member_functions.md

Theory: Member functions

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Writing member functions

Sometimes you need to store in objects not only the data itself but also a behavior. Member functions implement a common behavior for the whole set of objects which belong to the same class.

Member functions look like functions placed in the class body. In the example below, we declare a class with one function print():

class MyClass {
    fun print() = println("Hello from print")
}

This function is called a member function because it works with a specific object of a class and can access its fields. It's a class member:

class MyClassWithProperty(var property: Int) {
    fun printProperty() {
        println(this.property)
    }
}

As you may remember, the keyword this represents the current instance of the class. In the example above, it's an optional keyword, so you can omit it.

Just like functions, member functions can take arguments and return a value of any type including the same type as the defined class.

• Member Function 에 대한 정의는 그냥 뭐 메소드와 같은듯

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Calling member functions

Let's take a look at the example below. It prints the value of the field of the particular object:

val myObject = MyClassWithProperty(10)
myObject.printProperty()  // prints "10"

So to call a member function, you need to create an object of the class. Then, put a dot after the object name and write the function name with the arguments that you need in parentheses.

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An example with cats

As a more complex example, let's consider a class that represents cats.

A cat can sleep. Also, a cat can make one of the two sounds: "meow" or "zzz". It depends on its state. Finally, a cat can be awakened.

Here we've added comments for class members. Read them to better understand the logic of the class.

import kotlin.random.Random // library for getting random numbers

class Cat(val name: String) {

    /** The current state of the cat (by default a cat isn't sleeping). */
    var sleeping: Boolean = false

    /**
     * A cat says "meow" if it is not sleeping, otherwise, it says "zzz".
     * After a cat says "meow", it can sometimes fall asleep.
     */
    fun say() {
        if (sleeping) {
            println("zzz")
        } else {
            println("meow")

            if (Random.nextDouble() < 0.5) { //generates a double value between 0 and 1.0 
                sleeping = true
            }
        }
    }

    /** The function wakes the cat. */
    fun wakeUp() {
        sleeping = false
    }
}

Now, we can create an instance of a class and invoke its functions. Do not forget that a cat that you have just created is not sleeping.

fun main() {
     val pharaoh = Cat("Pharaoh")  // Create a cat named "Pharaoh"
 
     repeat (5) {
         pharaoh.say()  // it says "meow" or "zzz"
     }
 
     pharaoh.wakeUp()  // wake the cat up
     pharaoh.say()  // it says "meow"
 }

The output of the program can be different because of Random.nextDouble() inside the say function. Here is an example of a possible output:

meow
meow
meow
zzz
zzz
meow