The try Block:
The first step in constructing an exception handler is to enclose the code that might throw an exception within atry block. In general, a try block looks like the following:try {
code
}
catch and finally blocks . . .
code contains one or more legal lines of code that could throw an exception.To construct an exception handler for the
writeList method from the ListOfNumbers class, enclose the exception-throwing statements of the writeList method within a try block. There is more than one way to do this. You can put each line of code that might throw an exception within its own try block and provide separate exception handlers for each. Or, you can put all the writeList code within a single try block and associate multiple handlers with it. The following listing uses one try block for the entire method because the code in question is very short.private List<Integer> list;
private static final int SIZE = 10;
PrintWriter out = null;
try {
System.out.println("Entered try statement");
out = new PrintWriter(new FileWriter("OutFile.txt"));
for (int i = 0; i < SIZE; i++) {
out.println("Value at: " + i + " = " + list.get(i));
}
}
catch and finally statements . . .
try block, that exception is handled by an exception handler associated with it. To associate an exception handler with a try block, you must put a catch block after it;The catch Blocks:
You associate exception handlers with a
try block by providing one or more catch blocks directly after the try block. No code can be between the end of the try block and the beginning of the first catch block.try {
} catch (ExceptionType name) {
} catch (ExceptionType name) {
}
catch block is an exception handler and handles the type of exception indicated by its argument. The argument type, ExceptionType, declares the type of exception that the handler can handle and must be the name of a class that inherits from the Throwable class. The handler can refer to the exception with name.The following are two exception handlers for the
writeList method — one for two types of checked exceptions that can be thrown within the try statement:try {
} catch (FileNotFoundException e) {
System.err.println("FileNotFoundException: "
+ e.getMessage());
throw new SampleException(e);
} catch (IOException e) {
System.err.println("Caught IOException: "
+ e.getMessage());
}Catching More Than One Type of Exception with One Exception Handler:
In Java SE 7 and later, a singlecatch block can handle more than one type of exception. This feature can reduce code duplication and lessen the temptation to catch an overly broad exception.In the
catch clause, specify the types of exceptions that block can handle, and separate each exception type with a vertical bar (|):catch (IOException|SQLException ex) {
logger.log(ex);
throw ex;
}
catch block handles more than one exception type, then the catch parameter is implicitly final. In this example, the catch parameter ex is final and therefore you cannot assign any values to it within the catch block.The finally Block:
The finally block always executes when the try block exits. This ensures that the finally block is executed even if an unexpected exception occurs. But finally is useful for more than just exception handling it allows the programmer to avoid having cleanup code accidentally bypassed by a return, continue, or break. Putting cleanup code in a finally block is always a good practice, even when no exceptions are anticipated.
Note:
If the JVM exits while thetry or catch code is being executed, then the finally block may not execute. Likewise, if the thread executing the try or catch code is interrupted or killed, the finally block may not execute even though the application as a whole continues.Important:
The
finally block is a key tool for preventing resource leaks. When closing a file or otherwise recovering resources, place the code in a finally block to ensure that resource is always recovered.If you are using Java SE 7 or later, consider using the
try-with-resources statement in these situations, which automatically releases system resources when no longer needed
