Chapter: Java The Complete Reference - The Java Language - Exception Handling

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Using try and catch - Java

Although the default exception handler provided by the Java run-time system is useful for debugging, you will usually want to handle an exception yourself.

Using try and catch

 

Although the default exception handler provided by the Java run-time system is useful for debugging, you will usually want to handle an exception yourself. Doing so provides two benefits. First, it allows you to fix the error. Second, it prevents the program from automatically terminating. Most users would be confused (to say the least) if your program stopped running and printed a stack trace whenever an error occurred! Fortunately, it is quite easy to prevent this.

 

To guard against and handle a run-time error, simply enclose the code that you want to monitor inside a try block. Immediately following the try block, include a catch clause that specifies the exception type that you wish to catch. To illustrate how easily this can be done, the following program includes a try block and a catch clause that processes the ArithmeticException generated by the division-by-zero error:

 

class Exc2 {

 

public static void main(String args[]) { int d, a;

 

try { // monitor a block of code. d = 0;

 

a = 42 / d;

 

System.out.println("This will not be printed.");

 

} catch (ArithmeticException e) { // catch divide-by-zero error 

System.out.println("Division by zero.");

}

System.out.println("After catch statement.");

 

}

 

}

This program generates the following output:

 

Division by zero.

 

After catch statement.

Notice that the call to println( ) inside the try block is never executed. Once an exception is thrown, program control transfers out of the try block into the catch block. Put differently, catch is not “called,” so execution never “returns” to the try block from a catch. Thus, the line "This will not be printed." is not displayed. Once the catch statement has executed, program control continues with the next line in the program following the entire try / catch mechanism.

 

try and its catch statement form a unit. The scope of the catch clause is restricted to those statements specified by the immediately preceding try statement. A catch statement cannot catch an exception thrown by another try statement (except in the case of nestedtry statements, described shortly). The statements that are protected by try must be surrounded by curly braces. (That is, they must be within a block.) You cannot use try

 

on a single statement.

 

The goal of most well-constructed catch clauses should be to resolve the exceptional condition and then continue on as if the error had never happened. For example, in the next program each iteration of the for loop obtains two random integers. Those two integers are divided by each other, and the result is used to divide the value 12345. The final result is put into a. If either division operation causes a divide-by-zero error, it is caught, the value of a is set to zero, and the program continues.

 

// Handle an exception and move on. 


import java.util.Random;

 

class HandleError {

 

public static void main(String args[]) { int a=0, b=0, c=0;

 

Random r = new Random();

 

for(int i=0; i<32000; i++) { try {

 

b = r.nextInt(); c = r.nextInt();

a = 12345 / (b/c);

 

} catch (ArithmeticException e) { 

System.out.println("Division by zero."); a = 0; // set a to zero and continue

}

 

System.out.println("a: " + a);

 

}

 

}

}

Displaying a Description of an Exception

 

Throwable overrides the toString( ) method (defined by Object) so that it returns a string containing a description of the exception. You can display this description in a println( ) statement by simply passing the exception as an argument. For example, the catchblock in the preceding program can be rewritten like this:

 

catch (ArithmeticException e) { 

System.out.println("Exception: " + e); 

a = 0; // set a to zero and continue

 

}

 

When this version is substituted in the program, and the program is run, each divide-by-zero error displays the following message:

 

Exception: java.lang.ArithmeticException: / by zero

 

While it is of no particular value in this context, the ability to display a description of an exception is valuable in other circumstances—particularly when you are experimenting with exceptions or when you are debugging.


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