Exceptions and error handling in Java

Exceptions are a means to control error flow within a program. During the course of execution, your program may encounter errors for various reasons. A program will typical encounter various types of error, including:

• programming errors (e.g. trying to call a method on a null object);
• unexpected environmental errors (e.g. an expected setup file not being found, or the system running out of memory);
• expected errors such as invalid user input.

Writing a complex program would be tedious if we needed to worry about such errors with every line of code: for example, if we had to check whether an object reference was null every single time we tried to access it, or if we had to check for an error condition after every single call to read a byte from a file. If you've programmed in a low-level language such as C, you'll be aware that you often do need to check for invalid pointers or error result codes after many simple standard library functions.

In a language such as Java, exceptions make error handling less tedious. Exceptions work as follows:

• methods such as file access methods, string parsing methods etc declare that they can throw certain exceptions— effectively indicating that certain errors are likely to occur;
• the Java Virtual Machine, while executing your program, can also interrupt the normal program flow by "throwing" an exception, e.g. if you attempt to access a null object reference or invalid array index;
• blocks of code can be accompanied by error handlers called catch clauses: when an exception is thrown, the program flow will "jump" to the surrounding catch clause that can handle the error in question. (Here are some examples of exceptions thrown by the Java Virtual Machine.)

Exceptions therefore allow errors to be handled by shared error handling code, with more structured program flow, and without having to check for and micromanage error conditions after every individual line of code! We continue on the next page with an introduction to exceptions in Java.

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