Kotlin learning II: classes and objects

class

Create a class MyUtils, There is a value and the method of obtaining ：

class MyUtils {
    var a = 0

    fun get(): Int {
        return a
    }
}

stay MainActivity Class to get a And assign ：

        val myUtils = MyUtils()
        Log.w("MyUtils", "" + myUtils.get())
        myUtils.a = 9527
        Log.w("MyUtils", "" + myUtils.get())

Print the results ：

MyUtils: 0
MyUtils: 9527

Inherit

When you create a new project, you will find that the inheritance relationship is represented by a colon ：

class MainActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
    }
}

MainActivity Inherit AppCompatActivity , Remember that there are parentheses after the parent class .

Write your own parent class and remember to add open keyword , Only in this way can it be inherited ：

open class A {
}

B Inherit A：

class B : A() {
}

Constructors

kotlin Provides init Method , This is the main constructor , You can write some logic in it ：

class B(var num: Int) : A() {
    init {
        num = num + 1
        Log.w("num", "" + num)
    }
}

establish B Class, give an initial value num：

val b = B(30)

take A Also add a parameter ：

open class A(pos: Int) {
}

stay B You can pass a parameter to A：

class B(var num: Int) : A(num) {
    init {
        num = num + 1
        Log.w("num", "" + num)
    }
}