Question

我对Android开发很新。我创建了一个名为＆＃34; series＆＃34;的应用。如果我想运行我的应用程序，它会一直停止并告诉我那里有一个NullPointerException ...... 任何人都可以帮助我吗？

这是我的代码：

public class MainActivity extends AppCompatActivity {

int episode = 1;
int season = 1;
final Button add = (Button) findViewById(R.id.add);

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

    ListView resultListView = (ListView) findViewById(R.id.series_list);

    HashMap<String, String> nameAdresses = new HashMap<>();
    nameAdresses.put("The 100 ", "Season " + season + " Episode" + episode);
    nameAdresses.put("Supernatural ", "Season " + season + " Episode " + episode);

    List<HashMap<String, String>> listItems = new ArrayList<>();
    SimpleAdapter adapter = new SimpleAdapter(this, listItems, R.layout.list_item,
            new String[]{"First Line", "Second Line"},
            new int[]{R.id.text1, R.id.text2});

    Iterator it = nameAdresses.entrySet().iterator();
    while (it.hasNext()) {
        HashMap<String, String> resultsMap = new HashMap<>();
        Map.Entry pair = (Map.Entry) it.next();
        resultsMap.put("First Line", pair.getKey().toString());
        resultsMap.put("Second Line", pair.getValue().toString());
        listItems.add(resultsMap);
    }

    resultListView.setAdapter(adapter);

add.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            episode++;
        }
    });
   }
 }

logcat的：

E/AndroidRuntime: FATAL EXCEPTION: main
              Process: com.example.yurivanderburg.series, PID: 2389
              java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.yurivanderburg.series/com.example.yurivanderburg.series.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.Window$Callback android.view.Window.getCallback()' on a null object reference
                  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2548)
                  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2707)
                  at android.app.ActivityThread.-wrap12(ActivityThread.java)
                  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1460)
                  at android.os.Handler.dispatchMessage(Handler.java:102)
                  at android.os.Looper.loop(Looper.java:154)
                  at android.app.ActivityThread.main(ActivityThread.java:6077)
                  at java.lang.reflect.Method.invoke(Native Method)
                  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:865)
                  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:755)
               Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.Window$Callback android.view.Window.getCallback()' on a null object reference
                  at android.support.v7.app.AppCompatDelegateImplBase.<init>(AppCompatDelegateImplBase.java:120)
                  at android.support.v7.app.AppCompatDelegateImplV9.<init>(AppCompatDelegateImplV9.java:151)
                  at android.support.v7.app.AppCompatDelegateImplV11.<init>(AppCompatDelegateImplV11.java:31)
                  at android.support.v7.app.AppCompatDelegateImplV14.<init>(AppCompatDelegateImplV14.java:55)
                  at android.support.v7.app.AppCompatDelegateImplV23.<init>(AppCompatDelegateImplV23.java:33)
                  at android.support.v7.app.AppCompatDelegateImplN.<init>(AppCompatDelegateImplN.java:33)
                  at android.support.v7.app.AppCompatDelegate.create(AppCompatDelegate.java:201)
                  at android.support.v7.app.AppCompatDelegate.create(AppCompatDelegate.java:185)
                  at android.support.v7.app.AppCompatActivity.getDelegate(AppCompatActivity.java:519)
                  at android.support.v7.app.AppCompatActivity.findViewById(AppCompatActivity.java:190)
                  at com.example.yurivanderburg.series.MainActivity.<init>(MainActivity.java:21)
                  at java.lang.Class.newInstance(Native Method)
                  at android.app.Instrumentation.newActivity(Instrumentation.java:1078)
                  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2538)
                  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2707) 
                  at android.app.ActivityThread.-wrap12(ActivityThread.java) 
                  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1460) 
                  at android.os.Handler.dispatchMessage(Handler.java:102) 
                  at android.os.Looper.loop(Looper.java:154) 
                  at android.app.ActivityThread.main(ActivityThread.java:6077) 
                  at java.lang.reflect.Method.invoke(Native Method) 
                  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:865) 
                  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:755)

Answer 1

请将此行放在oncreateView ()

中

final Button add = (Button) findViewById(R.id.add);

之后

setContentView(R.layout.activity_main);

Answer 2

final Button add = (Button) findViewById(R.id.add);

删除此行并添加

Button add;

并在你的setContentView下面添加这一行

add = (Button) findViewById(R.id.add);

希望这可以帮助你

Answer 3

final Button add = (Button) findViewById(R.id.add);

您在OnCreate之外添加了此内容。这在Android中是不允许的，您无法在setContentView之前初始化视图对象。

Answer 4

在onCreate

中移动按钮

public class MainActivity extends AppCompatActivity {

int episode = 1;
int season = 1;

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

    Button add = (Button) findViewById(R.id.add);

    ListView resultListView = (ListView) findViewById(R.id.series_list);

    HashMap<String, String> nameAdresses = new HashMap<>();
    nameAdresses.put("The 100 ", "Season " + season + " Episode" + episode);
    nameAdresses.put("Supernatural ", "Season " + season + " Episode " + episode);

    List<HashMap<String, String>> listItems = new ArrayList<>();
    SimpleAdapter adapter = new SimpleAdapter(this, listItems, R.layout.list_item,
            new String[]{"First Line", "Second Line"},
            new int[]{R.id.text1, R.id.text2});

    Iterator it = nameAdresses.entrySet().iterator();
    while (it.hasNext()) {
        HashMap<String, String> resultsMap = new HashMap<>();
        Map.Entry pair = (Map.Entry) it.next();
        resultsMap.put("First Line", pair.getKey().toString());
        resultsMap.put("Second Line", pair.getValue().toString());
        listItems.add(resultsMap);
    }

    resultListView.setAdapter(adapter);

    add.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            episode++;
        }
    });
}
}

Android Studio Project上的NullPointerException

4 个答案: