Android 如何修复java语言空指针？

我在logcat中得到了javalang nullpointer错误，但是它没有显示行的位置，我也无法找出nullpointerexception的位置

以下是我的MainActivity.java：

package com.orar.cngcnasaud;

import android.app.Activity;
import android.content.Intent;
import android.content.res.Configuration;
import android.graphics.Color;
import android.media.MediaPlayer;
import android.os.Bundle;
import android.support.v4.app.ActionBarDrawerToggle;
import android.support.v4.view.GravityCompat;
import android.support.v4.widget.DrawerLayout;
import android.util.Log;
import android.view.MenuItem;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.AdapterView;
import android.widget.Button;
import android.widget.ListView;

public class MainActivity extends Activity implements OnClickListener {

    private ListView mDrawerList;
    private DrawerLayout mDrawer;
    private CustomActionBarDrawerToggle mDrawerToggle;
    private String[] menuItems;


    private static final String TAG = "AudioDemo";
    private static final String isPlaying = "Media is Playing"; 
    private static final String notPlaying = "Media has stopped Playing"; 

    MediaPlayer player;
    Button playerButton;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main_drawer);


        player.setLooping(false); // Set looping

        // Get the button from the view
        playerButton = (Button) this.findViewById(R.id.buttonmp);
        playerButton.setText(R.string.play_label);
        playerButton.setTextColor(Color.WHITE);
        playerButton.setOnClickListener((OnClickListener) this);



        player = MediaPlayer.create(this, R.raw.gc);



        // enable ActionBar app icon to behave as action to toggle nav drawer
        getActionBar().setDisplayHomeAsUpEnabled(true);
        getActionBar().setHomeButtonEnabled(true);

        mDrawer = (DrawerLayout) findViewById(R.id.drawer_layout);

        // set a custom shadow that overlays the main content when the drawer
        // opens
        mDrawer.setDrawerShadow(R.drawable.drawer_shadow, GravityCompat.START);

        _initMenu();
        mDrawerToggle = new CustomActionBarDrawerToggle(this, mDrawer);
        mDrawer.setDrawerListener(mDrawerToggle);
    }

        public void onClick(View v) {
            Log.d(TAG, "onClick: " + v);
            if (v.getId() == R.id.buttonmp) {
                playPause();
            }

    }




    private void demoPause() {
        // TODO Auto-generated method stub
        player.pause();
        playerButton.setText(R.string.play_label);

        Log.d(TAG, notPlaying);
}

    private void playPause() {
        // TODO Auto-generated method stub
        if(player.isPlaying()) {
          demoPause();
        } else {
          demoPlay();
        }   
    }

    private void demoPlay() {
        // TODO Auto-generated method stub
        player.start();
        playerButton.setText(R.string.stop_label);

        Log.d(TAG, isPlaying);
}




    private void _initMenu() {
        NsMenuAdapter mAdapter = new NsMenuAdapter(this);

        // Add Header
        mAdapter.addHeader(R.string.ns_menu_main_header);

        // Add first block

        menuItems = getResources().getStringArray(
                R.array.ns_menu_items);
        String[] menuItemsIcon = getResources().getStringArray(
                R.array.ns_menu_items_icon);

        int res = 0;
        for (String item : menuItems) {

            int id_title = getResources().getIdentifier(item, "string",
                    this.getPackageName());
            int id_icon = getResources().getIdentifier(menuItemsIcon[res],
                    "drawable", this.getPackageName());

            NsMenuItemModel mItem = new NsMenuItemModel(id_title, id_icon);
            if (res==1) mItem.counter=0; //it is just an example...
            if (res==3) mItem.counter=0; //it is just an example...
            mAdapter.addItem(mItem);
            res++;
        }



        mDrawerList = (ListView) findViewById(R.id.drawer);
        if (mDrawerList != null)
            mDrawerList.setAdapter(mAdapter);

        mDrawerList.setOnItemClickListener(new DrawerItemClickListener());

    }

    @Override
    protected void onPostCreate(Bundle savedInstanceState) {
        super.onPostCreate(savedInstanceState);
        // Sync the toggle state after onRestoreInstanceState has occurred.
        mDrawerToggle.syncState();
    }

    @Override
    public void onConfigurationChanged(Configuration newConfig) {
        super.onConfigurationChanged(newConfig);
        mDrawerToggle.onConfigurationChanged(newConfig);
    }




