Abstract Classes

An abstract class cannot be instantiated. It can hold state and normal functions, plus abstract members that every subclass must implement. Abstract classes and members are open automatically.

Example

abstract class Account(val holder: String) {
    var balance = 0.0
        protected set

    abstract val interestRate: Double          // must be provided
    abstract fun monthlyCharge(): Double

    fun deposit(amount: Double) { balance += amount }   // shared logic
    fun summary() = "$holder: balance ${balance}, rate ${interestRate}%, charge ${monthlyCharge()}"
}

class SavingsAccount(holder: String) : Account(holder) {
    override val interestRate = 3.5
    override fun monthlyCharge() = if (balance < 10000) 50.0 else 0.0
}

class CurrentAccount(holder: String) : Account(holder) {
    override val interestRate = 0.0
    override fun monthlyCharge() = 200.0
}

fun main() {
    val s = SavingsAccount("Asha"); s.deposit(5000.0)
    val c = CurrentAccount("Ravi Traders"); c.deposit(250000.0)
    println(s.summary())
    println(c.summary())
    // val a = Account("x")  // error: cannot create an instance of an abstract class
}

Output

Asha: balance 5000.0, rate 3.5%, charge 50.0
Ravi Traders: balance 250000.0, rate 0.0%, charge 200.0

What is an Abstract Class?

Example