public class MainActivity extends Activity {    private Button bu1;    private Camera camera;    private Parameters parameters;    private static boolean kaiguan = true;    public int back = 0;    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);                                setContentView(R.layout.activity_main);        LinearLayout container = (LinearLayout)findViewById(R.id.Acc);        new AdView(this,container).DisplayAd();        bu1=(Button)findViewById(R.id.bu1);        bu1.setOnClickListener(new OnClickListener() {                                        @Override            public void onClick(View v) {                // TODO Auto-generated method stub                if(kaiguan){                    bu1.setBackgroundResource(R.drawable.on);                    camera = Camera.open();                    parameters = camera.getParameters();                    parameters.setFlashMode(Parameters.FLASH_MODE_TORCH);                    camera.setParameters(parameters);                    kaiguan = false;                                                    }else{                    bu1.setBackgroundResource(R.drawable.off);                    parameters.setFlashMode(Parameters.FLASH_MODE_OFF);                    camera.setParameters(parameters);                    kaiguan = true;                    camera.release();                }            }        });    }    @Override    public boolean onCreateOptionsMenu(Menu menu) {        // Inflate the menu; this adds items to the action bar if it is present.        menu.add(0,1,1,"关于");        menu.add(1,2,2,"退出");        return super.onCreateOptionsMenu(menu);    }    public boolean onOptionsItemSelected(MenuItem item){        switch (item.getItemId()) {        case 1:            AlertDialog.Builder b = new Builder(MainActivity.this);            b.setTitle("关于");            b.setMessage("卡卡手电做出最简洁的应用                                                            作者：某人");            b.setCancelable(true);            b.setNegativeButton("确定", new DialogInterface.OnClickListener() {                public void onClick(DialogInterface d, int which) {                    d.dismiss();                }            });            b.show();            break;        case 2:            Myback();            break;        }        return super.onOptionsItemSelected(item);    }    public boolean onKeyDown(int keyCode, KeyEvent event) {        if(keyCode == KeyEvent.KEYCODE_BACK){            back++;            switch (back) {            case 1:                Toast.makeText(MainActivity.this, "再按一次退出卡卡手电", Toast.LENGTH_LONG).show();                break;            case 2:                back = 0;//初始化back值                Myback();                break;            }            return true;//设置成false让back失效    ，true表示 不失效        }        else{            return super.onKeyDown(keyCode, event);        }    }    public void Myback(){        if(kaiguan){//开关关闭时            MainActivity.this.finish();            android.os.Process.killProcess(android.os.Process.myPid());//关闭进程        }else if(!kaiguan){//开关打开时            camera.release();            MainActivity.this.finish();            android.os.Process.killProcess(android.os.Process.myPid());//关闭进程            kaiguan = true;//避免，打开开关后退出程序，再次进入不打开开关直接退出时，程序错误        }    }}