尝试在空对象引用上调用虚拟方法“void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)”

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【中文标题】尝试在空对象引用上调用虚拟方法“void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)”【英文标题】:Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference 【发布时间】:2015-08-11 01:28:44 【问题描述】:

问题如下。我有一个登录活动(在 android Studio 中),几天前运行良好。我不记得有任何更改,但是当我上次运行这个时,应用程序在我单击登录按钮后立即关闭。最后指出的是关于预执行 AsyncTask 的祝酒词。 而且我不明白为什么会出现 NullPointerException。 我的注册活动代码几乎相同,而且运行良好。

这是日志:

05-28 16:04:52.395    1218-1232/system_process V/WindowManager﹕ addAppToken: AppWindowToken5d89eb token=Token23ccc93a ActivityRecord2fe54865 u0 utanashati.reminder/.HomepageActivity t17 to stack=1 task=17 at 1
05-28 16:04:52.407  19927-19927/utanashati.reminder D/AndroidRuntime﹕ Shutting down VM
05-28 16:04:52.408  19927-19927/utanashati.reminder E/AndroidRuntime﹕ FATAL EXCEPTION: main
    Process: utanashati.reminder, PID: 19927
    java.lang.RuntimeException: Unable to start activity
ComponentInfoutanashati.reminder/utanashati.reminder.HomepageActivity: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2325)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2390)
            at android.app.ActivityThread.access$800(ActivityThread.java:151)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:135)
            at android.app.ActivityThread.main(ActivityThread.java:5257)
            at java.lang.reflect.Method.invoke(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:372)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
            at utanashati.reminder.HomepageActivity.onCreate(HomepageActivity.java:55)
            at android.app.Activity.performCreate(Activity.java:5990)
            at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1106)
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2278)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2390)
            at android.app.ActivityThread.access$800(ActivityThread.java:151)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:135)
            at android.app.ActivityThread.main(ActivityThread.java:5257)
            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:903)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)
05-28 16:04:52.410    1218-1232/system_process W/ActivityManager﹕ Force finishing activity 1 utanashati.reminder/.HomepageActivity
05-28 16:04:52.411    1218-1232/system_process W/ActivityManager﹕ Force finishing activity 2 utanashati.reminder/.LoginActivity

编辑 1

我睁大了眼睛,问题不在于LoginActivity,而在于HomepageActivity。代码如下:

import ...

public class HomepageActivity extends Activity implements AdapterView.OnItemSelectedListener 

    protected EditText mAddTaskText;
    protected Spinner mPrioritySpinner;
    protected Button mAddTaskButton;
    protected int intPriority = 0;
    protected String taskText;
    protected Timestamp taskTimestamp;
    protected Task userTask;
    protected JsonGenerator taskJSON;

    @Override
    protected void onCreate(Bundle savedInstanceState)                         // Starts activity. The state can be restored from savedInstanceState
        super.onCreate(savedInstanceState);                                     // Calls the superclass method (IMPORTANT)
        setContentView(R.layout.activity_homepage);                             // Sets layout from activity_homepage.xml

        mPrioritySpinner = (Spinner) findViewById(R.id.prioritySpinner);        // Creates an ArrayAdapter using the string array and a default spinner layout
        ArrayAdapter<CharSequence> adapter = ArrayAdapter.createFromResource(this,
                R.array.priorityList, android.R.layout.simple_spinner_item);    // Specifies the layout to use when the list of choices appears
        adapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);     // Applies the adapter to the spinner
        mPrioritySpinner.setAdapter(adapter);
        mPrioritySpinner.setOnItemSelectedListener(this);

        mAddTaskText = (EditText) findViewById(R.id.addTaskEditText);           // Finds View by its id in .xml file
        mAddTaskButton.setOnClickListener(new View.OnClickListener() 
            @Override
            public void onClick(View v) 
                Toast.makeText(HomepageActivity.this, "Done!", Toast.LENGTH_LONG).show();

                Calendar taskCalendar = Calendar.getInstance();             // Creates new calendar
                long taskTime = taskCalendar.getTimeInMillis();             // Gets time in milliseconds
                taskTimestamp = new Timestamp(taskTime);                    // Creates new Timestamp
                taskText = mAddTaskText.getText().toString();               // Gets description of the task

                userTask.setDate(taskTimestamp);                            // Sets date
                userTask.setText(taskText);                                 // Sets text

