The Java TM Tutorial

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Trail: Essential Java Classes
Lesson: Handling Errors with Exceptions

## The Throwable Class and Its Subclasses

> As you learned on the previous page, you can throw only objects that derive from the `Throwable` class. This includes direct descendants (that is, objects that derive directly from the `Throwable` class) as well as indirect descendants (objects that derive from children or grandchildren of the `Throwable` class).

> This diagram illustrates the class hierarchy of the `Throwable` class and its most significant subclasses.

> As you can see from the diagram, `Throwable` has two direct descendants: `Error` and `Exception`.

### Errors

> When a dynamic linking failure or some other "hard" failure in the virtual machine occurs, the virtual machine throws an `Error`. Typical Java programs should not catch `Error`s. In addition, it's unlikely that typical Java programs will ever throw `Error`s either.

### Exceptions

> Most programs throw and catch objects that derive from the `Exception` class. `Exception`s indicate that a problem occurred but that the problem is not a serious systemic problem. Most programs you write will throw and catch `Exception`s.

> The `Exception` class has many descendants defined in the Java packages. These descendants indicate various types of exceptions that can occur. For example, `IllegalAccessException` signals that a particular method could not be found, and `NegativeArraySizeException` indicates that a program attempted to create an array with a negative size.

> One `Exception` subclass has special meaning in the Java language: `RuntimeException`.

### Runtime Exceptions

> The `RuntimeException` class represents exceptions that occur within the Java virtual machine (during runtime). An example of a runtime exception is `NullPointerException`, which occurs when a method tries to access a member of an object through a null reference. A `NullPointerException` can occur anywhere a program tries to dereference a reference to an object. The cost of checking for the exception often outweighs the benefit of catching it.

> Because runtime exceptions are so ubiquitous and attempting to catch or specify all of them all the time would be a fruitless exercise (and a fruitful source of unreadable and unmaintainable code), the compiler allows runtime exceptions to go uncaught and unspecified.

> The Java packages define several `RuntimeException` classes. You can catch these exceptions just like other exceptions. However, a method is not required to specify that it throws `RuntimeExceptions`. In addition, you can create your own `RuntimeException` subclasses. Runtime Exceptions--The Controversy contains a thorough discussion of when and how to use runtime exceptions.

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