property type

   Getter returns field, which is correct. Property is var, so setter allowed.

3. Final Answer:

   No error, code works fine -> Option B

4. Quick Check:

   Setter and getter correct = no error [OK]

Hint: Setter with condition is valid; getter uses field correctly [OK]

Common Mistakes:

• Thinking getter can't use field
• Assuming setter must always assign value
• Confusing var and val for setters

5. You want a Kotlin property score that never stores a negative value. Which custom setter implementation ensures this correctly?

hard

A. var score: Int = 0
set(value) { field = value * -1 }

B. var score: Int = 0
set(value) { if (value < 0) field = 0 }

C. var score: Int = 0
set(value) { field = value }

D. var score: Int = 0
set(value) { field = if (value < 0) 0 else value }

Solution

1. Step 1: Understand requirement

   Property must never store negative values; negative inputs become zero.

2. Step 2: Evaluate each setter

   var score: Int = 0 set(value) { field = if (value < 0) 0 else value } sets field to 0 if value is negative, else stores value correctly.

3. Step 3: Check other options

   A stores negative of value, which can be positive or negative incorrectly; B only sets field if negative, ignoring positive values; C stores negatives unchanged.

4. Final Answer:

   var score: Int = 0 set(value) { field = if (value < 0) 0 else value } -> Option D

5. Quick Check:

   Setter with if-else ensures no negatives [OK]

Hint: Use if-else in setter to block negatives [OK]

Common Mistakes:

• Not assigning field for positive values
• Allowing negative values to be stored
• Incorrectly negating values