5.2. Exception Types

Java provides numerous exception classes, all of which are descendants of the Throwable class. All of them allow programs to differentiate among the various types of exceptions that can occur during the execution of a program.

Class Throwable

The Throwable class is the superclass of all errors and exceptions in Java.

Only objects that are instances of this class or one of its subclasses

• are thrown by the runtime environment,
• can be thrown by the Java throw statement, and
• can be an argument type in a catch clause.

Class Exception

An instance of Exception indicates a condition that an application may need to catch.

The class Exception and any subclasses that are not also subclasses of RuntimeException are called checked exceptions.

Checked exceptions need to be declared in a method or constructor's throws clause if

• they can be thrown by the execution of the method or constructor, and
• propagate outside the method or constructor boundary.

Class RuntimeException

RuntimeException is the superclass of those exceptions that can be thrown during the normal operation of the Java Virtual Machine.

RuntimeException and its subclasses are unchecked exceptions, which means that they do not need to be declared in a method or constructor's throws clause.

Most runtime exceptions can be avoided by checking certain conditions.

• NullPointerException checks if reference variable is null before invoking a method on it.

  public static Integer getHead(List<Integer> list) {
    if (list != null)
      return list.get(0);
  
    return null;
  }

• ArrayIndexOutOfBoundsException checks if the array index is less than the array length.

  public static int getValue(int[] array, int index) {
    if (index < array.length)
        return array[index];
  
    return -1;
  }

Class Error

extends the Throwable class.

An Error error is thrown by JVM. It indicates a serious problem that an application should not try to catch.

Here are a few examples:

• OutOfMemoryError is thrown when the JVM runs out of memory and cannot clean the memory using garbage collection.
• StackOverflowError is thrown when the memory allocated for the stack of the method calls is not enough to store another stack frame.
• NoClassDefFoundError is thrown when the JVM cannot find the definition of the class requested by the currently loaded class.
• AssertionError is thrown when an assertion has failed.