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一旦我按下手机上的后退按钮，我无法弄清楚为什么应用程序会不断崩溃。我试图在 WebView 上使用后退按钮返回页面

这是我的代码：

package com.***.****;

import android.app.Activity;
import android.os.Bundle;
import android.view.KeyEvent;
import android.view.Window;
import android.view.WindowManager;
import android.webkit.WebSettings;
import android.webkit.WebView;
import android.webkit.WebViewClient;


public class MainActivity extends Activity {

WebView myWebView;

@Override
protected void onCreate(Bundle savedInstanceState) {
    // TODO Auto-generated method stub
    super.onCreate(savedInstanceState);
    requestWindowFeature(Window.FEATURE_NO_TITLE);
    getWindow().setFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN,WindowManager.LayoutParams.FLAG_FULLSCREEN);
    setContentView(R.layout.activity_main);

    WebView myWebView = (WebView) findViewById(R.id.webview);
    myWebView.setWebViewClient(new WebViewClient());
    WebSettings webSettings = myWebView.getSettings();
    webSettings.setJavaScriptEnabled(true);
    myWebView.loadUrl("*****");
}

 @Override
 public boolean onKeyDown(int keyCode, KeyEvent event) {
    // Check if the key event was the Back button and if there's history
    if ((keyCode == KeyEvent.KEYCODE_BACK) && myWebView.canGoBack()) {
        myWebView.goBack();
        return true;
    }
    // If it wasn't the Back key or there's no web page history, bubble up to the default
    // system behavior (probably exit the activity)
    return super.onKeyDown(keyCode, event);
}
}

这是我单击后退按钮时出现的情况

11-28 14:13:17.768: E/AndroidRuntime(3636): FATAL EXCEPTION: main
11-28 14:13:17.768: E/AndroidRuntime(3636): java.lang.NullPointerException
11-28 14:13:17.768: E/AndroidRuntime(3636):     at com.triplec.googledeveloper.MainActivity.onKeyDown(MainActivity.java:35)
11-28 14:13:17.768: E/AndroidRuntime(3636):     at android.view.KeyEvent.dispatch(KeyEvent.java:2756)
11-28 14:13:17.768: E/AndroidRuntime(3636):     at android.app.Activity.dispatchKeyEvent(Activity.java:2428)
11-28 14:13:17.768: E/AndroidRuntime(3636):     at com.android.internal.policy.impl.PhoneWindow$DecorView.dispatchKeyEvent(PhoneWindow.java:2076)
11-28 14:13:17.768: E/AndroidRuntime(3636):     at android.view.ViewRootImpl.deliverKeyEventPostIme(ViewRootImpl.java:4192)
11-28 14:13:17.768: E/AndroidRuntime(3636):     at android.view.ViewRootImpl.handleImeFinishedEvent(ViewRootImpl.java:4121)
11-28 14:13:17.768: E/AndroidRuntime(3636):     at android.view.ViewRootImpl$ViewRootHandler.handleMessage(ViewRootImpl.java:3169)
11-28 14:13:17.768: E/AndroidRuntime(3636):     at android.os.Handler.dispatchMessage(Handler.java:99)
11-28 14:13:17.768: E/AndroidRuntime(3636):     at android.os.Looper.loop(Looper.java:137)
11-28 14:13:17.768: E/AndroidRuntime(3636):     at android.app.ActivityThread.main(ActivityThread.java:5328)
11-28 14:13:17.768: E/AndroidRuntime(3636):     at java.lang.reflect.Method.invokeNative(Native Method)
11-28 14:13:17.768: E/AndroidRuntime(3636):     at java.lang.reflect.Method.invoke(Method.java:511)
11-28 14:13:17.768: E/AndroidRuntime(3636):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1102)
11-28 14:13:17.768: E/AndroidRuntime(3636):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:869)
11-28 14:13:17.768: E/AndroidRuntime(3636):     at dalvik.system.NativeStart.main(Native Method)
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1 回答 1

0

这是因为 myWebView 为空。一个快速的解决方法是：

@Override
public boolean onKeyDown(int keyCode, KeyEvent event) {
    if(myWebView != null){
        // Check if the key event was the Back button and if there's history
        if ((keyCode == KeyEvent.KEYCODE_BACK) && myWebView.canGoBack()) {
            myWebView.goBack();
            return true;
        }
        // If it wasn't the Back key or there's no web page history, bubble up to the default
        // system behavior (probably exit the activity)
    }
    else{
        Log.e(MainActivity.class.getSimpleName(), "myWebView is null : won't do anything");
    }
    return super.onKeyDown(keyCode, event);
}
于 2013-11-28T22:22:31.583 回答