An exception is a problem that arises during the execution of a program. An exception can occur for many different reasons, including the following:
- A user has entered invalid data.
- A file that needs to be opened cannot be found.
- A network connection has been lost in the middle of communications, or the JVM has run out of memory.
Some of these exceptions are caused by user error, others by programmer error, and others by physical resources that have failed in some manner.
To understand how exception handling works in Java, you need to understand the three categories of exceptions:
- Checked exceptions: A checked exception is an exception that is typically a user error or a problem that cannot be foreseen by the programmer. For example, if a file is to be opened, but the file cannot be found, an exception occurs. These exceptions cannot simply be ignored at the time of compilation.
- Runtime exceptions: A runtime exception is an exception that occurs that probably could have been avoided by the programmer. As opposed to checked exceptions, runtime exceptions are ignored at the time of compliation.
- Errors: These are not exceptions at all, but problems that arise beyond the control of the user or the programmer. Errors are typically ignored in your code because you can rarely do anything about an error. For example, if a stack overflow occurs, an error will arise. They are also ignored at the time of compilation.
All exception classes are subtypes of the java.lang.Exception class. The exception class is a subclass of the Throwable class. Other than the exception class there is another subclass called Error which is derived from the Throwable class.
Errors are not normally trapped form the Java programs. These conditions normally happen in case of severe failures, which are not handled by the java programs. Errors are generated to indicate errors generated by the runtime environment. Example : JVM is out of Memory. Normally programs cannot recover from errors.
The Exception class has two main subclasses : IOException class and RuntimeException Class.
Here is a list of most common checked and unchecked Java’s Built-in Exceptions.
Following is the list of important medthods available in the Throwable class.
|SN||Methods with Description|
|1||public String getMessage()
Returns a detailed message about the exception that has occurred. This message is initialized in the Throwable constructor.
|2||public Throwable getCause()
Returns the cause of the exception as represented by a Throwable object.
|3||public String toString()
Returns the name of the class concatenated with the result of getMessage()
|4||public void printStackTrace()
Prints the result of toString() along with the stack trace to System.err, the error output stream.
|5||public StackTraceElement  getStackTrace()
Returns an array containing each element on the stack trace. The element at index 0 represents the top of the call stack, and the last element in the array represents the method at the bottom of the call stack.
|6||public Throwable fillInStackTrace()
Fills the stack trace of this Throwable object with the current stack trace, adding to any previous information in the stack trace.
A method catches an exception using a combination of the try and catch keywords. A try/catch block is placed around the code that might generate an exception. Code within a try/catch block is referred to as protected code, and the syntax for using try/catch looks like the following:
- Java NullPointerException on createStatement (stackoverflow.com)
- java.net.SocketException when sending a heavy request on server (stackoverflow.com)
- Under the JVM hood – Classloaders (javacodegeeks.com)