你好！我是安卓应用程序开发的新手。我试图创建一个简单的计数器，但在构建后出现错误"Unable to instantiate activity"

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+"");
}
});
}

错误:

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

我想这是你在启动时显示的第一个屏幕/活动/类，即它是你的启动器活动。如果是的,

启动器活动不能抽象。因为当一个应用程序从Android设备的主屏幕启动时，Android操作系统会在你声明为启动器活动的应用程序中创建一个活动实例。抽象类不能被实例化，只能被子类化。

请去掉类名前的abstract。

Answer 2

将这一行添加到您的manifest文件

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