Android Studio错误：不幸的是App停止了工作

Question

我是Android开发的新手，我正在制作一个基本的网络浏览器，并在我的手机上测试我的应用程序，该消息显示不幸的应用程序停止工作并且没有打开应用程序。

我发布了我已创建的代码文件

这是java类

package com.example.dell.browser;

import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.ImageButton;

public class MainActivity extends Activity {

public String inputURL = "https://www.google.co.in/";
Button searchButton = (Button)findViewById(R.id.searchButton);
ImageButton googleButton = (ImageButton)findViewById(R.id.googleButton);
ImageButton youtubeButton = (ImageButton)findViewById(R.id.youtubeButton);
ImageButton facebookButton = (ImageButton)findViewById(R.id.facebookButton);
ImageButton wikiButton = (ImageButton)findViewById(R.id.wikiButton);

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    searchButton.setOnClickListener(new View.OnClickListener(){
        @Override
        public void onClick(View view) {
            inputURL = ((EditText)findViewById(R.id.urlText)).getText().toString();
            // Calling the webView Intent
            Intent webPage = new Intent(MainActivity.this, WebPage.class);
            webPage.putExtra("searchedURL", inputURL);
            webPage.putExtra("hint", "searchButton");
            startActivity(webPage);
        }
    });

    googleButton.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            // Calling the webView Intent with google homepage URL
            Intent googlePage = new Intent(MainActivity.this, WebPage.class);
            googlePage.putExtra("RequestedURL", "https://www.google.co.in");
            googlePage.putExtra("hint", "imageButton");
            startActivity(googlePage);
        }
    });

    youtubeButton.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            // Calling webView Intent with youtube URL
            Intent youtuePage = new Intent(MainActivity.this, WebPage.class);
            youtuePage.putExtra("RequestedURL", "https://www.youtube.com");
            youtuePage.putExtra("hint", "imageButton");
            startActivity(youtuePage);
        }
    });

    facebookButton.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            // Calling webView Intent with facebook URL
            Intent facebookPage = new Intent(MainActivity.this, WebPage.class);
            facebookPage.putExtra("RequestedURL", "https://www.facebook.com");
            facebookPage.putExtra("hint", "imageButton");
            startActivity(facebookPage);
        }
    });

    wikiButton.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            // Calling webView Intent with wikipedia URL
            Intent wikiPage = new Intent(MainActivity.this, WebPage.class);
            wikiPage.putExtra("RequestedURL", "https://en.wikipedia.org");
            wikiPage.putExtra("hint", "imageButton");
            startActivity(wikiPage);
        }
    });

}
}

这是WebView Activity的java类

package com.example.dell.browser;

import android.app.Activity;
import android.os.Bundle;
import android.support.annotation.Nullable;
import android.webkit.WebView;
import android.webkit.WebViewClient;

public class WebPage extends Activity {

@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    Bundle urlrecieved = getIntent().getExtras();
    Bundle searchTypeURL = getIntent().getExtras();
    Bundle hintRecieved = getIntent().getExtras();
    if (urlrecieved==null)
        return;

    String enteredURL = urlrecieved.getString("RequestedURL");
    String searchURL = searchTypeURL.getString("searchedURL");
    String clue = hintRecieved.getString("hint");

    WebView pageView = findViewById(R.id.webPage_area);
    pageView.setWebViewClient(new WebViewClient());
    pageView.getSettings().setJavaScriptEnabled(true);
    pageView.setVerticalScrollBarEnabled(false);
    pageView.setHorizontalScrollBarEnabled(false);

    switch (clue){
        case "searchButton":
            pageView.loadUrl(searchURL);
            break;

        case "imageButton":
            switch (enteredURL){
                case "https://www.google.co.in": pageView.loadUrl("https://www.google.co.in");
                break;
                case "https://www.youtube.com": pageView.loadUrl("https://www.youtube.com");
                break;
                case "https://www.facebook.com": pageView.loadUrl("https://www.facebook.com");
                break;
                case "https://en.wikipedia.org": pageView.loadUrl("https://en.wikipedia.org");
                break;
            }
    }





}
}

这就是LogCat所说的

11-08 04:33:55.084 10874-10874/? E/AndroidRuntime: FATAL EXCEPTION: main
                                               Process: com.example.dell.browser, PID: 10874
                                               java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.dell.browser/com.example.dell.browser.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:2568)
                                                   at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2727)
                                                   at android.app.ActivityThread.-wrap12(ActivityThread.java)
                                                   at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1478)
                                                   at android.os.Handler.dispatchMessage(Handler.java:102)
                                                   at android.os.Looper.loop(Looper.java:154)
                                                   at android.app.ActivityThread.main(ActivityThread.java:6121)
                                                   at java.lang.reflect.Method.invoke(Native Method)
                                                   at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:889)
                                                   at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:779)
                                                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:2328)
                                                   at com.example.dell.browser.MainActivity.<init>(MainActivity.java:14)
                                                   at java.lang.Class.newInstance(Native Method)
                                                   at android.app.Instrumentation.newActivity(Instrumentation.java:1079)
                                                   at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2558)
                                                   at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2727) 
                                                   at android.app.ActivityThread.-wrap12(ActivityThread.java) 
                                                   at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1478) 
                                                   at android.os.Handler.dispatchMessage(Handler.java:102) 
                                                   at android.os.Looper.loop(Looper.java:154) 
                                                   at android.app.ActivityThread.main(ActivityThread.java:6121) 
                                                   at java.lang.reflect.Method.invoke(Native Method) 
                                                   at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:889) 
                                                   at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:779) 
11-08 04:33:59.766 10912-10912/? E/AndroidRuntime: FATAL EXCEPTION: main
                                               Process: com.example.dell.browser, PID: 10912
                                               java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.dell.browser/com.example.dell.browser.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:2568)
                                                   at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2727)
                                                   at android.app.ActivityThread.-wrap12(ActivityThread.java)
                                                   at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1478)
                                                   at android.os.Handler.dispatchMessage(Handler.java:102)
                                                   at android.os.Looper.loop(Looper.java:154)
                                                   at android.app.ActivityThread.main(ActivityThread.java:6121)
                                                   at java.lang.reflect.Method.invoke(Native Method)
                                                   at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:889)
                                                   at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:779)
                                                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:2328)
                                                   at com.example.dell.browser.MainActivity.<init>(MainActivity.java:14)
                                                   at java.lang.Class.newInstance(Native Method)
                                                   at android.app.Instrumentation.newActivity(Instrumentation.java:1079)
                                                   at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2558)
                                                   at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2727) 
                                                   at android.app.ActivityThread.-wrap12(ActivityThread.java) 
                                                   at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1478) 
                                                   at android.os.Handler.dispatchMessage(Handler.java:102) 
                                                   at android.os.Looper.loop(Looper.java:154) 
                                                   at android.app.ActivityThread.main(ActivityThread.java:6121) 
                                                   at java.lang.reflect.Method.invoke(Native Method) 
                                                   at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:889) 
                                                   at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:779)