java - 単純な KeyListener アプリのエラー

Question

基本的に私はビギナープログラマーです。C# と C++ を少し知っていますが、現在は Android アプリを構築するための Java を学んでいます。私はブランドニオ制作のYouTubeからJavaを学んでいます。とても役に立ちます。http://www.youtube.com/user/BrandonioProductions?feature=watch

これは完全なコードです：

import java.applet.Applet;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Rectangle;
import java.awt.event.KeyEvent;
import java.awt.event.KeyListener;
import java.util.ArrayList;

public class KeyListenerApp 
extends Applet
implements KeyListener{
private Rectangle rect; //The rectangle that moving
private ArrayList<Integer> keysDown;
public void init()
{
    rect = new Rectangle(0,0,50,50);
    this.addKeyListener(this);
}
public void paint(Graphics g)
{
    setSize(500,500);
    Graphics2D g2 = (Graphics2D)g;
    g2.fill(rect);
}
@Override
public void keyPressed(KeyEvent e) {
    if(keysDown.contains(new Integer(e.getKeyCode())))
    {
        keysDown.add(new Integer(e.getKeyCode()));
    }
    moveRect();
}
@Override
public void keyReleased(KeyEvent e) {
    keysDown.remove(new Integer(e.getKeyCode()));
}
public void moveRect()
{
    int x = rect.x;
    int y = rect.y;
    if (keysDown.contains(KeyEvent.VK_UP))
        y -= 2;
    if (keysDown.contains(KeyEvent.VK_DOWN))
        y += 2;
    if (keysDown.contains(KeyEvent.VK_LEFT))
        x -= 2;
    if (keysDown.contains(KeyEvent.VK_RIGHT))
        x += 2;
    rect.setLocation(x, y);
    repaint();
}
@Override
public void keyTyped(KeyEvent e) {
}
}

これは完全なエラーです:

Exception in thread "AWT-EventQueue-1" java.lang.NullPointerException
    at KeyListenerApp.keyPressed(KeyListenerApp.java:34)
    at java.awt.Component.processKeyEvent(Unknown Source)
    at java.awt.Component.processEvent(Unknown Source)
    at java.awt.Container.processEvent(Unknown Source)
    at java.awt.Component.dispatchEventImpl(Unknown Source)
    at java.awt.Container.dispatchEventImpl(Unknown Source)
    at java.awt.Component.dispatchEvent(Unknown Source)
    at java.awt.KeyboardFocusManager.redispatchEvent(Unknown Source)
    at java.awt.DefaultKeyboardFocusManager.dispatchKeyEvent(Unknown Source)
    at java.awt.DefaultKeyboardFocusManager.preDispatchKeyEvent(Unknown Source)
    at java.awt.DefaultKeyboardFocusManager.typeAheadAssertions(Unknown Source)
    at java.awt.DefaultKeyboardFocusManager.dispatchEvent(Unknown Source)
    at java.awt.Component.dispatchEventImpl(Unknown Source)
    at java.awt.Container.dispatchEventImpl(Unknown Source)
    at java.awt.Component.dispatchEvent(Unknown Source)
    at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
    at java.awt.EventQueue.access$200(Unknown Source)
    at java.awt.EventQueue$3.run(Unknown Source)
    at java.awt.EventQueue$3.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
    at java.awt.EventQueue$4.run(Unknown Source)
    at java.awt.EventQueue$4.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
    at java.awt.EventQueue.dispatchEvent(Unknown Source)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
    at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    at java.awt.EventDispatchThread.run(Unknown Source)
Exception in thread "AWT-EventQueue-1" java.lang.NullPointerException
    at KeyListenerApp.keyReleased(KeyListenerApp.java:44)
    at java.awt.Component.processKeyEvent(Unknown Source)
    at java.awt.Component.processEvent(Unknown Source)
    at java.awt.Container.processEvent(Unknown Source)
    at java.awt.Component.dispatchEventImpl(Unknown Source)
    at java.awt.Container.dispatchEventImpl(Unknown Source)
    at java.awt.Component.dispatchEvent(Unknown Source)
    at java.awt.KeyboardFocusManager.redispatchEvent(Unknown Source)
    at java.awt.DefaultKeyboardFocusManager.dispatchKeyEvent(Unknown Source)
    at java.awt.DefaultKeyboardFocusManager.preDispatchKeyEvent(Unknown Source)
    at java.awt.DefaultKeyboardFocusManager.typeAheadAssertions(Unknown Source)
    at java.awt.DefaultKeyboardFocusManager.dispatchEvent(Unknown Source)
    at java.awt.Component.dispatchEventImpl(Unknown Source)
    at java.awt.Container.dispatchEventImpl(Unknown Source)
    at java.awt.Component.dispatchEvent(Unknown Source)
    at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
    at java.awt.EventQueue.access$200(Unknown Source)
    at java.awt.EventQueue$3.run(Unknown Source)
    at java.awt.EventQueue$3.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
    at java.awt.EventQueue$4.run(Unknown Source)
    at java.awt.EventQueue$4.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
    at java.awt.EventQueue.dispatchEvent(Unknown Source)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
    at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    at java.awt.EventDispatchThread.run(Unknown Source)

