Exception Hierarchy

Throwable is the base class of all exceptions in java. Throwable has two direct sub classes namely Exception and Error. These two classes creates clean separation of exception types. Exception and all its sub classes are that can be handled within the program. Error and all its sub classes are expected to be handled by the program. Error and its sub type are used by Java’s run time system to indicate problems related to run time environment like out of memory or stack overflow. For example, out of memory problem(OutOfMemoryError) cannot be fixed using program code and is not expected to be handled by the program.

In the below image java’s own exception classes and it hierarchy is shown. Only important Errors and Exceptions are shown. With every release of java there are numerous new exceptions added to the package and we have a lot of them now. Green colored classes are exceptions that can be handled in a program. Red colored classes are that cannot be handled. Packaging wise, every Java package has its set of exceptions organized within itself. For example, java.io.* contains all IO related exceptions in java.io.* itself, so thats where we gotta search for exception classes to have a look at it.

RuntimeException is a sub class of Exception class. RuntimeException and all it sub classes are need not be declared in throws clause of a method. These are understood to be regular exceptions that can occur in any context and java says that we need not declare them in thows of a method. All exceptions other than RuntimeException and its subclasses must be declared in throws clause of a method. To highlight that, in the image RuntimeException and its subclassess are shown in dark green. RuntimeException and all its sub classes are called un-checked exceptions, since they need not be declared using throws in method signature and all other java exceptions are checked exceptions.