這兩天做一個IP地址的控件，在鍵盤監聽遇到一點麻煩，癥狀就是無法截獲鍵盤字符輸入的事件，經過艱苦的調查，才知道原來是對swing的key event機制了解不深，將截獲代碼寫在了keyPressed
處理方法中，而不是keyTyped。 而鍵盤輸入一個字符時，key pressed, key typed 和 key released 事件都會產生（特定情況），所以在key pressed代碼中event.consume()只是截獲了key pressed事件，
但是key typed事件仍然會傳遞到控件。
附上詳細的java doc描述，戒除浮躁，java doc還是要常看的，尤其是遇到問題的時候，盲目的試驗，不如仔細地研究java doc或者源碼
 
public class KeyEvent
extends InputEvent

An event which indicates that a keystroke occurred in a component.

This low-level event is generated by a component object (such as a text field) when a key is pressed, released, or typed. The event is passed to every KeyListener or KeyAdapter object which registered to receive such events using the component's addKeyListener method. (KeyAdapter objects implement the KeyListener interface.) Each such listener object gets this KeyEvent when the event occurs.

"Key typed" events are higher-level and generally do not depend on the platform or keyboard layout. They are generated when a Unicode character is entered, and are the preferred way to find out about character input. In the simplest case, a key typed event is produced by a single key press (e.g., 'a'). Often, however, characters are produced by series of key presses (e.g., 'shift' + 'a'), and the mapping from key pressed events to key typed events may be many-to-one or many-to-many. Key releases are not usually necessary to generate a key typed event, but there are some cases where the key typed event is not generated until a key is released (e.g., entering ASCII sequences via the Alt-Numpad method in Windows). No key typed events are generated for keys that don't generate Unicode characters (e.g., action keys, modifier keys, etc.).

The getKeyChar method always returns a valid Unicode character or CHAR_UNDEFINED. Character input is reported by KEY_TYPED events: KEY_PRESSED and KEY_RELEASED events are not necessarily associated with character input. Therefore, the result of the getKeyChar method is guaranteed to be meaningful only for KEY_TYPED events.

getKeyChar 方法的返回結果只有在KEY_TYPED事件處理代碼可以保證，也即該方法應該在KEY_PRESSED 代碼處理中調用

For key pressed and key released events, the getKeyCode method returns the event's keyCode. For key typed events, the getKeyCode method always returns VK_UNDEFINED.

"Key pressed" and "key released" events are lower-level and depend on the platform and keyboard layout. They are generated whenever a key is pressed or released, and are the only way to find out about keys that don't generate character input (e.g., action keys, modifier keys, etc.). The key being pressed or released is indicated by the getKeyCode method, which returns a virtual key code.

Virtual key codes are used to report which keyboard key has been pressed, rather than a character generated by the combination of one or more keystrokes (such as "A", which comes from shift and "a").

For example, pressing the Shift key will cause a KEY_PRESSED event with a VK_SHIFT keyCode, while pressing the 'a' key will result in a VK_A keyCode. After the 'a' key is released, a KEY_RELEASED event will be fired with VK_A. Separately, a KEY_TYPED event with a keyChar value of 'A' is generated.

Pressing and releasing a key on the keyboard results in the generating the following key events (in order): 

    KEY_PRESSED
    KEY_TYPED (is only generated if a valid Unicode character could be generated.)
    KEY_RELEASED
But in some cases (e.g. auto-repeat or input method is activated) the order could be different (and platform dependent).

Notes:

  • Key combinations which do not result in Unicode characters, such as action keys like F1 and the HELP key, do not generate KEY_TYPED events.
  • Not all keyboards or systems are capable of generating all virtual key codes. No attempt is made in Java to generate these keys artificially.
  • Virtual key codes do not identify a physical key: they depend on the platform and keyboard layout. For example, the key that generates VK_Q when using a U.S. keyboard layout will generate VK_A when using a French keyboard layout.
  • Not all characters have a keycode associated with them. For example, there is no keycode for the question mark because there is no keyboard for which it appears on the primary layer.
  • In order to support the platform-independent handling of action keys, the Java platform uses a few additional virtual key constants for functions that would otherwise have to be recognized by interpreting virtual key codes and modifiers. For example, for Japanese Windows keyboards, VK_ALL_CANDIDATES is returned instead of VK_CONVERT with the ALT modifier.
  • As specified in Focus Specification key events are dispatched to the focus owner by default.

WARNING: Aside from those keys that are defined by the Java language (VK_ENTER, VK_BACK_SPACE, and VK_TAB), do not rely on the values of the VK_ constants. Sun reserves the right to change these values as needed to accomodate a wider range of keyboards in the future.