Question

此代码停止工作，我不明白为什么。当它尝试执行StartActivity（Intent）[在代码中标记，在OnClick内部时]它返回此并崩溃：

“java.lang.NullPointerException：尝试在空对象引用上调用虚拟方法'void android.view.View.setOnClickListener（android.view.View $ OnClickListener）' 在android.app.ActivityThread.performLaunchActivity（ActivityThread.java:2255）“

这是活动：

public class MainActivity extends ListActivity implements View.OnClickListener {

TextView responseField;

private ScoresDataSource datasource;

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

    responseField = (TextView)findViewById(R.id.textView);
    findViewById(R.id.buttonScore).setOnClickListener(this);
    findViewById(R.id.buttonAdd).setOnClickListener(this);

    datasource = new ScoresDataSource(this);
    datasource.open();


    List<Score> scores = datasource.getAllScores();

    ArrayAdapter<Score>
            adapter = new ArrayAdapter<Score>(
            this,
            android.R.layout.simple_list_item_1,
            scores);
    setListAdapter(adapter);    }

@Override
protected void onResume() {
    datasource.open();
    super.onResume();
}
@Override
protected void onPause() {
    datasource.close();
    super.onPause();
}


@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.main, menu);
    return true;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    // Handle action bar item clicks here. The action bar will
    // automatically handle clicks on the Home/Up button, so long
    // as you specify a parent activity in AndroidManifest.xml.
    int id = item.getItemId();
    if (id == R.id.action_settings) {
        return true;
    }
    return super.onOptionsItemSelected(item);
}

public void onClick(View view)
{
    ArrayAdapter<Score> adapter =
            (ArrayAdapter)getListAdapter();
    Score c = null;

    switch (view.getId()){
        case R.id.buttonScore:

            Intent scorePage = new Intent(this, ScoreboardActivity.class);
            startActivity(scorePage);
       //[CRASHES HERE]     <---------------


            break;
        case R.id.buttonAdd:

            EditText nameField = (EditText)findViewById(R.id.name);
            EditText scoreField = (EditText)findViewById(R.id.score);
            String name = nameField.getText().toString();
            String score = scoreField.getText().toString();

            try {
                int scoreInt = Integer.parseInt(score);
                responseField.setText("SCORE RECEIVED: " + System.getProperty("line.separator") + name + "  " + score);
                c = datasource.createScore(score, name);
                adapter.add(c);
            }
            catch(Exception e){
                responseField.setText("Not Valid Score");
            }

            adapter.notifyDataSetChanged();
 break; }

}
}

这是活动的xml表：

<LinearLayout
android:orientation="vertical"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
xmlns:android="http://schemas.android.com/apk/res/android">



    <Button
        android:layout_width="271dp"
        android:layout_height="wrap_content"
        android:text="AddScore"
        android:id="@+id/buttonAdd" />


<EditText
    android:layout_width="187dp"
    android:text="Player"
    android:layout_height="wrap_content"
    android:id="@+id/name" />

<EditText
    android:layout_width="184dp"
    android:text="Score"
    android:layout_height="wrap_content"
    android:id="@+id/score" />

<TextView
    android:layout_width="226dp"
    android:layout_height="70dp"
    android:text=""
    android:id="@+id/textView" />

<Button
    android:layout_width="222dp"
    android:layout_height="73dp"
    android:text="ScoreBoard"
    android:id="@+id/buttonScore" />

<ListView
    android:layout_width="39dp"
    android:layout_height="24dp"
    android:id="@android:id/list"
    android:layout_gravity="center_horizontal"
    android:visibility="invisible" />

Android Manifest：

<application
    android:allowBackup="true"
    android:icon="@drawable/ic_launcher"
    android:label="@string/app_name"
    android:theme="@style/AppTheme" >
    <activity
        android:name=".MainActivity"
        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=".ScoreboardActivity"
        android:label="Scoreboard" >
    </activity>
</application>

目标活动：

public class ScoreboardActivity extends ListActivity implements View.OnClickListener {



private ScoresDataSource datasource;

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

    findViewById(R.id.buttonAdd).setOnClickListener(this);
    findViewById(R.id.buttonDelete).setOnClickListener(this);

    datasource = new ScoresDataSource(this);
    datasource.open();


    List<Score> scores = datasource.getAllScores();

    ArrayAdapter<Score>
            adapter = new ArrayAdapter<Score>(
            this,
            android.R.layout.simple_list_item_1,
            scores);

//Sorting
    adapter.sort(new Comparator<Score>() {
        @Override
        public int compare(Score sc1, Score sc2) {
            return Integer.toString(sc1.getScore()).compareTo(Integer.toString(sc2.getScore()));         
        }
    });

    setListAdapter(adapter);
}

@Override
protected void onResume() {
    datasource.open();
    super.onResume();
}
@Override
protected void onPause() {
    datasource.close();
    super.onPause();
}


@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.main, menu);
    return true;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    // Handle action bar item clicks here. The action bar will
    // automatically handle clicks on the Home/Up button, so long
    // as you specify a parent activity in AndroidManifest.xml.
    int id = item.getItemId();
    if (id == R.id.action_settings) {
        return true;
    }
    return super.onOptionsItemSelected(item);
}

public void onClick(View view)
{
    ArrayAdapter<Score> adapter =
            (ArrayAdapter)getListAdapter();
    Score c = null;
    Random r = new Random();
    switch (view.getId()){
        case R.id.buttonBack:
        Intent main = new Intent(this, MainActivity.class);
        startActivity(main);
            break;

        case R.id.buttonDelete:
            if(adapter.getCount()>0){
                c = adapter.getItem(0);
                datasource.deleteScore(c);
                adapter.remove(c);
            }
            break;
    }
    adapter.notifyDataSetChanged();
}

}

Answer 1

您的错误告诉您，您正尝试在空对象上设置点击侦听器。

如果您说启动下一个活动时会触发错误，那么在ScoreboardActivity上，尝试为其中一个视图设置点击监听器时会出错：

findViewById(R.id.buttonAdd).setOnClickListener(this);
findViewById(R.id.buttonDelete).setOnClickListener(this);

检查您的activity_scores.xml，看看您是否确实拥有这些ID。

Android StartActivity（意图） - ＆gt; NullPointerException（与onclicklistener有关）

1 个答案: