Programming Tutorials

this keyword in Java

By: Jagan in Java Tutorials on 2007-09-08  

Sometimes a method will need to refer to the object that invoked it. To allow this, Java defines the this keyword. this can be used inside any method to refer to the current object. That is, this is always a reference to the object on which the method was invoked. You can use this anywhere a reference to an object of the current class' type is permitted.

To better understand what this refers to, consider the following version of Box():

// A redundant use of this.
Box(double w, double h, double d) {
    this.width = w;
    this.height = h;
    this.depth = d;

The use of this is redundant, but perfectly correct. Inside Box(), this will always refer to the invoking
object. While it is redundant in this case, this is useful in other contexts, one of which is explained in the next section.

Instance Variable Hiding

As you know, it is illegal in Java to declare two local variables with the same name inside the same or enclosing scopes. Interestingly, you can have local variables, including formal parameters to methods, which overlap with the names of the class' instance variables. However, when a local variable has the same name as an instance variable, the local variable hides the instance variable.

This is why width, height, and depth were not used as the names of the parameters to the Box() constructor inside the Box class. If they had been, then width would have referred to the formal parameter, hiding the instance variable width. While it is usually easier to simply use different names, there is another way around this situation. Because this lets you refer directly to the object, you can use it to resolve any name space collisions that might occur between instance variables and local variables. For example, here is another version of Box(), which uses width, height, and depth for parameter names and then uses this to access the instance variables by the same name:

    // Use this to resolve name-space collisions.
Box(double width, double height, double depth) {
    this.width = width;
    this.height = height;
    this.depth = depth;

A word of caution: The use of this in such a context can sometimes be confusing, and some programmers are careful not to use local variables and formal parameter names that hide instance variables. Of course, other programmers believe the contrary that it is a good convention to use the same names for clarity, and use this to overcome the instance variable hiding. It is a matter of taste which approach you adopt.

Although this is of no significant value in the examples just shown, it is very useful in certain situations.

This is an extract from the book: Java 2 - The Complete Reference by Herbert Schildt.

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