Question

这里是主要活动代码.......................................... .................................................. ..........

  package com.josephvarkey996gmail.test1;
    import android.content.Intent;
    import android.os.Bundle;
    import android.support.v7.app.AppCompatActivity;
    import android.util.Log;
    import android.view.Menu;
    import android.view.MenuItem;
    import android.view.View;
    import android.widget.ImageButton;

    import com.parse.ParseAnalytics;
    import com.parse.ParseUser;

    public abstract class MainActivity extends AppCompatActivity {

        public static final String TAG = MainActivity.class.getSimpleName();
     public ImageButton imgbutton;
        protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_main);

            ParseAnalytics.trackAppOpenedInBackground(getIntent());


            ParseUser currentUser = ParseUser.getCurrentUser();
            if(currentUser==null) {
                navigatetologin();
            }
            if(currentUser!=null)
            {
                Log.i(TAG ,currentUser.getUsername());


            }
            // Enable Local Datastore.
    imgbutton= (ImageButton)findViewById(R.id.frnd);
            imgbutton.setOnClickListener(new View.OnClickListener() {
                @Override
                public void onClick(View v) {
                    Intent intent = new Intent(MainActivity.this, Editfriends.class);
                    startActivity(intent);
                }
            });

        }

        @Override
        public boolean onCreateOptionsMenu(Menu menu) {
            // Inflate the menu; this adds items to the action bar if it is present.
            getMenuInflater().inflate(R.menu.menu_main, menu);
            return true;
        }

        @Override
        public boolean onOptionsItemSelected(MenuItem item) {
            // Handle action bar item clicks here. The action bar will
            // automatically handle clicks on the Home/Up button, so long
            // as you specify a parent activity in AndroidManifest.xml.
            int id = item.getItemId();

            //noinspection SimplifiableIfStatement
            if (id == R.id.action_logout) {
               ParseUser.logOut();
                navigatetologin();
            }
            else if (id==R.id.action_edit_friends){
                Intent intent = new Intent(this, Editfriends.class);
                intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
                intent.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TASK);
                startActivity(intent);
            }

            return super.onOptionsItemSelected(item);
        }

        private void navigatetologin() {
            Intent intent = new Intent(this, LoginActivity.class);
            intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
            intent.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TASK);
            startActivity(intent);
        }
    }

这是Editfriend活动.........................

package com.josephvarkey996gmail.test1;

import android.app.ListActivity;
import android.os.Bundle;
import android.support.v7.app.AlertDialog;
import android.util.Log;
import android.view.Menu;
import android.view.MenuItem;
import android.view.Window;
import android.widget.ArrayAdapter;

import com.parse.FindCallback;
import com.parse.ParseException;
import com.parse.ParseQuery;
import com.parse.ParseUser;

import java.util.List;


public class Editfriends extends ListActivity {
public  static final String Tag=Editfriends.class.getSimpleName();
    protected List<ParseUser> mUser;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        requestWindowFeature(Window.FEATURE_INDETERMINATE_PROGRESS);
        setContentView(R.layout.activity_editfriends);
    }

    @Override
    protected void onResume() {
        super.onResume();
        setProgressBarIndeterminateVisibility(true);
        ParseQuery<ParseUser> query= ParseUser.getQuery();
        query.orderByAscending(ParseConstants.key_Username);
        query.setLimit(1000);
        query.findInBackground(new FindCallback<ParseUser>() {
            @Override
            public void done(List<ParseUser> users, ParseException e) {
                setProgressBarIndeterminateVisibility(true);
                if (e== null){
                      mUser=users;
                    String[] username = new String[mUser.size()];
                    int i=0;
                    for(ParseUser user:mUser){
                        username[i]=user.getUsername();
                        i++;
                    }
                   ArrayAdapter<String> adapter = new ArrayAdapter<>(Editfriends.this,android.R.layout.simple_list_item_checked,username);
                    setListAdapter(adapter);
                }
                else
                {
                    Log.e(Tag,e.getMessage());
                    AlertDialog.Builder builder=new AlertDialog.Builder(Editfriends.this);
                    builder.setMessage(e.getMessage()).setTitle(R.string.error_title).setPositiveButton(android.R.string.ok, null);
                    AlertDialog dialog = builder.create();
                    dialog.show();
                }
            }
        });

    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.menu_editfriends, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();

        //noinspection SimplifiableIfStatement
        if (id == R.id.action_settings) {
            return true;
        }

        return super.onOptionsItemSelected(item);
    }
}

HERE是logcat，我是新手。所以不能理解wat在logcat中说的 ........................

 Process: com.josephvarkey996gmail.test1, PID: 2736
    java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.josephvarkey996gmail.test1/com.josephvarkey996gmail.test1.MainActivity}: java.lang.InstantiationException: class com.josephvarkey996gmail.test1.MainActivity cannot be instantiated
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2236)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2390)
            at android.app.ActivityThread.access$800(ActivityThread.java:151)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:135)
            at android.app.ActivityThread.main(ActivityThread.java:5257)
            at java.lang.reflect.Method.invoke(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:372)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)
     Caused by: java.lang.InstantiationException: class com.josephvarkey996gmail.test1.MainActivity cannot be instantiated
            at java.lang.Class.newInstance(Class.java:1587)
            at android.app.Instrumentation.newActivity(Instrumentation.java:1066)
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2226)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2390)
            at android.app.ActivityThread.access$800(ActivityThread.java:151)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:135)
            at android.app.ActivityThread.main(ActivityThread.java:5257)
            at java.lang.reflect.Method.invoke(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:372)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)
07-21 18:23:09.419    2736-2748/com.josephvarkey996gmail.test1 I/art﹕ Background sticky concurrent mark sweep GC freed 3678(165KB) AllocSpace objects, 0(0B) LOS objects, 0% free, 1170KB/1170KB, paused 7.592ms total 95.745ms

Answer 1

变化

public abstract class MainActivity extends AppCompatActivity

到

public class MainActivity extends AppCompatActivity

Answer 2

MainActivity是abstract class。所以你需要扩展这个类或删除抽象声明。

Java编程语言中的抽象涉及创建一个或多个接口或抽象类来隐藏实现的信息。

扩展抽象类：

public class MainActivity2 extends MainActivity {

    protected void onCreate(Bundle savedInstanceState) {
       //This calls the superclass method: MainActivity onCreate
        super.onCreate(savedInstanceState);
    }
}

删除抽象类：

public class MainActivity extends AppCompatActivity {

    public void onCreate(Bundle savedInstanceState) {
        setContentView(R.layout.activity_main);
        ...
    }
}

什么是错误，应用程序崩溃

2 个答案: