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Trail: Learning the Java Language
Lesson: Classes and Objects
Section: More on Classes
Classes and Objects
Classes
Declaring Classes
Declaring Member Variables
Defining Methods
Providing Constructors for Your Classes
Passing Information to a Method or a Constructor
Objects
Creating Objects
Using Objects
More on Classes
Returning a Value from a Method
Using the this Keyword
Controlling Access to Members of a Class
Understanding Instance and Class Members
Initializing Fields
Summary of Creating and Using Classes and Objects
Questions and Exercises
Questions and Exercises
Nested Classes
Inner Class Example
Summary of Nested Classes
Questions and Exercises
Enum Types
Questions and Exercises
Annotations
Questions and Exercises
Home Page > Learning the Java Language > Classes and Objects
Using the this Keyword

> Within an instance method or a constructor, `this` is a reference to the current object — the object whose method or constructor is being called. You can refer to any member of the current object from within an instance method or a constructor by using `this`.

> ### Using `this` with a Field

> The most common reason for using the `this` keyword is because a field is shadowed by a method or constructor parameter.

> For example, the `Point` class was written like this

> >     public class Point {
> >         public int x = 0;
> >         public int y = 0;

> >         //constructor
> >         public Point(int a, int b) {
> >     	x = a;
> >     	y = b;
> >         }
> >     }

> but it could have been written like this:

> >     public class Point {
> >         public int x = 0;
> >         public int y = 0;

> >         //constructor
> >         public Point(int x, int y) {
> >     	this.x = x;
> >     	this.y = y;
> >         }
> >     }

> Each argument to the second constructor shadows one of the object's fields—inside the constructor `x` is a local copy of the constructor's first argument. To refer to the `Point` field `x`, the constructor must use `this.x`.

> ### Using `this` with a Constructor

> From within a constructor, you can also use the `this` keyword to call another constructor in the same class. Doing so is called an explicit constructor invocation. Here's another `Rectangle` class, with a different implementation from the one in the Objects section.

> >     public class Rectangle {
> >         private int x, y;
> >         private int width, height;

> >         public Rectangle() {
> >             this(0, 0, 0, 0);
> >         }
> >         public Rectangle(int width, int height) {
> >             this(0, 0, width, height);
> >         }
> >         public Rectangle(int x, int y, int width, int height) {
> >             this.x = x;
> >             this.y = y;
> >             this.width = width;
> >             this.height = height;
> >         }
> >         ...
> >     }

> This class contains a set of constructors. Each constructor initializes some or all of the rectangle's member variables. The constructors provide a default value for any member variable whose initial value is not provided by an argument. For example, the no-argument constructor calls the four-argument constructor with four 0 values and the two-argument constructor calls the four-argument constructor with two 0 values. As before, the compiler determines which constructor to call, based on the number and the type of arguments.

> If present, the invocation of another constructor must be the first line in the constructor.

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