Android 应用程序无法启动活动组件信息

【问题标题】：Android app unable to start activity componentinfoAndroid 应用程序无法启动活动组件信息
【发布时间】：2011-10-17 13:03:57
【问题描述】：

我是一名新的 Android 程序员，最近，我的很多项目都出现了这个错误：

07-31 23:45:19.592: ERROR/AndroidRuntime(716): java.lang.RuntimeException: Unable to start     

activity ComponentInfo{com.amrit.musifind/com.amrit.musifind.Main}: 

java.lang.NullPointerException

谁能帮我解决这个问题？

这是整个 logcat 堆栈跟踪：

07-31 23:45:18.512: INFO/ActivityManager(63): Starting activity: Intent {     
act=android.intent.action.MAIN cat=[android.intent.category.LAUNCHER] flg=0x10200000   
cmp=com.amrit.musifind/.Main }
07-31 23:45:18.682: INFO/ActivityManager(63): Start proc com.amrit.musifind for activity   
com.amrit.musifind/.Main: pid=716 uid=10025 gids={3003}
07-31 23:45:18.962: INFO/jdwp(716): received file descriptor 20 from ADB
07-31 23:45:19.102: DEBUG/ddm-heap(716): Got feature list request
07-31 23:45:19.572: DEBUG/AndroidRuntime(716): Shutting down VM
07-31 23:45:19.572: WARN/dalvikvm(716): threadid=3: thread exiting with uncaught   
exception (group=0x4001aa28)
07-31 23:45:19.572: ERROR/AndroidRuntime(716): Uncaught handler: thread main exiting due 
to uncaught exception
07-31 23:45:19.592: ERROR/AndroidRuntime(716): java.lang.RuntimeException: Unable to    
start activity ComponentInfo{com.amrit.musifind/com.amrit.musifind.Main}:    
java.lang.NullPointerException
07-31 23:45:19.592: ERROR/AndroidRuntime(716):     at    
android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2401)
07-31 23:45:19.592: ERROR/AndroidRuntime(716):     at    
android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2417)
07-31 23:45:19.592: ERROR/AndroidRuntime(716):     at    
android.app.ActivityThread.access$2100(ActivityThread.java:116)
07-31 23:45:19.592: ERROR/AndroidRuntime(716):     at   
android.app.ActivityThread$H.handleMessage(ActivityThread.java:1794)
07-31 23:45:19.592: ERROR/AndroidRuntime(716):     at   
android.os.Handler.dispatchMessage(Handler.java:99)
07-31 23:45:19.592: ERROR/AndroidRuntime(716):     at       
android.os.Looper.loop(Looper.java:123)
07-31 23:45:19.592: ERROR/AndroidRuntime(716):     at   
android.app.ActivityThread.main(ActivityThread.java:4203)
07-31 23:45:19.592: ERROR/AndroidRuntime(716):     at    
java.lang.reflect.Method.invokeNative(Native Method)
07-31 23:45:19.592: ERROR/AndroidRuntime(716):     at   
java.lang.reflect.Method.invoke(Method.java:521)
07-31 23:45:19.592: ERROR/AndroidRuntime(716):     at   
com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:791)
07-31 23:45:19.592: ERROR/AndroidRuntime(716):     at   
com.android.internal.os.ZygoteInit.main(ZygoteInit.java:549)
07-31 23:45:19.592: ERROR/AndroidRuntime(716):     at   
dalvik.system.NativeStart.main(Native Method)
07-31 23:45:19.592: ERROR/AndroidRuntime(716): Caused by: java.lang.NullPointerException
07-31 23:45:19.592: ERROR/AndroidRuntime(716):     at   
com.amrit.musifind.Main.onCreate(Main.java:44)
07-31 23:45:19.592: ERROR/AndroidRuntime(716):     at 
android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1123)
07-31 23:45:19.592: ERROR/AndroidRuntime(716):     at     
android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2364)
07-31 23:45:19.592: ERROR/AndroidRuntime(716):     ... 11 more
07-31 23:45:19.612: INFO/Process(63): Sending signal. PID: 716 SIG: 3
07-31 23:45:19.612: INFO/dalvikvm(716): threadid=7: reacting to signal 3
07-31 23:45:19.642: INFO/dalvikvm(716): Wrote stack trace to '/data/anr/traces.txt'
07-31 23:45:21.422: INFO/Process(716): Sending signal. PID: 716 SIG: 9
07-31 23:45:21.442: INFO/ActivityManager(63): Process com.amrit.musifind (pid 716) has   
died.
07-31 23:45:21.583: WARN/UsageStats(63): Unexpected resume of com.android.launcher while    
already resumed in com.amrit.musifind
07-31 23:45:21.712: WARN/InputManagerService(63): Window already focused, ignoring focus    
gain of: com.android.internal.view.IInputMethodClient$Stub$Proxy@439559d0
07-31 23:45:23.482: DEBUG/AndroidRuntime(725): >>>>>>>>>>>>>> AndroidRuntime START    
<<<<<<<<<<<<<<
07-31 23:45:23.482: DEBUG/AndroidRuntime(725): CheckJNI is ON
07-31 23:45:23.812: DEBUG/AndroidRuntime(725): --- registering native functions ---
07-31 23:45:23.832: INFO/jdwp(725): received file descriptor 20 from ADB
07-31 23:45:24.193: DEBUG/ddm-heap(725): Got feature list request
07-31 23:45:25.052: DEBUG/AndroidRuntime(725): Shutting down VM
07-31 23:45:25.052: DEBUG/dalvikvm(725): DestroyJavaVM waiting for non-daemon threads to   
exit
07-31 23:45:25.062: DEBUG/dalvikvm(725): DestroyJavaVM shutting VM down
07-31 23:45:25.062: DEBUG/dalvikvm(725): HeapWorker thread shutting down
07-31 23:45:25.062: DEBUG/dalvikvm(725): HeapWorker thread has shut down
07-31 23:45:25.073: DEBUG/jdwp(725): JDWP shutting down net...
07-31 23:45:25.073: DEBUG/jdwp(725): +++ peer disconnected
07-31 23:45:25.073: INFO/dalvikvm(725): Debugger has detached; object registry had 1   
entries
07-31 23:45:25.093: DEBUG/dalvikvm(725): VM cleaning up
07-31 23:45:25.113: DEBUG/dalvikvm(725): LinearAlloc 0x0 used 665652 of 4194304 (15%)
07-31 23:47:22.712: DEBUG/dalvikvm(104): GC freed 9173 objects / 524184 bytes in 149ms