    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        /*
         * The action bar home/up should open or close the drawer.
         * ActionBarDrawerToggle will take care of this.
         */
        if (mDrawerToggle.onOptionsItemSelected(item)) {
            return true;
        }

        // Handle your other action bar items...
        return super.onOptionsItemSelected(item);
    }

    private class CustomActionBarDrawerToggle extends ActionBarDrawerToggle {

        public CustomActionBarDrawerToggle(Activity mActivity,DrawerLayout mDrawerLayout){
            super(
                mActivity,
                mDrawerLayout, 
                R.drawable.ic_drawer,
                R.string.ns_menu_open, 
                R.string.ns_menu_close);
        }

        @Override
        public void onDrawerClosed(View view) {
            getActionBar().setTitle(getString(R.string.ns_menu_close));
            invalidateOptionsMenu(); // creates call to onPrepareOptionsMenu()
        }

        @Override
        public void onDrawerOpened(View drawerView) {
            getActionBar().setTitle(getString(R.string.ns_menu_open));
            invalidateOptionsMenu(); // creates call to onPrepareOptionsMenu()
        }
    }

    private class DrawerItemClickListener implements ListView.OnItemClickListener {

        @Override
        public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
            mDrawerList.setItemChecked(position, true);         
            mDrawer.closeDrawer(mDrawerList);
            if (position == 1) {
                Intent intent = new Intent(MainActivity.this, Istoric.class);
                startActivity(intent);
                mDrawer.closeDrawers();
            }
            else if (position == 2) {
                Intent intent2 = new Intent(MainActivity.this, Profesori.class);
                startActivity(intent2);
                mDrawer.closeDrawers();
            }
            if (position == 3) {
                Intent intent3 = new Intent(MainActivity.this, Elevi.class);
                startActivity(intent3);
                mDrawer.closeDrawers();
            }
            if (position == 4) {
                Intent intent4 = new Intent(MainActivity.this, ShowImageActivity.class);
                startActivity(intent4);
                mDrawer.closeDrawers();
            }

            if (position == 5) {
                Intent intent5 = new Intent(MainActivity.this, Despre.class);
                startActivity(intent5);
                mDrawer.closeDrawers();
            }
            if (position == 6) {
                Intent intent6 = new Intent(MainActivity.this, Feedback.class);
                startActivity(intent6);
                mDrawer.closeDrawers();
            }

        }



    }
        }

请帮忙，谢谢

在onCreate（）中

您使用了player对象，但它为null。您需要首先创建一个实例，但之后将使用

 player = MediaPlayer.create(this, R.raw.gc);

只需移动最后一行，并将其设置在setLooping（）行之前。

在onCreate（）中

您使用了player对象，但它为null。您需要首先创建一个实例，但之后将使用

 player = MediaPlayer.create(this, R.raw.gc);

只需移动最后一行，并将其设置在setLooping（）行之前。

在onCreate（）中

您使用了player对象，但它为null。您需要首先创建一个实例，但之后将使用

 player = MediaPlayer.create(this, R.raw.gc);

只需移动最后一行，并将其设置在setLooping（）行之前。

在onCreate（）中

您使用了player对象，但它为null。您需要首先创建一个实例，但之后将使用

 player = MediaPlayer.create(this, R.raw.gc);

只需移动最后一行，并将其设置在setLooping（）行之前。

错误是：

Caused by: java.lang.NullPointerException
E/AndroidRuntime(1136):     at com.orar.cngcnasaud.MainActivity.onCreate(MainActivity.java:40)

第40行使用的任何对象都为空。

错误为：

Caused by: java.lang.NullPointerException
E/AndroidRuntime(1136):     at com.orar.cngcnasaud.MainActivity.onCreate(MainActivity.java:40)

第40行使用的任何对象都为空。

错误为：

Caused by: java.lang.NullPointerException
E/AndroidRuntime(1136):     at com.orar.cngcnasaud.MainActivity.onCreate(MainActivity.java:40)

第40行使用的任何对象都为空。

错误为：

Caused by: java.lang.NullPointerException
E/AndroidRuntime(1136):     at com.orar.cngcnasaud.MainActivity.onCreate(MainActivity.java:40)

第40行使用的任何对象都是空的