奥利奥的空指针异常[重复]

Question

这个问题在这里已有答案：

• What is a NullPointerException, and how do I fix it? 12个答案

在Oreo中运行应用程序时，我一直收到Null Pointer异常，但一切都在KitKat中没有错误。该错误表示异常位于OnClickListner上。代码是：

   package com.example.magicid;

    import android.support.v7.app.AppCompatActivity;
    import android.os.Bundle;
    import android.view.View;
    import android.widget.Button;
    import android.widget.EditText;
    import android.widget.TextView;

    public class MainActivity extends AppCompatActivity {

    EditText etID;
    Button btnSubmit;
    TextView tvResults;

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

        etID = findViewById(R.id.edID);
        btnSubmit = findViewById(R.id.btnSubmit);
        tvResults = findViewById(R.id.tvResults);


        btnSubmit.setOnClickListener(new 
    View.OnClickListener() {
            @Override
            public void onClick(View v)
            {

                String idNumber = 
    etID.getText().toString().trim();

                String dob = idNumber.substring(0, 6);

                int gender = 

 Integer.parseInt(Character.toString(idNumber.charAt(6)));

                String sGender;

                if (gender < 5)
                    sGender = getString(R.string.female);
                else
                    sGender = getString(R.string.male);

                 int nationality = 


   Integer.parseInt(Character.toString(idNumber.charAt(10)));

                String sNationality;

                if (nationality == 0)
                    sNationality = 
    getString(R.string.sacit);
                else
                    sNationality = 
    getString(R.string.permanent);

                String text = getString(R.string.dob) + dob 
    + "\n" +
                        getString(R.string.gender) + sGender 
    + "\n" +
                        getString(R.string.nationality) + 
    sNationality;

                tvResults.setText(text);


            }
        });

    }
    }

任何帮助，将不胜感激

Logcat Error
2019-03-12 12:08:16.503 2158-2158/com.example.magicid E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.magicid, PID: 2158
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.magicid/com.example.magicid.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2817)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2892)
        at android.app.ActivityThread.-wrap11(Unknown Source:0)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1593)
        at android.os.Handler.dispatchMessage(Handler.java:105)
        at android.os.Looper.loop(Looper.java:164)
        at android.app.ActivityThread.main(ActivityThread.java:6541)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:240)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:767)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
        at com.example.magicid.MainActivity.onCreate(MainActivity.java:26)
        at android.app.Activity.performCreate(Activity.java:6975)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1213)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2770)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2892) 
        at android.app.ActivityThread.-wrap11(Unknown Source:0) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1593) 
        at android.os.Handler.dispatchMessage(Handler.java:105) 
        at android.os.Looper.loop(Looper.java:164) 
        at android.app.ActivityThread.main(ActivityThread.java:6541) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:240) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:767) 

Answer 1

有错误的布局文件夹的可能性请确保你把你的activity_main（和如此）

res->layout