Java™ Application Development on Linux - Dator

3.2 Fundamental Language Elements75Java provides the try/catch/throw mechanism for more sophisticatederror handling. It avoids a lot of unnecessary checking and passing on of errors.The only parts of a Java program that need to deal with an error are those thatknow what to do with it.The throw in Java is really just a nonlocal “goto”—it will branch the executionof your program to a location which can be quite far away from themethod where the exception was thrown. But it does so in a very structuredand well-defined manner.In our simple example of A calling B calling C calling D, D implementedas a Java method can throw an exception when it runs into an error. Controlwill pass to the first enclosing block of code on the call stack that contains acatch for that kind of exception. So A can have code that will catch an exception,and B and C need not have any error handling code at all. Example 3.17demonstrates the syntax.Example 3.17 A simple try/catch blocktry {for (i = 0; i < max; i++) {someobj.methodB(param1, i);}} catch (Exception e) {// do the error handling here:System.out.println("Error encountered. Try again.");}// continues execution here after successful completion// but also after the catch if an error occursIn the example, if any of the calls to methodB() in the for loop goawry—that is, anywhere inside methodB() or whatever methods it may call anexception is thrown (and assuming those called methods don’t have their owntry/catch blocks), then control is passed up to the catch clause in our example.The for loop is exited unfinished, and execution continues first with thecatch clause and then with the statements after the catch.How does an error get thrown in the first place? One simply creates anException object and then throws the exception (Example 3.18).