这是第一个活动：start.java

package com.amrit.musifind;

import java.io.BufferedReader;
import java.io.InputStreamReader;

import org.apache.http.HttpResponse;
import org.apache.http.client.HttpClient;
import org.apache.http.client.methods.HttpGet;
import org.apache.http.impl.client.DefaultHttpClient;

import com.amrit.musifind.R;

import android.app.ListActivity;
import android.content.Intent;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.AdapterView;
import android.widget.AdapterView.OnItemClickListener;
import android.widget.ListAdapter;
import android.widget.ListView;
import android.widget.SimpleAdapter;
import android.widget.Toast;
import android.app.Activity;
import android.os.Bundle;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;

public abstract class start extends Activity implements OnClickListener {

Button buttonGo, buttonReset;
EditText EditTextinput;
String input = "";
String ur = "http://www.tastekid.com/ask/ws?q=";
String l = "&f=musifin2125&k=mjjlnzkyzwuz&format=JSON";
String url = "" ;

/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);

    buttonGo = (Button) findViewById(R.id.buttonGo);
    buttonReset = (Button) findViewById(R.id.buttonReset);

    EditTextinput = (EditText) findViewById(R.id.EditTextinput);

    //Button listener
    buttonReset.setOnClickListener(this);
    buttonGo.setOnClickListener(this);
}

public void onClick (View src){
    switch(src.getId()){
        case R.id.buttonGo:
            input = EditTextinput.getText().toString();
            url = ur + input + l  ;

