Android Studio：应用程序在我的智能手机上运行后崩溃[重复]

Question

每次启动应用程序时，我的手机上都会显示一条错误消息：“应用程序已结束，抱歉”。我现在能做什么？我已经在网上搜索过，我试图解决它，但我没有找到答案。

这是我的 logcat 输出：

FATAL EXCEPTION: main
Process: mycompany.kopfrechnenti1907_1839, PID: 524
java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{mycompany.kopfrechnenti1907_1839/mycompany.kopfrechnenti1907_1839.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:2363)
  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2512)
  at android.app.ActivityThread.access$900(ActivityThread.java:158)
  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1364)
  at android.os.Handler.dispatchMessage(Handler.java:102)
  at android.os.Looper.loop(Looper.java:148)
  at android.app.ActivityThread.main(ActivityThread.java:5521)
  at java.lang.reflect.Method.invoke(Native Method)
  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:804)
  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:666)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.Window$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:155)
  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.AppCompatDelegate.create(AppCompatDelegate.java:203)
  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 mycompany.kopfrechnenti1907_1839.MainActivity.<init>(MainActivity.java:10)
  at java.lang.Class.newInstance(Native Method)
  at android.app.Instrumentation.newActivity(Instrumentation.java:1069)
  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2353)
  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2512) 
  at android.app.ActivityThread.access$900(ActivityThread.java:158) 
  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1364) 
  at android.os.Handler.dispatchMessage(Handler.java:102) 
  at android.os.Looper.loop(Looper.java:148) 
  at android.app.ActivityThread.main(ActivityThread.java:5521) 
  at java.lang.reflect.Method.invoke(Native Method) 

 

import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.widget.RatingBar;
import android.widget.Button;

public class MainActivity extends AppCompatActivity {

    Button btnplus = (Button)findViewById(R.id.btnplus);
    RatingBar ratingbar = (RatingBar)findViewById(R.id.ratingBar);


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

    public void plusonclick()
    {
        if (ratingbar.getRating() == 1)
        {
            setContentView(R.layout.activity_plusrechnenlvl1);
        }
    }

}

我的英语不是最好的，但我希望你能理解我:)

非常感谢！

七月

Answer 1

你必须在onCreate之后写下以下内容

setContentView()
Button btnplus = (Button)findViewById(R.id.btnplus);
    RatingBar ratingbar = (RatingBar)findViewById(R.id.ratingBar);

如下所示。

public class MainActivity extends AppCompatActivity {

    Button btnplus;
    RatingBar ratingbar;


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

     btnplus = (Button)findViewById(R.id.btnplus);
     ratingbar = (RatingBar)findViewById(R.id.ratingBar);
    }

    public void plusonclick()
    {
        if (ratingbar.getRating() == 1)
        {
            setContentView(R.layout.activity_plusrechnenlvl1);
        }
    }

}