Class

Overview

Class is a set of data and procedure. Kinx is a prototype based language but class keyword is prepared as a syntax sugar of creating an object instance.

Basic Style

The class definition is basically that there are the case of having arguments and the case of no arguments.

class A { /* ... */ }
class A() { /* ... */ }
class A(a, b) { /* ... */ }

In the above case, class A is the same as class A().

Inheritance

The : is used for inheritance of class. About arguments for inheritance, it is also same as a simple class definition, and the arguments with ( and ) are able to be omitted. The examples of inheritance are below.

class A : B { /* ... */ }
class A() : B { /* ... */ }
class A : B(true) { /* ... */ }
class A(a, b) : B(b) { /* ... */ }

Instantiate

The class object is instantiated by new operator. When instantiating the object, you have to use a function call style like new ClassName(). It makes no sense with new ClassName only.

class A { /* ... */ }
var a = new A();

Keyword this and @

this means the instance itself.

class A {
    public flagOnAlt() {
        this.flagOn();
    }
    public flagOn() {
        this.flag = true;
    }
}

You can use @ instead of this., which is this plus dot. By this, you can write the following for the above code.

class A {
    public flagOnAlt() {
        @flagOn();
    }
    public flagOn() {
        @flag = true;
    }
}

Data

Private data and public data

The variable declared in class scope is a local private variable. A property of this is a public variable.

class A {
    var flag_ = false;  // a local private variable.
    public flagOn() {
        @flagOnActual();
    }
    public flagOnActual() {
        @flag = true;   // a public variable.
    }
}

var a = new A();
a.flagOn();
System.println(a.flag ? "true" : "false");  // => true

Methods

Methods are defined with public and private keyword. The function defined with private is used only in a local scope, which is inside a class scope.

On the other hand, the function defined with public can be accessed by an instance’s property.

class A {
    private method1() {
        /* ... */
    }
    public method2() {
        method1();  // Okay, the `method1` is accessible inside the class scope.
        /* ... */
    }
    private method3() {
        method2();  // The `method2` is also accessible inside the class scope.
        @method2(); // The `method2` is also accessible through `this`.
    }
}

var a = new A();
// a.method1();     // Error, the `method1` is accessible through the instance.
a.method2();        // Okay, the `method2` is accessible through the instance.

Special Methods

The class keyword will define some special methods.

initialize

If initialize method is defined, the initialize method is automatically called right after instantiated. The initialize method can be defined either public or private.

class A {
    private initialize() {
        System.println("called");
    }
}

var a = new A();    // => "called"

instanceOf

A class instance has an instanceOf method automatically. This method accepts a class name as a variable, and it will return true if a target object is an instance of a specified class or a base class.

class C {};
class B {};
class A : B {};

var a = new A();
System.println(a.instanceOf(A) ? "true" : "false"); // => "true"
System.println(a.instanceOf(B) ? "true" : "false"); // => "true"
System.println(a.instanceOf(C) ? "true" : "false"); // => "false"

Examples

Example 1. Normal case (1)

Code

class A(name) {
    public method() {
        System.println(name, " called.");
    }
}
class B : A("class B") {}
class C() : A("class C") {}
class D(name) : A("class "+name) {}
new B().method();
new C().method();
new D("D").method();

Result

class B called.
class C called.
class D called.

Example 2. Normal case (2)

Code

class A {
    var flag_ = false;  // a local private variable.
    public flagOn() {
        @flagOnActual();
    }
    public flagOnActual() {
        @flag = true;   // a public variable.
    }
}

var a = new A();
a.flagOn();
System.println(a.flag ? "true" : "false");  // => true

Result

true

Example 3. initialize method

Code

class A {
    private initialize() {
        System.println("called");
    }
}

var a = new A();    // => "called"

Result

called

Example 3. instanceOf method

Code

class C {};
class B {};
class A : B {};

var a = new A();
System.println(a.instanceOf(A) ? "true" : "false"); // => "true"
System.println(a.instanceOf(B) ? "true" : "false"); // => "true"
System.println(a.instanceOf(C) ? "true" : "false"); // => "false"

Result

true
true
false