A compiler for the Java programming language checks, at compile time, that a program contains handlers for checked exceptions, by analyzing which checked exceptions can result from execution of a method or constructor. For each checked exception which is a possible result, the throws clause for the method (�8.4.4) or constructor (�8.8.4) must mention the class of that exception or one of the superclasses of the class of that exception. This compile-time checking for the presence of exception handlers is designed to reduce the number of exceptions which are not properly handled.
The unchecked exceptions classes are the class RuntimeException and its subclasses, and the class Error and its subclasses. All other exception classes are checked exception classes. The Java API defines a number of exception classes, both checked and unchecked. Additional exception classes, both checked and unchecked, may be declared by programmers. See �11.5 for a description of the exception class hierarchy and some of the exception classes defined by the Java API and Java virtual machine.
The checked exception classes named in the throws clause are part of the contract between the implementor and user of the method or constructor. The throws clause of an overriding method may not specify that this method will result in throwing any checked exception which the overridden method is not permitted, by its throws clause, to throw. When interfaces are involved, more than one method declaration may be overridden by a single overriding declaration. In this case, the overriding declaration must have a throws clause that is compatible with all the overridden declarations (�9.4).
Static initializers (�8.7), class variable initializers, and instance initializers or instance variable initializers within named classes and interfaces (�8.3.2), must not result in a checked exception; if one does, a compile-time error occurs. No such restriction applies to instance initializers or instance variable initializers within anonymous classes (�15.9.5).