Classes and Objects - Beginning Java 8 Fundamentals - page 200

What Is this?
Java has a keyword called this . It is a reference to the current instance of a class. It can be used only in the context of
an instance. It can never be used in the context of a class because it means the current instance, and no instance exists
in the context of a class.
The keyword this is used in many contexts. I will cover most of its uses in this chapter, not necessarily in this
section. However, you need to note that when it appears in Java code, it means the current instance of the class for
which the code is being executed.
Let's consider the following snippet of code that declares a class ThisTest1 :
public class ThisTest1 {
int varA = 555;
int varB = varA; // Assign value of varA to varB
int varC = this.varA; // Assign value of varA to varC
}
The ThisTest1 class declares three instance variables: varA , varB , and varC . The instance variable varA is
initialized to 555. The instance variable varB is initialized to the value of varA , which is 555. The instance variable
varC is initialized to the value of varA , which is 555. Note the difference in initialization expressions for varB and varC .
I used unqualified varA when I initialized varB . I used this.varA when I initialized varC . However, the effect is the
same. Both varB and varC are initialized with the value of varA . When I use this.varA , it means the value of varA for
the current instance, which is 555. In this simple example, it was not necessary to use the keyword this . In the above
case, the unqualified varA refers to the varA for the current instance. However, there are some cases where you must
use the keyword this . I will discuss such cases shortly.
Since the use of the keyword this is illegal in the context of a class, you cannot use it when you initialize a class
variable, like so:
// Would not compile
public class ThisTest2 {
static int varU = 555;
static int varV = varU;
static int varW = this.varU; // A compile-time error
}
When you compile the code for the class ThisTest2 , you receive the following compiler error:
"ThisTest2.java": non-static variable this cannot be referenced from a static context at line 4,
column 21
The compiler error is loud and clear that you cannot use the keyword this in a static context. Note that static and
non-static words are synonymous with “class” and “instance” terms in Java. Static context is the same as class context
and non-static context is the same as instance context. The above code can be corrected by removing the qualifier
this from the initialization expression for varW , as follows:
public class CorrectThisTest2 {
static int varU = 555;
static int varV = varU;
static int varW = varU; // Ok
}