1. The Java Keyword this Marie J. Garlitos

2. Instance Methods First we introduce instance methods: –any method not declared with a static keyword –operates on an instance of the class (an object) instead of operating on the class itself –this object instance is often referred to as the receiving instance of the instance method

3. Instance Methods A simple class Circle 1 2

4. Using Instance Methods To use an instance method from outside of the class in which it is defined we must prepend a reference to the instance that is to be operated on: //create a Circle object and store in c Circle c = new Circle(); c.r = 2.0; //set an instance field of object //invoke an instance method of the object double a = c.area(); Notice the method does not have a parameter. How does it know what data to operate on?

5. The Object Reference this All instance methods are implemented with an implicit parameter not shown in the method signature. The implicit argument is named this It holds a reference to the object through which the method is invoked. In our example, that object is a Circle.

6. 3 Common Uses of this accessing shadowed fields invoking another overloaded constructor in the same class passing this as a parameter

7. Accessing Shadowed Fields In some cases a field in a class can be shadowed or hidden inside a method by a parameter or a local variable by the same name. For example, when a method parameter has the same name as one of the fields of the class, you must use this to refer to the field.

8. Accessing Shadowed Fields Looking at our previous example class Circle, we add the method setRadius(). The local variable r shadows the instance field r. We must use this.r to refer to the instance field set to argument r of the method setRadius().

9. Accessing Shadowed Fields In general, avoid shadowing variables Shadow only instance fields with local variables that serve as their temporary copies in an instance method or constructor. Copy the local variables back to the instance fields before leaving the method

10. Invoking other Constructors of Same Class A class can define two or more overloaded constructors with same name and different argument lists. Sometimes it is useful for one overloaded constructor to invoke another in the same class.

11. Invoking other Constructors of Same Class In our given example class calcData, the statement this(30); //line number 12 is used by an overloaded constructor public calcData()//line no. 10; also //called a noarg //constructor (no parms.) to invoke another overloaded constructor while passing an integer value of 30 as a parameter.

12. Invoking other Constructors of Same Class That constructor stores the value of the incoming parameter (30) in the instance variable named myData (line number 16) Control is then returned to the noarg constructor (line 10) which in turn returns control to main() method. New object has been constructed and the instance field myData (line 02) belonging to that object contains the value 30.

13. Passing this as a Parameter When one object invokes a method in another object and passes a reference to itself as a parameter (also referred to as registration). The method in the second object saves the reference that it receives as an incoming parameter. This makes it possible for a method in the second object to make a callback to the first object later when necessary.

14. To illustrate: Department d contains a method ( newFaculty() ) which invokes a method belonging to Faculty a passing a reference to itself as a parameter (line 26). Passing this as a Parameter

15. Faculty a saves that reference in an instance variable for later use enabling it to make a callback to Department d. The main point is that the this reference is available to all instance methods belonging to an object and can be used whenever there is a need for reference to the object on which the method is invoked.

16. Why use this ? To summarize: –To provide more information, more clarity. –To distinguish between fields and local/ parameters variables of the same –To make it possible for one overloaded constructor to invoke another overloaded constructor in the same class. –To pass a reference to the current object to a method belonging to a different object.

17. References Jia, Xiaoping. Object Oriented Software Development using Java- 2 nd edition, Adison Wesley 2003 Baldwin, Richard. The Essence of OOP using Java, The this and super Keywords. 2006. http://www.developer.com/java/article.php/1440571 Flanagan, David. Java in a Nutshell 3 rd edition, O’Reilly and Associates, Inc 1999, 1997 and 1996.