Classes and Objects - Beginning Java 8 Fundamentals - page 204

public void setId(int id) {
this.id = id;
}
public int getId() {
return this.id;
{
}

The Student class declares an instance variable id . In its setId() method, it also names the parameter id , and
uses this.id to refer to the instance variable. It also uses this.id to refer to the instance variable id in its getId()
method. Note that there is no name hiding occurring in the getId() method and you could use the simple name id ,
which means the instance variable id .
I will discuss the use of the keyword this again in this chapter and subsequent chapters. Table 6-2 lists the parts of
a class, the context in which they occur, and the permitted use of the keyword this , the instance variable, and the class
variable. I have not yet covered all parts of a class that are listed in Table 6-2 . I will cover them shortly in this chapter.

Table 6-2. The Context Type and Allowed Use of the Keyword this, an Instance Variable, and a Class Variable

Part of a Class Context Can use this

keyword?

Can use instance
variable?

Can use class
variable?

Instance variable initialization Instance Yes Yes Yes

Class variable initialization Class No No Yes

Instance initializer Instance Yes Yes Yes

Class initializer
(Also called static initializer)

Class No No Yes

Constructor Instance Yes Yes Yes

Instance method
(Also called non-static method)

Instance Yes Yes Yes

Class method
(Also called static method)

Class No No Yes

The keyword this is a final (a constant is called final in Java because Java uses the final keyword to declare
a constant) reference to the current instance of the class in which it appears. Because it is final , you cannot change
its value. Because this is a keyword, you cannot declare a variable named this . The following code will generate a
compiler error:

public class ThisError {
void m1() {
// An error. Cannot name a variable this
int this = 10;

// An error. Cannot assign a value to this because it is a constant.
this = null;
}
}