checked Exception, unchecked Exception and Error

1. checked exception
a. 都是Exception的子类，若子类可能抛出X异常，则父类也必须throws X异常，效率低，耦合度高。
b. 需要强制catch，否则编译器直接报错。

/**
 * The class <code>Exception</code> and its subclasses are a form of 
 * <code>Throwable</code> that indicates conditions that a reasonable 
 * application might want to catch.
 *
 * @author  Frank Yellin
 * @version 1.32, 11/17/05
 * @see     java.lang.Error
 * @since   JDK1.0
 */
public class Exception extends Throwable {
    static final long serialVersionUID = -3387516993124229948L;

    /**
     * Constructs a new exception with <code>null</code> as its detail message.
     * The cause is not initialized, and may subsequently be initialized by a
     * call to {@link #initCause}.
     */
    public Exception() {
	super();
    }

    /**
     * Constructs a new exception with the specified detail message.  The
     * cause is not initialized, and may subsequently be initialized by
     * a call to {@link #initCause}.
     *
     * @param   message   the detail message. The detail message is saved for 
     *          later retrieval by the {@link #getMessage()} method.
     */
    public Exception(String message) {
	super(message);
    }

    /**
     * Constructs a new exception with the specified detail message and
     * cause.  <p>Note that the detail message associated with
     * <code>cause</code> is <i>not</i> automatically incorporated in
     * this exception's detail message.
     *
     * @param  message the detail message (which is saved for later retrieval
     *         by the {@link #getMessage()} method).
     * @param  cause the cause (which is saved for later retrieval by the
     *         {@link #getCause()} method).  (A <tt>null</tt> value is
     *         permitted, and indicates that the cause is nonexistent or
     *         unknown.)
     * @since  1.4
     */
    public Exception(String message, Throwable cause) {
        super(message, cause);
    }

    /**
     * Constructs a new exception with the specified cause and a detail
     * message of <tt>(cause==null ? null : cause.toString())</tt> (which
     * typically contains the class and detail message of <tt>cause</tt>).
     * This constructor is useful for exceptions that are little more than
     * wrappers for other throwables (for example, {@link
     * java.security.PrivilegedActionException}).
     *
     * @param  cause the cause (which is saved for later retrieval by the
     *         {@link #getCause()} method).  (A <tt>null</tt> value is
     *         permitted, and indicates that the cause is nonexistent or
     *         unknown.)
     * @since  1.4
     */
    public Exception(Throwable cause) {
        super(cause);
    }
}

2. unchecked exception(RuntimeException继承Exception)
a. 都是RuntimeException的子类。
b. 非必须catch，是无法预料的。比如，list..size()，若list=null， 则会抛出NullPointerException，该异常是一个RuntimeException， 也就是unchecked exception。

public class RuntimeException extends Exception

3. Error
a. 都是Error的子类。
b. 非必须catch，是无法预料的。

Error和Exception都继承自Throwable。不同处:
Exceptions: 表示一种设计或实现问题。也就是说，它表示如果程序运行正常，从不会发生的情况。程序员可以对异常代码进行改正。
1．可以是可控制的(checked)和不可控制的(unchecked)
2．表示一个由程序员导致的错误
3．应该在应用程序级被处理

Errors: 表示恢复不是不可能但很困难，是一种严重问题，比如说内存溢出。
1．总是不可控制的(unchecked)
2．经常用来用于表示系统错误或低层资源的错误
3．如何可能的话，应该在系统级被捕捉