            Intent Main = new Intent(this, ToClass.class);
            Main.putExtra("userurl", url);
            startActivity(Main);
            break;

        case R.id.buttonReset:
            EditTextinput.setText("");
            break;
        }
    }
}

第二个活动：Main.java

package com.amrit.musifind;

import java.util.ArrayList;
import java.util.HashMap;

import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;
import org.w3c.dom.Document;
import org.w3c.dom.Element;
import org.w3c.dom.NodeList;

import com.amrit.musifind.JSONfunctions;
import com.amrit.musifind.R;

import android.app.ListActivity;
import android.content.Intent;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.AdapterView;
import android.widget.AdapterView.OnItemClickListener;
import android.widget.ListAdapter;
import android.widget.ListView;
import android.widget.SimpleAdapter;
import android.widget.Toast;
import android.app.Activity;
import android.os.Bundle;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;

public class Main extends ListActivity {
/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.listplaceholder);

    Intent intent = getIntent();
    String url = intent.getExtras().getString("userurl");

    ArrayList<HashMap<String, String>> mylist = new ArrayList<HashMap<String, String>>    
    ();

    JSONObject json = JSONfunctions.getJSONfromURL(url);

    try{
        JSONObject earthquakes = json.getJSONObject("Similar");
        JSONArray info = earthquakes.getJSONArray("Results");

        for (int i = 0; i < info.length(); i++) {
            HashMap<String, String> map = new HashMap<String, String>();
            JSONObject e = info.getJSONObject(i);

            map.put("id", String.valueOf(i));
            map.put("name", "Name:" + e.getString("Name"));
            map.put("type", "Type: " + e.getString("Type"));
            mylist.add(map);
        }

        JSONArray  results = json.getJSONArray("Results");

        for(int i=0;i<results.length();i++){                        
            HashMap<String, String> map = new HashMap<String, String>();    
            JSONObject e = results.getJSONObject(i);

            map.put("id",  String.valueOf(i));
            map.put("name", "Name:" + e.getString("name"));
            map.put("type", "Type: " +  e.getString("type"));
            mylist.add(map);            
        }       
    } catch(JSONException e) {
         Log.e("log_tag", "Error parsing data "+e.toString());
    }

    ListAdapter adapter = new SimpleAdapter(this, mylist , R.layout.main, 
         new String[] { "name", "type" }, 
         new int[] { R.id.item_title, R.id.item_subtitle });

    setListAdapter(adapter);

    final ListView lv = getListView();
    lv.setTextFilterEnabled(true);  
    lv.setOnItemClickListener(new OnItemClickListener() {
        public void onItemClick(AdapterView<?> parent, View view, int position, long id) {              
            @SuppressWarnings("unchecked")
            HashMap<String, String> o = (HashMap<String, String>) lv.getItemAtPosition(position);                   
            Toast.makeText(Main.this, "ID '" + o.get("id") + "' was clicked.", 
                Toast.LENGTH_SHORT).show(); 
            }
        });
    }
}

最后，我的清单文件：AndroidManifest.xml

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.amrit.musifind"
    android:versionCode="1"
    android:versionName="1.0">
    <uses-sdk android:minSdkVersion="4" />

    <application android:icon="@drawable/icon" android:label="@string/app_name">
        <activity android:name=".start" android:label="@string/app_name">
            <intent-filter>
                <action android:name="android.intent.action.start" />
                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
        <activity android:name= ".Main">
            <intent-filter>
                <action android:name="android.intent.action.Main" />
            </intent-filter>
        </activity>
    </application>

    <uses-permission android:name="android.permission.INTERNET" />

</manifest>

非常感谢任何帮助，如果不解决此问题，我将无法测试我的应用程序！

【问题讨论】：

  • 能否请您发布完整的堆栈跟踪？
  • 我该怎么做？抱歉，我对 android 很陌生 :)
  • 您可以在 logcat 输出中找到调用堆栈。您已经在帖子的开头包含了其中的一部分（带有空指针异常的那个）。
  • 我在设置内容视图之前尝试分配点击侦听器时得到了这个，所以如果 view.setOnClickListener 出现在 setContentView 调用之前设置视图，如果您的代码出现此错误也这样做然后只需更改顺序以首先设置视图然后分配侦听器。

