Classes

class Point {
    int x;
    int y;

    Point(int x, int y) {          // the constructor: the class name, no return type
        this.x = x;
        this.y = y;
    }

    int sum() {
        return x + y;              // fields by their bare name, or this.x
    }

    String toString() {            // used by println and string joining
        return "(" + x + ", " + y + ")";
    }
}

Point p = new Point(1, 2);
println(p, p.sum());               // (1, 2) 3

• Fields have a type (dyn, int, String, a class...) and an optional initial value: int hp = 20;.
• Methods need a return type: void, dyn, int...
• One constructor at most. Without one, new C() takes no arguments (or the parent's).
• There is no overloading, no private/public, no nested classes.

An object has exactly the fields of its class: reading or writing an unknown field is an error (No field 'z' in Point). For free-form data use a table.

Inheritance

class Point3 extends Point {
    int z;

    Point3(int x, int y, int z) {
        super(x, y);               // the parent's constructor
        this.z = z;
    }

    int sum() {
        return super.sum() + z;    // the parent's method
    }
}

dyn q = new Point3(1, 2, 3);
println(q.sum());                  // 6
println(isa(q, Point), type(q));   // true Point3

Methods are virtual: q.sum() calls Point3.sum even through a Point variable.

A class extends only another Jumper class. A Java interface is implemented by passing a function (see Java interop).

Field initializers of the whole chain run before any constructor, parents first.

Static members

class Registry {
    static int count = 0;
    static void add() { count++; }
}

Registry.add();
Registry.add();
println(Registry.count);   // 2

Static fields are set up when the class statement runs.

Classes as values

A class is a value: keep it in a variable or a table, create objects from it later.

class Cat { String name; Cat(String name) { this.name = name; } }

dyn kinds = { cat: Cat };
dyn tom = new kinds.cat("Tom");
println(tom.name);   // Tom