The Java Tutorials have been written for JDK 8. Examples and practices described in this page don't take advantage of improvements introduced in later releases and might use technology no longer available.
See Java Language Changes for a summary of updated language features in Java SE 9 and subsequent releases.
See JDK Release Notes for information about new features, enhancements, and removed or deprecated options for all JDK releases.
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
.
this
with a FieldThe 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 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
.
this
with a ConstructorFrom 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, 1, 1); } 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 creates a 1x1 Rectangle
at coordinates 0,0. The two-argument constructor calls the four-argument constructor, passing in the width and height but always using the 0,0 coordinates. 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.