Constructors and this

Building objects properly

Setting fields one by one after new is clumsy and error-prone — forget one line and lesson 5-1's defaults leave a null hiding in the object, waiting to crash some later method. Real codebases therefore treat "an object is valid the instant it exists" as a rule. A constructor is a special method that runs during new and sets the object up in one step. It has the same name as the class and no return type:

class Point {
  int x;
  int y;

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

Point p = new Point(3, 4);

this means "the object being built", like Python's self, so this.x = x copies the parameter into the field of the same name. It is the direct translation of Python's __init__.

Once you define any constructor, callers must use it: new Point() stops compiling unless you also define a no-argument constructor.

Constructors overload too

A class may define several constructors with different parameter lists — the overloading rule from lesson 4-2 applies. A common pattern is a convenience constructor that fills in defaults by delegating to the full one with this(...):

class Point {
  int x;
  int y;

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

  Point() {
    this(0, 0);   // the origin
  }
}

this(0, 0) calls the other constructor and must be the first line. One warning worth its weight: Java gives you a free no-argument constructor only while you define none yourself — the moment you write Point(int x, int y), plain new Point() stops compiling unless you add it back explicitly, as above.