标签： android android-activity runtime-error


【解决方案1】：

这个问题已经回答了，但我想补充更多关于原因的信息。

Android 应用无法启动活动组件信息

此错误通常带有适当的日志。您可以阅读日志并轻松解决此问题。

这是一个示例日志。在其中您可以清楚地看到ClassCastException。所以出现这个问题是因为TextView 不能转换为EditText。

原因：java.lang.ClassCastException: android.widget.TextView 无法转换为 android.widget.EditText

11-04 01:24:10.403: D/AndroidRuntime(1050): Shutting down VM
11-04 01:24:10.403: W/dalvikvm(1050): threadid=1: thread exiting with uncaught exception (group=0x41465700)
11-04 01:24:10.543: E/AndroidRuntime(1050): FATAL EXCEPTION: main
11-04 01:24:10.543: E/AndroidRuntime(1050): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.troysantry.tipcalculator/com.troysantry.tipcalculator.TipCalc}: java.lang.ClassCastException: android.widget.TextView cannot be cast to android.widget.EditText
11-04 01:24:10.543: E/AndroidRuntime(1050):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2211)
11-04 01:24:10.543: E/AndroidRuntime(1050):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2261)
11-04 01:24:10.543: E/AndroidRuntime(1050):     at android.app.ActivityThread.access$600(ActivityThread.java:141)
11-04 01:24:10.543: E/AndroidRuntime(1050):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1256)
11-04 01:24:10.543: E/AndroidRuntime(1050):     at android.os.Handler.dispatchMessage(Handler.java:99)
11-04 01:24:10.543: E/AndroidRuntime(1050):     at android.os.Looper.loop(Looper.java:137)
11-04 01:24:10.543: E/AndroidRuntime(1050):     at android.app.ActivityThread.main(ActivityThread.java:5103)
11-04 01:24:10.543: E/AndroidRuntime(1050):     at java.lang.reflect.Method.invokeNative(Native Method)
11-04 01:24:10.543: E/AndroidRuntime(1050):     at java.lang.reflect.Method.invoke(Method.java:525)
11-04 01:24:10.543: E/AndroidRuntime(1050):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:737)
11-04 01:24:10.543: E/AndroidRuntime(1050):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
11-04 01:24:10.543: E/AndroidRuntime(1050):     at dalvik.system.NativeStart.main(Native Method)
11-04 01:24:10.543: E/AndroidRuntime(1050): Caused by: java.lang.ClassCastException: android.widget.TextView cannot be cast to android.widget.EditText
11-04 01:24:10.543: E/AndroidRuntime(1050):     at com.troysantry.tipcalculator.TipCalc.onCreate(TipCalc.java:45)
11-04 01:24:10.543: E/AndroidRuntime(1050):     at android.app.Activity.performCreate(Activity.java:5133)
11-04 01:24:10.543: E/AndroidRuntime(1050):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
11-04 01:24:10.543: E/AndroidRuntime(1050):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2175)
11-04 01:24:10.543: E/AndroidRuntime(1050):     ... 11 more
11-04 01:29:11.177: I/Process(1050): Sending signal. PID: 1050 SIG: 9
11-04 01:31:32.080: D/AndroidRuntime(1109): Shutting down VM
11-04 01:31:32.080: W/dalvikvm(1109): threadid=1: thread exiting with uncaught exception (group=0x41465700)
11-04 01:31:32.194: E/AndroidRuntime(1109): FATAL EXCEPTION: main
11-04 01:31:32.194: E/AndroidRuntime(1109): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.troysantry.tipcalculator/com.troysantry.tipcalculator.TipCalc}: java.lang.ClassCastException: android.widget.TextView cannot be cast to android.widget.EditText
11-04 01:31:32.194: E/AndroidRuntime(1109):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2211)
11-04 01:31:32.194: E/AndroidRuntime(1109):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2261)
11-04 01:31:32.194: E/AndroidRuntime(1109):     at android.app.ActivityThread.access$600(ActivityThread.java:141)
11-04 01:31:32.194: E/AndroidRuntime(1109):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1256)
11-04 01:31:32.194: E/AndroidRuntime(1109):     at android.os.Handler.dispatchMessage(Handler.java:99)
