Question

我正在构建我的第一个应用程序之一，这次我使用的是Android Studio 1.5。

该应用程序是一个非常简单的Web浏览器，我已经为此做了很多修复。一切都工作正常，直到我决定按“输入”键比点击一个愚蠢的“go”按钮更简单，更方便。

哦，男孩不是我错了

该应用程序构建良好，但一旦我将其启动到我的手机，它会立即崩溃。

下面是我的MainActivity.java，下面是生成的异常。

正如您可能看到的那样，我已经广泛检查了“null”对象属性，并且错误仍然存在

- CODE -

package eu.depa.browsing.stack;

import android.net.Uri;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.KeyEvent;
import android.view.View;
import android.view.View.OnKeyListener;
import android.webkit.WebChromeClient;
import android.webkit.WebView;
import android.webkit.WebViewClient;
import android.widget.EditText;

public class MainActivity extends AppCompatActivity implements OnKeyListener{
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);                         //what .xml it's tied to

    //getSupportActionBar().hide();                                 //hide ugly ass fat bar
                                                                    //TODO fix exception generated by this

    WebView webview = (WebView) findViewById(R.id.webView); //actual web window
    EditText toptextbar = (EditText) findViewById(R.id.toptextbar);   //address bar

    webview.getSettings().setJavaScriptEnabled(true);               // Enable javascript
    webview.setWebChromeClient(new WebChromeClient());              // Set WebView client
    webview.setWebViewClient(new WebViewClient() {
        @Override
        public boolean shouldOverrideUrlLoading(WebView view, String url) {
            view.loadUrl(url);
            return true;
        }
    });
}

WebView webview = (WebView) findViewById(R.id.webView);           //actual web window
EditText toptextbar = (EditText) findViewById(R.id.toptextbar);   //address bar


@Override
public boolean onKey(View v, int keyCode, KeyEvent event) {
    if (v != null && event != null && toptextbar.getText() != null && keyCode == KeyEvent.KEYCODE_ENTER){
        String text = String.valueOf(toptextbar.getText());
        if (!(text.contains(" ")) && text.contains(".")) {
            if (text.split("//")[0].equals("http:") || text.split("//")[0].equals("https:"))
                webview.loadUrl(text);
            else
                webview.loadUrl("http://" + text);
        } else {
            String search = "";
            String[] parts = text.split(" ");
            for (String item : parts)
                search = search + item + "%20";
            webview.loadUrl("http://duckduckgo.com/" + search);
        }
        toptextbar.setText(webview.getUrl());
    }
    return false;
}
}

- EXCEPTION--我甚至认为这不是我应该提供的

12-17 14:12:15.291 19699-19699/eu.depa.browsing.stack E/AndroidRuntime: FATAL EXCEPTION: main
    Process: eu.depa.browsing.stack, PID: 19699
    java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{eu.depa.browsing.stack/eu.depa.browsing.stack.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.view.Window.findViewById(int)' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2256)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2424)
        at android.app.ActivityThread.access$900(ActivityThread.java:155)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1323)
        at android.os.Handler.dispatchMessage(Handler.java:102)
        at android.os.Looper.loop(Looper.java:139)
        at android.app.ActivityThread.main(ActivityThread.java:5308)
        at java.lang.reflect.Method.invoke(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:372)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:950)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:745)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.view.Window.findViewById(int)' on a null object reference
        at android.app.Activity.findViewById(Activity.java:2129)
        at eu.depa.browsing.stack.MainActivity.<init>(MainActivity.java:37)
        at java.lang.reflect.Constructor.newInstance(Native Method)
        at java.lang.Class.newInstance(Class.java:1606)
        at android.app.Instrumentation.newActivity(Instrumentation.java:1066)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2246)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2424) 
        at android.app.ActivityThread.access$900(ActivityThread.java:155) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1323) 
        at android.os.Handler.dispatchMessage(Handler.java:102) 
        at android.os.Looper.loop(Looper.java:139) 
        at android.app.ActivityThread.main(ActivityThread.java:5308) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at java.lang.reflect.Method.invoke(Method.java:372) 
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:950) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:745)

Answer 1

删除以下字段初始化代码，在调用findViewById()之前无法调用onCreate()：

WebView webview = (WebView) findViewById(R.id.webView);           //actual web window
EditText toptextbar = (EditText) findViewById(R.id.toptextbar);   //address bar

字段初始化代码是指onCreate()和onKey()方法之间的代码。离开田地＆＃39;声明：

private WebView webview;
private EditText toptextbar;

Answer 2

Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.view.Window.findViewById(int)' on a null object reference

检查您提供的ID，您确定它们存在于XML中吗？

特别检查webview为第37行：

at eu.depa.browsing.stack.MainActivity.<init>(MainActivity.java:37)

Answer 3

您的xml文件应如下所示：

webview xml id 必须 webView ：

<RelativeLayout
    android:layout_width="match_parent"
    android:layout_height="wrap_content">

    <WebView
        android:id="@+id/webView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"></WebView>

</RelativeLayout>

希望这会对你有所帮助。

Android Studio：修复java.lang.NullPointerException

3 个答案: