DEBUGGING A JAVA PROGRAM - Kansas State University

[Pages:9]DEBUGGING A JAVA PROGRAM

After you have deployed a Java program you must check that it compiles and runs correctly. A program bug is an error in the program. Debugging is the process of finding and fixing these errors.

Types of Program Errors A compiler error results when the compiler doesn't understand a program statement. Such errors are most often the results of misspelled words, bad spacing, wrong letter case or missing punctuation. The best way to minimize these errors is to pay close attention to detail as you type out your programs.

Examples doubel height; double height Double height; public class Hello World

Misspelled keyword double Missing semicolon (;) Wrong letter case Incorrect spacing

A run-time error is not caught by the compiler but causes the computer to abnormally halt during the program's run.

Example Entering input that is not a floating-point number causes this code to fail.

java.util.Scanner in = new java.util.Scanner( System.in ); System.out.print( "Enter the circle's radius: " ); double radius = in.nextDouble( );

A logic error is an error in the program's underlying logic so that the program output is incorrect.

Example This statement incorrectly calculates the mileage on a car. There is no obvious error but the program will output the wrong answer.

mpg = gallons / miles;

Debugging a Java Program

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Debugging Compiler Errors If the compiler encounters an error in your program, it issues a diagnostic message (or diagnostic). The diagnostic is a clue that an error exists, but don't rely too much on its accuracy. A diagnostic can cite the wrong line number; its explanation can be obscure or even totally wrong.

Example Here is a typical Java diagnostic:

MPG1.java:10: cannot find symbol symbol : variable Miles location: class MPG1

Miles = 341; ^

The first line contains: Java file name

line number of the offending statement

description of the error

MPG1.java:10: cannot find symbol

The second line contains: the unexpected symbol

what kind of symbol it is

symbol : variable Miles

The third line tells you the class in which the error appears. The fourth line reproduces the offending Java statement and the fifth line places a caret mark (^) beneath the offending symbol.

Debugging a Java Program

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The compiler does not produce a class file to execute unless the Java file is error free. A compilation with no errors is called a clean compile.

Follow these steps to debug your compiler errors:

Begin

Compile program

Any diagnostics?

NO

YES

NO NO

Fix the FIRST one

Done Fix it

Any more?

YES

Does it make sense?

YES

The picture recommends that you fix the first error and recompile if the remaining errors don't make sense. This is because the remaining diagnostics may be spurious ? a spurious diagnostic identifies a correct statement as being incorrect. An avalanche of spurious diagnostics is called error cascade.

Here's why the compiler gives spurious diagnostics. It reads the program left to right, top to bottom and generates a diagnostic if it encounters something unexpected. After this first error, the compiler tries to recover by skipping the program's text until reaching a symbol it understands. If the skipped program text has information in it that the compiler needs to parse subsequent statements, those statements will give diagnostics even though they are correct.

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Therefore, always concentrate on the first diagnostic. It is the only one that you can trust to be authentic.

Example The following application has been seeded with one error ? the opening brace ({) at line 4 has been removed. This single error results in 28 diagnostics issued by the compiler.

1 public class MPGx

2{

3

public static void main( String [] args )

4

5

// declare data

6

double miles; // miles driven in car

7

double gallons; // number of gallons used

8

double mpg;

// miles per gallon

9

// initialize data

10

miles = 341;

11

gallons = 15.5;

12

// calculate mpg

13

mpg = miles / gallons;

14

// output results

15

System.out.print ( miles + " mi. / " );

16

System.out.print ( gallons + " gal." );

17

System.out.println( " = " + mpg + " mpg" );

18

}

19 }

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Debugging Run-Time Errors If your running program encounters an exception ? that is, a situation that it hasn't been programmed to deal with ? it halts and generates an error message.

Example Here is a Java run-time error message that results from entering input of the wrong data type:

Exception in thread "main" java.util.InputMismatchException at java.util.Scanner.throwFor(Scanner.java:909) at java.util.Scanner.next(Scanner.java:1530) at java.util.Scanner.nextDouble(Scanner.java:2456) at MPG.main(MPG.java:15)

The lines tell you exactly where the exception was thrown and how it was propagated back through the chain of method calls. For example, the last line above shows:

the method that threw it the Java file name

the line number of the statement

at MPG.main(MPG.java:15)

Debugging Logic Errors A program run that moves successfully to completion is called a normal termination. Even so, the program may not be correct if it has a logic error. Logic errors do not produce error messages; the only way to detect them is to inspect the program's output.

Example Suppose a program prints:

15.5 mi. / 341.0 gal. = 22.0 mpg

Since 15.5 0.045 22 , this output is clearly wrong. 341

You hunt down the error by comparing the program's output to what it is supposed to output.

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You should know the latter from having done your problem analysis and program specification. Also, use input data for which you know the correct answer. Grab a pencil and paper and double check your calculations.

For the former, use a symbolic debugger to step through the program and view its internal memory.

By comparing your worked answer to the debugger's output, you can discover the exact statement where the program begins to go wrong.

Exercises

1. Use jGRASP to enter, save and compile the MPGx application (page 4). Observe the error cascade. Is the line number cited by the first diagnostic correct?

2. Use jGRASP to enter, save and compile the application below. What is the diagnostic? What line number does it cite? Is the explanation accurate?

1 public class StudyDiagnostic

2{

3

public static void main( String [] args )

4

{

5

double cost, qty;

6

java.util.Scanner in = new java.util.Scanner( System.in );

7

System.out.print( "Enter a quantity: " );

8

qty = in.nextDouble( );

9

cost = $15,000;

10

System.out.println( "Total: " + qty * cost );

11

}

12 }

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Use jGRASP to correct the StudyDiagnostic application of the previous problem. Compile and run it to make sure it is correct and then complete each of the following problems.

3. On line 5 change double to doubel. Recompile the application and observe the diagnostic. Restore the application so that it is correct.

4. On line 8 change qty to Qty. Recompile the application and observe the diagnostic. What does the diagnostic cannot find symbol mean? Restore the application so that it is correct.

5. On line 1 change StudyDiagnostic to Study Diagnostic. Recompile the application and observe the diagnostic. Restore the application so that it is correct.

6. Practice debugging this program. It has approximately 10 compiler errors. Identify them all.

public class Multiplierx {

public static void main( String [] args ) {

/ / declare data double firstnum; double secondnum; double product; // input data java.util.Scanner in = new Scanner( System.in ); System.out.print( "Enter first number to multiply: " ); double firstnum = in.nextdouble( ); System.out.print( "Enter second number to multiply: " ); double secondnum = in.nextdouble( ); // calculate result product = 2ndnumber * 1stnumber; // output results System.out.print( firstNum + " * " ); System.out.print( secondNum + " = " ); System.out.println( product ); } }

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7. Practice debugging this program. It has approximately 22 compiler errors, 1 logic error and 1 usability error. Identify them all.

import java.util.scanner;

This program finds the average of three input values.

public class Arithmeticx {

public static void main( String [] args ) // declare variables double number1, number2, number3; // input data scanner in = newscanner( ); System.out.print( "Number 1? " ); System.out.print( "Number 2? " ); System.out.print( "Number 3? " ); number 1 = in.nextDouble; number 2 = in.nextDouble; number 3 = in.nextDouble; // calculate average average = num1 + num2 + num3 / 3; // print results system.out.print( Number1 + ", " + Number2 + " & " + Number3 ); system.out.println( " averages to " + average );

}

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