11-04 01:31:32.194: E/AndroidRuntime(1109):     at android.os.Looper.loop(Looper.java:137)
11-04 01:31:32.194: E/AndroidRuntime(1109):     at android.app.ActivityThread.main(ActivityThread.java:5103)
11-04 01:31:32.194: E/AndroidRuntime(1109):     at java.lang.reflect.Method.invokeNative(Native Method)
11-04 01:31:32.194: E/AndroidRuntime(1109):     at java.lang.reflect.Method.invoke(Method.java:525)
11-04 01:31:32.194: E/AndroidRuntime(1109):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:737)
11-04 01:31:32.194: E/AndroidRuntime(1109):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
11-04 01:31:32.194: E/AndroidRuntime(1109):     at dalvik.system.NativeStart.main(Native Method)
11-04 01:31:32.194: E/AndroidRuntime(1109): Caused by: java.lang.ClassCastException: android.widget.TextView cannot be cast to android.widget.EditText
11-04 01:31:32.194: E/AndroidRuntime(1109):     at com.troysantry.tipcalculator.TipCalc.onCreate(TipCalc.java:44)
11-04 01:31:32.194: E/AndroidRuntime(1109):     at android.app.Activity.performCreate(Activity.java:5133)
11-04 01:31:32.194: E/AndroidRuntime(1109):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
11-04 01:31:32.194: E/AndroidRuntime(1109):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2175)
11-04 01:31:32.194: E/AndroidRuntime(1109):     ... 11 more
11-04 01:36:33.195: I/Process(1109): Sending signal. PID: 1109 SIG: 9
11-04 02:11:09.684: D/AndroidRuntime(1167): Shutting down VM
11-04 02:11:09.684: W/dalvikvm(1167): threadid=1: thread exiting with uncaught exception (group=0x41465700)
11-04 02:11:09.855: E/AndroidRuntime(1167): FATAL EXCEPTION: main
11-04 02:11:09.855: E/AndroidRuntime(1167): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.troysantry.tipcalculator/com.troysantry.tipcalculator.TipCalc}: java.lang.ClassCastException: android.widget.TextView cannot be cast to android.widget.EditText
11-04 02:11:09.855: E/AndroidRuntime(1167):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2211)
11-04 02:11:09.855: E/AndroidRuntime(1167):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2261)
11-04 02:11:09.855: E/AndroidRuntime(1167):     at android.app.ActivityThread.access$600(ActivityThread.java:141)
11-04 02:11:09.855: E/AndroidRuntime(1167):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1256)
11-04 02:11:09.855: E/AndroidRuntime(1167):     at android.os.Handler.dispatchMessage(Handler.java:99)
11-04 02:11:09.855: E/AndroidRuntime(1167):     at android.os.Looper.loop(Looper.java:137)
11-04 02:11:09.855: E/AndroidRuntime(1167):     at android.app.ActivityThread.main(ActivityThread.java:5103)
11-04 02:11:09.855: E/AndroidRuntime(1167):     at java.lang.reflect.Method.invokeNative(Native Method)
11-04 02:11:09.855: E/AndroidRuntime(1167):     at java.lang.reflect.Method.invoke(Method.java:525)
11-04 02:11:09.855: E/AndroidRuntime(1167):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:737)
11-04 02:11:09.855: E/AndroidRuntime(1167):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
11-04 02:11:09.855: E/AndroidRuntime(1167):     at dalvik.system.NativeStart.main(Native Method)
11-04 02:11:09.855: E/AndroidRuntime(1167): Caused by: java.lang.ClassCastException: android.widget.TextView cannot be cast to android.widget.EditText
11-04 02:11:09.855: E/AndroidRuntime(1167):     at com.troysantry.tipcalculator.TipCalc.onCreate(TipCalc.java:44)
11-04 02:11:09.855: E/AndroidRuntime(1167):     at android.app.Activity.performCreate(Activity.java:5133)
11-04 02:11:09.855: E/AndroidRuntime(1167):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
11-04 02:11:09.855: E/AndroidRuntime(1167):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2175)
11-04 02:11:09.855: E/AndroidRuntime(1167):     ... 11 more

