java.lang.NullPointerException:尝试调用虚拟方法''；关于空对象引用

我想/尝试在首次启动应用程序时显示弹出窗口，但出现此错误

java.lang.NullPointerException：尝试在空对象引用上调用虚拟方法“void android.widget.PopupWindow.showAtLocation（android.view.view，int，int，int）”

我做覆盖活动，在第一次启动时用弹出窗口启动应用程序

MainActivity.java

public class MainActivity extends AppCompatActivity implements NetworkStateReceiver.NetworkStateReceiverListener {

    //PopUp
    Context popupContext;
    Activity popupActivity;
    PopupWindow popupWindow;
    RelativeLayout popupLayout;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        popupWindow.showAtLocation(popupLayout, Gravity.CENTER, 0, 0);

        (***)

        //Start PopUp
        popupContext = getApplicationContext();
        popupActivity =  MainActivity.this;
        LayoutInflater inflater = (LayoutInflater) popupContext.getSystemService(LAYOUT_INFLATER_SERVICE);
        View popupView = inflater.inflate(R.layout.activity_start, null);
        popupWindow = new PopupWindow(popupView,
                RelativeLayout.LayoutParams.MATCH_PARENT,
                RelativeLayout.LayoutParams.MATCH_PARENT);
        if (Build.VERSION.SDK_INT >= 21){
            popupWindow.setElevation(5.0f);
        }
    }
}

错误消息

05-08 09:48:18.612 7398-7398/id.nax.androidstate E/AndroidRuntime: FATAL EXCEPTION: main
    Process: id.nax.androidstate, PID: 7398
    java.lang.RuntimeException: Unable to start activity ComponentInfo{id.nax.androidstate/id.nax.androidstate.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.PopupWindow.showAtLocation(android.view.View, int, int, int)' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3319)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3415)
        at android.app.ActivityThread.access$1100(ActivityThread.java:229)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1821)
        at android.os.Handler.dispatchMessage(Handler.java:102)
        at android.os.Looper.loop(Looper.java:148)
        at android.app.ActivityThread.main(ActivityThread.java:7406)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1230)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1120)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.PopupWindow.showAtLocation(android.view.View, int, int, int)' on a null object reference
        at id.nax.androidstate.MainActivity.onCreate(MainActivity.java:53)
        at android.app.Activity.performCreate(Activity.java:6904)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1136)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3266)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3415) 
        at android.app.ActivityThread.access$1100(ActivityThread.java:229) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1821) 
        at android.os.Handler.dispatchMessage(Handler.java:102) 
        at android.os.Looper.loop(Looper.java:148) 
        at android.app.ActivityThread.main(ActivityThread.java:7406) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1230) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1120) 

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有没有建议解决我的代码问题或更好的方法来创建弹出窗口？

如果对空对象调用此方法，则需要在创建弹出窗口之后而不是之前调用它。 把电话放在电话线后： popupWindow=新的popupWindow（popupView， RelativeLayout.LayoutParams.MATCH_父级，

---

RelativeLayout.LayoutParams.MATCH_PARENT）

在实际创建

弹出窗口之前，您正在尝试调用
showAtLocation（…）

    popupWindow.showAtLocation(popupLayout, Gravity.CENTER, 0, 0);

    (***)

    //Start PopUp
    popupContext = getApplicationContext();
    popupActivity =  MainActivity.this;
    LayoutInflater inflater = (LayoutInflater) popupContext.getSystemService(LAYOUT_INFLATER_SERVICE);
    View popupView = inflater.inflate(R.layout.activity_start, null);
    popupWindow = new PopupWindow(popupView,
            RelativeLayout.LayoutParams.MATCH_PARENT,
            RelativeLayout.LayoutParams.MATCH_PARENT);
    if (Build.VERSION.SDK_INT >= 21){
        popupWindow.setElevation(5.0f);
    }

您可以这样做，我还对代码进行了重构，使其更干净

    // The View you're making the popup in regards to.
    View parent = findViewById(R.id....);

    View popupView = LayoutInflater.from(this).inflate(R.layout.activity_start, null);
    popupWindow = new PopupWindow(popupView,
            RelativeLayout.LayoutParams.MATCH_PARENT,
            RelativeLayout.LayoutParams.MATCH_PARENT);

    if (Build.VERSION.SDK_INT >= 21){
        popupWindow.setElevation(5.0f);
    }
    popupWindow.showAtLocation(parent, Gravity.CENTER, 0, 0);

现在我得到了新的错误信息
java.lang.NullPointerException:尝试在空对象引用上调用虚拟方法“android.os.IBinder android.view.view.getWindowToken（）”
@ForceGaming什么行？----------->
popupWindow.showAtLocation（PopulLayout，Gravity.CENTER，0，0）
@ForceGaming好的，我相信问题是你通过了
popupLayout
。您需要传递一个使弹出窗口成为其子对象的
视图。例如，您可以传递一个使弹出窗口靠近的
按钮
。您可能忘记创建
PopUpWindow
实例。不要使用
getApplicationContext（）
使用
main活动。这将
创建
PopUpWindow
。