Variables - Kinds, Scope, Lifetime

What are the three kinds of variables?

1.

2.

3.

What is variable "scope" and how does it apply to each kind of variable?

What is variable "lifetime" and how does it apply to each kind of variable?

What is hiding?

public class X {

  private static int a = 10;   //"static" or "class" variable
                               //with class scope. It exists
			       //the whole time the program is running
  
  private int b;               //"instance" variable with class scope 
                               //It exists during the lifetime
			       //of the object which contains it
  
  
  public X(int b) {            // b is a "local" variable with
                               // block scope and goes away at the
    this.b = b;                // end of the constructor
    
  }
  
  
  public void changeB(int c) {   //c and d are a "local" variables
                                 //with block scope which "go away"
    int d = 5;	                 //at the end of the method
	
    b = c;		         //Change value of "instance" variable b to c  
    
    b += d;                      
    
  }   
  
  public void sillyChangeB(int c) {   
                                     
    int d = 5;	                  
			  
    int b = c;                   //b is a "local" variable which "hides"
                                 //the "instance" variable b so its
    b += d;                      //value is NOT changed
    
  }      
  
  public int getB() {
  
    return b;
    
  }
  
  public static void main(String[] args) {
  
    System.out.println("a = " + a);   //a exists even when there a no
                                      //X class objects and unlike instance
				      //variables the main method can access it
  
    X myX = new X(5);      //Creates a new X object referred to by myX
    
    myX.changeB(3);
    
    System.out.println(myX.getB());
    
    myX.sillyChangeB(6);
    
    System.out.println(myX.getB());
    
    myX = new X(7);       //Creates a new X object referred to by myX
                          //The garbage collector gets rid of the old one
    
    myX = null;           //Tells the garbage collector to pick up this
                          //object, too, because we don't need it anymore
			  
  }
}
/**PROGRAM OUTPUT:
csc% java X
a = 10
8
8
*/