一些常见错误。

1.findViewById() 不存在的视图

就像在布局 XML 中不存在按钮 ID 时使用 findViewById(R.id.button) 一样。

2。演员阵容错误。

如果你错误地转换了某个类，那么你会得到这个错误。就像您将RelativeLayout 转换为LinearLayout 或EditText 转换为TextView。

3。活动未在manifest.xml 中注册

如果你没有在manifest.xml注册Activity，就会出现这个错误。

4。 findViewById() 在顶层声明

以下代码不正确。这将产生错误。因为你应该在调用setContentView() 之后执行findViewById()。因为 View 可以在创建之后就在那里。

public class MainActivity extends Activity {

  ImageView mainImage = (ImageView) findViewById(R.id.imageViewMain); //incorrect way

  @Override
  protected void onCreate(Bundle savedInstanceState){
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    mainImage = (ImageView) findViewById(R.id.imageViewMain); //correct way
    //...
  }
}

5。开始abstractActivity 类。

当你尝试启动一个抽象的 Activity 时，你会得到这个错误。所以只需 remove abstract keyword 在活动类名称之前。

6。使用 kotlin 但未配置 kotlin。

如果您的 Activity 是用 Kotlin 编写的，并且您尚未在应用中设置 kotlin。那么你会得到错误。您可以按照Android Link 或Kotlin Link 中所写的简单步骤进行操作。你也可以查看this answer。

更多信息

Read about Downcast and Upcast

Read about findViewById() method of Activity class

Super keyword in Java

How to register activity in manifest

【讨论】：

  • 另一个原因可能是，如果在没有附加 super.attachBaseContext(..) 的情况下调用 resources.updateConfiguration()。
  • 我的问题是由错误的 Intent 类型引起的。以发送 String 并尝试获取 Int 为例。
  • 这是一个很好的答案。写得很好。我想补充一下，您能否也添加导航的原因。我将此导航图添加到活动中的片段标记中。但我得到的原因是android.view.InflateException: Binary XML file line #17 .... Error inflating class fragment
【解决方案2】：

您的空指针异常似乎在这一行：

String url = intent.getExtras().getString("userurl");

因为 intent.getExtras() 在 Intent 没有任何额外内容时返回 null。

你必须意识到这段代码：

Intent Main = new Intent(this, ToClass.class);
Main.putExtra("userurl", url);
startActivity(Main);

不会启动您在 Main.java 中编写的 Activity，它会尝试启动名为 ToClass 的 Activity，如果该 Activity 不存在，您的应用会崩溃。

此外，没有 "android.intent.action.start" 这样的东西，所以清单应该看起来更像：

<activity android:name=".start" android:label="@string/app_name">
    <intent-filter>
        <action android:name="android.intent.action.MAIN" />
        <category android:name="android.intent.category.LAUNCHER" />
    </intent-filter>
</activity>
<activity android:name= ".Main">
</activity>

我希望这可以解决您遇到的一些问题，但我强烈建议您查看一些用于 android 开发的“入门”教程并从那里开始构建。

【讨论】：

  • 我删除了您指出的代码，并编辑了我的清单文件，但我仍然强制关闭。这是来自 logcat 的新错误：08-01 12:28:01.522: ERROR/AndroidRuntime(807): java.lang.RuntimeException: Unable to instance activity ComponentInfo{com.amrit.musifind/com.amrit.musifind.start} : java.lang.InstantiationException: com.amrit.musifind.start
  • 再一次，我需要的不仅仅是调用堆栈的第一行
  • 我修好了，我有“公共抽象类”而不仅仅是“公共类”。谢谢！
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