                /* Creating JsonGenerator */
                ObjectMapper mapper = new ObjectMapper();
                try 
                    mapper.writeValue(taskJSON, userTask);
                
                catch (IOException e) 
                    Toast.makeText(HomepageActivity.this, "Could not create JSON", Toast.LENGTH_LONG).show();
                

                /* Getting out email and password */
                String userPassword = ((EmailPassword) HomepageActivity.this.getApplication()).getPassword();
                String userEmail = ((EmailPassword) HomepageActivity.this.getApplication()).getUserEmail();
                Toast.makeText(HomepageActivity.this, userEmail + " " + userPassword, Toast.LENGTH_LONG).show();

                /* HTTP stuff */
                HttpPoster get = new HttpPoster();
                get.execute(userEmail, userPassword, taskJSON.toString());
            
        );
    

    public int getData (String username, String password, String taskJSON) 
        try 
            HttpPost httpPost = new HttpPost("http://something.com/" + username + "/tasks");
            String dataToEncode = username + ":" + password;
            String encodedData = Base64.encodeToString(dataToEncode.getBytes(), Base64.NO_WRAP);
            httpPost.setHeader("Authorization", encodedData);

            try 
                StringEntity taskEntity = new StringEntity(taskJSON, "UTF-8");
                httpPost.setEntity(taskEntity);
            
            catch (UnsupportedEncodingException e) 
                Toast.makeText(HomepageActivity.this, "Unsupported encoding", Toast.LENGTH_LONG).show();
            

            HttpClient client = new DefaultHttpClient();
            HttpResponse response = client.execute(httpPost);
            StatusLine statusLine = response.getStatusLine();
            int statusCode = statusLine.getStatusCode();
            if (statusCode == 200) 
                return 1;
            
            else if (statusCode == 404)  return 2; 
            else if (statusCode == 500)  return 3; 
            else if (statusCode == 409)  return 4; 
            else  return statusCode; 
        
        catch (IOException e) 
            e.printStackTrace();
        
        return 0;
    

    public void onItemSelected(AdapterView<?> parent, View view,
                               int pos, long id) 
        String priority = parent.getItemAtPosition(pos).toString();             // Gets chosen priority
        Toast.makeText(HomepageActivity.this, priority, Toast.LENGTH_LONG).show();
        while (!((priority.equals("Low")) || (priority.equals("Medium")) || (priority.equals("High")))) 
            Toast.makeText(HomepageActivity.this, "Something bad happened. Try to choose again", Toast.LENGTH_LONG).show();
        
        if (priority.equals("Low")) 
            intPriority = 0;
        
        else if (priority.equals("Medium")) 
            intPriority = 1;
        
        else if (priority.equals("High")) 
            intPriority = 2;
        
        userTask.setPriority(intPriority);                                      // Sets chosen priority
    

    public void onNothingSelected(AdapterView<?> parent) 
        userTask.setPriority(intPriority);                                      // Sets default priority ("0")
    

    public class HttpPoster extends AsyncTask<String, Void, Integer> 

        @Override
        protected void onPreExecute() 
            super.onPreExecute();
        

        @Override
        protected Integer doInBackground(String... params) 
            return getData(params[0], params[1], params[3]);
        

        @Override
        protected void onPostExecute(Integer result) 
            super.onPostExecute(result);
            if (result == 1) 
                Toast.makeText(HomepageActivity.this, "Login successful", Toast.LENGTH_LONG).show();
                Intent takeUserHome = new Intent(HomepageActivity.this, HomepageActivity.class);
                startActivity(takeUserHome);
            
            else if (result == 2) 
                Toast.makeText(HomepageActivity.this, "No such user", Toast.LENGTH_LONG).show();
            
            else if (result == 3) 
                Toast.makeText(HomepageActivity.this, "Internal server error: unable to send email", Toast.LENGTH_LONG).show();
            
            else if (result == 4) 
                Toast.makeText(HomepageActivity.this, "Task already exists", Toast.LENGTH_LONG).show();
            
            else 
                Toast.makeText(HomepageActivity.this, result.toString(), Toast.LENGTH_LONG).show();
            
