Constructor basics

Learn the class constructor method.

Constructor basics

A constructor lets an instance receive initial values when it is created. The constructor's name matches the class name, it has no return type and it is called with new. A constructor assigns starting values to fields.

Example: class Point { int x; Point() { x = 0; } }. Here Point() is a constructor. When new Point() is called, this constructor runs and assigns zero to x. A constructor does not use return; no return type is written. Its name must exactly match the class name.

Constructor invocation

A constructor is called with the new keyword. The call creates a new instance in memory and runs the constructor body. When the body finishes, the instance is ready to use.

Example: Point p = new Point();. Here new Point() calls the Point constructor. The body runs x = 0; and p.x becomes zero. If no constructor is defined, Java adds an empty one automatically; that is why new Point() always works.

Exercise Add a constructor to the Point class

Define a class named Point. Add int x and int y fields. Write a no-argument constructor; its body assigns zero to both x and y. In main create a Point instance and print x and y on separate lines.

Show example solution

class Point {
    int x;
    int y;
    Point() {
        x = 0;
        y = 0;
    }
    public static void main(String[] args) {
        Point p = new Point();
        System.out.println(p.x);
        System.out.println(p.y);
    }
}

Test yourself

Which is a constructor definition?
- Point() { }
- void Point() { }
- constructor Point() { }
- def Point(): pass
Show answer

Point() { } — A constructor's name matches the class name and has no return type.

Complete the constructor definition

class Point {
    int x;
    ____ {
        x = 0;
    }
}

Show answer

class Point {
    int x;
    Point {
        x = 0;
    }
}

Match the constructor parts

Point

Constructor name
()

Parameter list
{ }

Body
new

Call keyword

Next lesson: Parameterized constructor

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