最初にここにコードを投稿しなかったことをお詫びします。質問とは別のものにすれば、より快適になると思います。

ありがとうございました

作業コードは次のとおりです。

import java.applet.Applet;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Rectangle;
import java.awt.event.KeyEvent;
import java.awt.event.KeyListener;
import java.util.ArrayList;


public class KeyListenerApp 
extends Applet
implements KeyListener{

private Rectangle rect; //The rectangle that moving

private ArrayList<Integer> keysDown;

public void init()
{
    rect = new Rectangle(0,0,50,50);
    this.addKeyListener(this);
    keysDown = new ArrayList<Integer>();
}

public void paint(Graphics g)
{
    setSize(500,500);
    Graphics2D g2 = (Graphics2D)g;
    g2.fill(rect);
}

@Override
public void keyPressed(KeyEvent e) {

    if(keysDown.contains(new Integer(e.getKeyCode())) == false)
    {
        keysDown.add(new Integer(e.getKeyCode()));
    }

    moveRect();
}

@Override
public void keyReleased(KeyEvent e) {
    keysDown.remove(new Integer(e.getKeyCode()));

}

public void moveRect()
{
    int x = rect.x;
    int y = rect.y;

    if (keysDown.contains(KeyEvent.VK_UP))
        y -= 2;

    if (keysDown.contains(KeyEvent.VK_DOWN))
        y += 2;

    if (keysDown.contains(KeyEvent.VK_LEFT))
        x -= 2;

    if (keysDown.contains(KeyEvent.VK_RIGHT))
        x += 2;

    rect.setLocation(x, y);
    repaint();
}

@Override
public void keyTyped(KeyEvent e) {


}

}

エラーの原因:

if(keysDown.contains(new Integer(e.getKeyCode())) == false)
{
    keysDown.add(new Integer(e.getKeyCode()));
}

このコードは、修正する前は次のようでした。

if(keysDown.contains(new Integer(e.getKeyCode())))
{
    keysDown.add(new Integer(e.getKeyCode()));
}

エラーが発生したのは、コードで次のように言ったためです。何かが存在する場合は彼を追加し、彼が存在しない場合にのみ追加することになっていました。

score 0 · Accepted Answer

private ArrayList<Integer> keysDown;

このオブジェクトのインスタンスを作成することはないため、NullPointerException.

score 0 · Accepted Answer

キーダウンは NULL です! 初期化されていません。あなたはするべきです：

keysDown = new ArrayList<Integer>();

それを使用する前に何らかの方法で、リストにASCIIキーを入力してください。

java - 単純な KeyListener アプリのエラー

2 に答える 2

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