        
    

还有XML文件:

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_
    android:layout_
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    android:paddingBottom="@dimen/activity_vertical_margin"
    tools:context="utanashati.testapp.HomepageActivity">

    <EditText
        android:layout_
        android:layout_
        android:hint="Add a new task..."
        android:id="@+id/addTaskEditText"
        android:nestedScrollingEnabled="false"
        android:minLines="1"
        android:maxLines="1" />

    <Spinner
        android:layout_
        android:layout_
        android:id="@+id/prioritySpinner"
        android:layout_alignRight="@+id/addTaskButton"
        android:layout_alignEnd="@+id/addTaskButton"
        android:layout_below="@+id/addTaskEditText" />

    <Button
        android:layout_
        android:layout_
        android:text="Add task"
        android:id="@+id/addTaskButton"
        android:layout_below="@+id/prioritySpinner"
        android:layout_centerHorizontal="true" />

</RelativeLayout>

【问题讨论】:

您发布的堆栈跟踪指向HomepageActivity 中的问题,而不是您发布代码的LoginActivity 你在 HomepageActivity 活动中的错误,你能把这段代码放在这里吗? @ci_ 哇,你让我大开眼界,谢谢 :) 你能指定解决这个问题的方法吗 【参考方案1】:
 mAddTaskButton.setOnClickListener(new View.OnClickListener()

您有一个点击列表器,但您尚未使用布局绑定初始化 mAddTaskButton

【讨论】:

【参考方案2】:

检查您在 Java 和 XML 中是否有匹配的 ID

【讨论】:

欢迎来到 SO。这可能应该是评论,而不是答案...【参考方案3】:

遇到同样的错误, 检查这个:小错误

检查 findviewbyid(R.id.yourID); 如果您输入的 id 是否正确。

【讨论】:

【参考方案4】:

setOnClickListener 放入onStart 方法为我解决了这个问题。 查看“Android 生命周期概念”以获得进一步说明

【讨论】:

【参考方案5】:

我遇到了同样的问题,似乎我没有启动与点击侦听器一起使用的按钮,换句话说,id 没有启动

【讨论】:

“没有”?领带,你是说?【参考方案6】:

只需在类的顶部将按钮定义为 lateinit var:

 lateinit var buttonOk: Button

当您想在另一个布局中使用按钮时,您应该在该布局中定义它。例如,如果您想在布局中使用名称为“dialogview”的按钮,则应编写:

 buttonOk = dialogView.findViewById<Button>(R.id.buttonOk)

在此之后,您可以对按钮使用 setonclicklistener 并且不会出现任何错误。 你可以看到这个问题的正确答案:Android Kotlin findViewById must not be null

【讨论】:

【参考方案7】:

查看此解决方案。它对我有用...... 检查引发错误的按钮的 id...它可能与您应用程序的其他页面中的任何一个相同。 如果是,则更改它们的id,然后应用程序就可以完美运行了。

我在两个不同的 XML 代码中有两个相同的按钮 ID....我更改了 ID。现在它完美运行!! 希望有效

【讨论】:

【参考方案8】:

您正在调用的按钮似乎不在您在setContentView(R.layout.your_layout) 中使用的布局中 检查一下。

【讨论】:

【参考方案9】:

确保在使用时: 按钮 "varName" =findViewById("btID"); 你输入了正确的“btID”。我不小心从另一个类似的活动中输入了一个按钮的 id,它显示了同样的错误。希望对您有所帮助。

【讨论】:

【参考方案10】:

没错,Mustafa....它的工作原理..它指向两个布局

    setContentView(R.layout.your_layout) v23(your_layout)。

您应该同时采用 Button 活动布局... 成功解决这个问题

【讨论】:

【参考方案11】:

mAddTaskButton 为空,因为您从未使用以下方法对其进行初始化:

mAddTaskButton = (Button) findViewById(R.id.addTaskButton);

在您致电mAddTaskButton.setOnClickListener()之前。

【讨论】:

我也有同样的问题,但是我已经正确初始化了。 投射到按钮是多余的

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