Objects

Everything in Java is within classes and objects. Java objects hold a state, state are variables which are saved together within an object, we call them fields or member variables.

Let start with an example:

class Point {
    int x;
    int y;
}

This class defined a point with x and y values.

In order to create an instance of this class, we need to use the keyword new.

Point p = new Point();

In this case, we used a default constructor (constructor that doesn't get arguments) to create a Point. All classes that don't explicitly define a constructor has a default constructor that does nothing.

We can define our own constructor:

class Point {
    int x;
    int y;

    Point(int x, int y) {
        this.x = x;
        this.y = y;
    }
}

This means we can not longer use the default constructor new Point(). We can now only use the defined constructor new Point(4, 1).

We can define more than one constructor, so Point can be created in several ways. Let's define the default constructor again.

class Point {
    int x;
    int y;

    Point() {
        this(0, 0);
    }

    Point(int x, int y) {
        this.x = x;
        this.y = y;
    }
}

Notice the usage of the this keyword here. We can use it within a constructor to call a different constructor (in order to avoid code duplication). It can only be the first line within the constructor.

We also used the this keyword as a reference of the current object we are running on.

After we defined p we can access x and y.

p.x = 3;
p.y = 6;

Methods

We can now define methods on Point.

class Point {
    ... // Our code previously
    void printPoint() {
        System.out.println("(" + x + "," + y + ")");
    }

    Point center(Point other) {
        // Returns the center between this point the other point
        // Notice we are using integer, we won't get an accurate value
        return new Point((x + other.x) / 2, (y + other.y) / 2);
    }
}

Public and Private

Although we'll talk about modifiers later on, it's important to understand the different between private and public variables and methods.

When using the keyword private before a variable or a method, it means only the class itself can access the variable or method, when we're using public it means everybody can access it. We usually see constructors defined as public, variables defined as private and methods are separated, some public and some private.

Exercise

Write a new method in Point called scale, that will make the point closer by half to (0,0). So for example, point (8, 4) after scale will be (4, 2).