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Programming errors on using the while-statement

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• Infinite loop
  • Facts:

    The while-statement is very powerful With more power, comes more responsibility, because You can create a never ending program using loop statements !!!

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  • Infinite loop:

    Infinite loop = a loop statement that does not end

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  • Example:

    public class While02 { public static void main(String[] args) { int a; a = 1; while ( a <= 10 ) // While-statement { System.out.println(a); // Print a // NO a++ statement !!! } System.out.println("Done"); System.out.println("Exit: a = " + a); } }

    Output:

    1 1 .... (never ending printing of 1's)

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  • Example Program: (Demo above code)                                                
    • Prog file: click here

    How to run the program:

    Right click on link and save in a scratch directory To compile:   javac While02.java To run:          java While02

    Note:

    You must type control-C in the terminal window to terminate the Java program when you do run it !

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• Prime directive of the while-statement
  • Very important programming rule:

    The while-body must contain statements that will affect (change) the outcome of the loop-continuation-condition of the while-loop !!!

    Otherwise, you will certainly have an infinite loop in the program.

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• Common errors with while-loops
  • Common error 1: bogus semicolon

    Example:

    public class Error01 { public static void main(String[] args) { int a; a = 1; while ( a <= 10 ) ; // BOGUS semicolon !! { System.out.println(a); // Print a a++; // Increment a } System.out.println("Done"); System.out.println("Exit: a = " + a); } }

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  • The program is read syntactically as follows:

    public class Error01 { public static void main(String[] args) { int a; a = 1; while ( a <= 10 ) ; // While body contains no statements { System.out.println(a); // Print a a++; // Increment a } System.out.println("Done"); System.out.println("Exit: a = " + a); } }

    Result:

    The program contains an infinite loop that keep testing the loop-continuation-condition The program will run forever but will not make any progress !!!

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  • Example Program: (Demo above code)                                                
    • Prog file: click here

    How to run the program:

    Right click on link and save in a scratch directory To compile:   javac Error01.java To run:          java Error01 Use control-C to terminate the program !!!

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  • Common error 2: forgetting to use statement block

    Example:

    public class Error02 { public static void main(String[] args) { int a; a = 1; while ( a <= 10 ) System.out.println(a); // Print a a++; // Increment a System.out.println("Done"); System.out.println("Exit: a = " + a); } }

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  • The program is read syntactically as follows:

    public class Error02 { public static void main(String[] args) { int a; a = 1; while ( a <= 10 ) System.out.println(a); // Body of while-loop a++; // Outside the while loop !! System.out.println("Done"); System.out.println("Exit: a = " + a); } }

    Result:

    The program contains an infinite loop that repeatedly prints 1, 1, 1, ...

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  • Example Program: (Demo above code)                                                
    • Prog file: click here

    How to run the program:

    Right click on link and save in a scratch directory To compile:   javac Error02.java To run:          java Error02 Use control-C to terminate the program !!!