Within a block, variables can be declared at any point, but are valid only after they are
declared. Thus, if you define a variable at the start of a method, it is available to all of the code
within that method. Conversely, if you declare a variable at the end of a block, it is effectively
useless, because no code will have access to it. For example, this fragment is invalid because
count cannot be used prior to its declaration:
// This fragment is wrong!
count = 100; // oops! cannot use count before it is declared!
int count;
Here is another important point to remember: variables are created when their scope is
entered, and destroyed when their scope is left. This means that a variable will not hold its
value once it has gone out of scope. Therefore, variables declared within a method will not
hold their values between calls to that method. Also, a variable declared within a block will
lose its value when the block is left. Thus, the lifetime of a variable is confined to its scope.
If a variable declaration includes an initializer, then that variable will be reinitialized each
time the block in which it is declared is entered. For example, consider the next program.
// Demonstrate lifetime of a variable.
class LifeTime {
public static void main(String args[]) {
int x;
for(x = 0; x < 3; x++) {
int y = -1; // y is initialized each time block is entered
System.out.println("y is: " + y); // this always prints -1
y = 100;
System.out.println("y is now: " + y);
}
}
}
The output generated by this program is shown here:
y
is: -1
y
is now: 100
y
is: -1
y
is now: 100
y
is: -1
y
is now: 100
As you can see, y is reinitialized to 1 each time the inner for loop is entered. Even though it
is subsequently assigned the value 100, this value is lost.
One last point: Although blocks can be nested, you cannot declare a variable to have the
same name as one in an outer scope. For example, the following program is illegal:
// This program will not compile
class ScopeErr {
public static void main(String args[]) {
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