- Consider the following program:
public class Demo1 { public static void main( String[] args ) { System.out.println("Mystery:"); { int x = 123; // Variable x System.out.println( x ); // (1) } System.out.println( x ); // (2) This statement will produce an error // Why ??? } }Experiment:
- Save the program below and compile it
- You will see that the Java compiler will
say that variable x
at the marked line
cannot be found.
- However, the same statement above does not produce any error !!!
- Example Program:
(Demo above code)
- Prog file: click here
- This example shows you that:
- A variable has a
limited scope
(scope = region in the program where the variable can be used)
- The variable x is
accessible at located
marked (1)
- But the variable x is not accessible at located marked (2)
- The variable x is
accessible at located
marked (1)
- A variable has a
limited scope
(scope = region in the program where the variable can be used)
- Consider a different kind of
variable in the following program:
public class Demo2 { static int x = 123; // Variable x public static void main( String[] args ) { System.out.println("Mystery:"); { System.out.println( x ); // (1) } System.out.println( x ); // (2) } }Experiment:
- Save the program below and compile it
- You will see that the Java compiler will report no errors
- Example Program:
(Demo above code)
- Prog file: click here
- This example shows you that:
- This particular variable has a
wider scope than the
previous variable
(scope = region in the program where the variable can be used)
- The variable x is accessible at located marked (1) and (2) !!!
- This particular variable has a
wider scope than the
previous variable
(scope = region in the program where the variable can be used)
- When a variable can be access from
more places in a program,
the information stored in such variable will be
long term.
When a variable can be access from only within a certain place in a program, the information stored in such variable will be short term.
(I.e., the information will not be retained when the program exection leaves the specific area in the program).
- You may think
(as a beginner) that
the wider the area
where a variable is
accessible, would be the
better choice
- That is not the case !!!
- If a variable is accessible
in a more places in a program,
then looking for errors that
is related with the use
of that variable
will be harder
(because you need to look in more places).
- Computer memory is
a limited resource
When information stored in a variable will no longer be used, the memory cells used to store the information must be "recycled" (= used to store other information that is being used by the program).
- If a variable is accessible
in a more places in a program,
then looking for errors that
is related with the use
of that variable
will be harder
(because you need to look in more places).
- That is not the case !!!
- Now consider this example
with 2 different variable x:
public class Demo3 { static int x = 123; // Variable x (1) public static void main( String[] args ) { int x = 777; // Variable x (2) { System.out.println( x ); // (1) x = ? } System.out.println( x ); // (2) x = ? } }Experiment:
- Save the program below and compile and run it.
- You will see
777 777
- Conclusion: both print statements are using the variable x defined on line (2).
- Example Program:
(Demo above code)
- Prog file: click here
- Now consider another example
with 2 different variable x:
public class Demo4 { static int x = 123; // Variable x (1) public static void main( String[] args ) { { int x = 777; // Variable x (2) System.out.println( x ); // (1) x = ? } System.out.println( x ); // (2) x = ? } }Experiment:
- Save the program below and compile and run it.
- You will see
777 123
- Conclusion:
- The first print statement uses the variable x defined on line (2).
- The second print statement uses the variable x defined on line (1).
- Example Program:
(Demo above code)
- Prog file: click here