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The Color class
For example, the Color class defines a number of public static final variables containing the information that represents generic colors such as ORANGE, PINK, and MAGENTA. (By convention, constants in Java are written with all upper-case characters, but that is not a technicalrequirement.)
If you need generic colors and not custom colors, you can easily access and use these color values without the requirement to mix red, green, and blue toproduce the desired color values.
C. 6.283185307179586
A public static final variable
This program declares a public static final member variable named twoPI in the class named Worker , and properly initializes it when it is declared as shown in the following code fragment.
class Worker{ public static final double twoPI
= 2 * Math.PI;}/}// end class Worker From that point forward in the program, this member variable named twoPI behaves like a constant, meaning that any code that attempts to change its value will cause a compiler error (as in the program in Question 4 ) ..
Accessing the static variable
The following single statement that appears in the main method of the controlling class instantiates a new object of the Worker class, accesses, and displays the public static final member variable named twoPI .
public static void main(
String args[]){
System.out.println(new Worker().twoPI);
} }//end main() (Note for future discussion that the variable named twoPI is accessed via a reference to an object instantiated from the class named Worker .)
This causes the double value 6.283185307179586 to be displayed on the standard output device.
A. Compiler Error
A final variable
When a member variable of a class (not a local variable) is declared final , its value must be established when the variable is declared. This programattempts to assign a value to a final member variable after it has been declared, producing the following compiler error under JDK 1.3.
Ap113.java:20: cannot assign a value to
final variable twoPItwoPI = 2 * Math.PI; D. 22
Two uses of the this keyword
This program illustrates two different uses of the this keyword.
Disambiguating a reference to a variable
Consider first the use of this that is shown in the following code fragment.
class Worker{
private int data;public Worker(int data){this.data = data;
}//end constructor Very common usage
The code in the above fragment is commonly used by many Java programmers. All aspiring Java programmers need to know how to read suchcode, even if they elect not to use it. In addition, understanding this code should enhance your overall understanding of the use and nature of the this keyword.
A parameterized constructor
The above fragment shows a parameterized constructor for the class named Worker . This constructor illustrates a situation where there is a local parameter named data that has the same name as an instance variable belonging to the object.
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