你好。 我是 android 应用程序开发的新手。 我尝试创建一个简单的计数器，但在构建后出现错误“无法实例化活动”

Question

public abstract class MainActivity extends AppCompatActivity {

    Button decrement;
    Button increment;
    TextView counter_view;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
       // Log.i("tag", "onCreate: Created Successfully");

        increment=findViewById(R.id.inc_btn);
        decrement=findViewById(R.id.dec_btn);
        counter_view=findViewById(R.id.counter);

        increment.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                String cnt_text=counter_view.getText().toString();
                int cnt_no= Integer.parseInt(cnt_text);
                cnt_no=cnt_no+1;
                counter_view.setText(cnt_no+"");
            }
        });

        decrement.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                String cnt_text=counter_view.getText().toString();
                int cnt_no=Integer.parseInt(cnt_text);
                cnt_no=cnt_no-1;
                counter_view.setText(cnt_no+"");
            }
        });
    }

Error:

2021-02-10 00:29:06.870 16714-16714/? E/Zygote: v2
2021-02-10 00:29:06.871 16714-16714/? E/Zygote: accessInfo : 0
2021-02-10 00:29:07.017 16714-16714/? E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.counter_app, PID: 16714
    java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.counter_app/com.example.counter_app.MainActivity}: java.lang.InstantiationException: java.lang.Class<com.example.counter_app.MainActivity> cannot be instantiated
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2849)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3045)
        at android.app.ActivityThread.-wrap14(ActivityThread.java)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1642)
        at android.os.Handler.dispatchMessage(Handler.java:102)
        at android.os.Looper.loop(Looper.java:154)
        at android.app.ActivityThread.main(ActivityThread.java:6776)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1518)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1408)
     Caused by: java.lang.InstantiationException: java.lang.Class<com.example.counter_app.MainActivity> cannot be instantiated
        at java.lang.Class.newInstance(Native Method)
        at android.app.Instrumentation.newActivity(Instrumentation.java:1086)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2839)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3045) 
        at android.app.ActivityThread.-wrap14(ActivityThread.java) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1642) 
        at android.os.Handler.dispatchMessage(Handler.java:102) 
        at android.os.Looper.loop(Looper.java:154) 
        at android.app.ActivityThread.main(ActivityThread.java:6776) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1518) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1408) 

Answer 1

add to your manifest file this line

<activity android:name="your.package.name.MainActivity"/>

Answer 2

I suppose this is your first screen/activity/class that is displayed on launch ie it is your launcher activity. If yes,

the launcher activity cannot be abstract. Because when an app is launched from the home screen on an Android device, the Android OS creates an instance of the activity in the application you have declared to be the launcher activity. And abstract classes can not be instantiated, they can only be sub-classed.

Please remove the word abstract before your class name.