Android退出整个应用程序

import java.util.LinkedList;
import java.util.List;
import android.app.Activity;
import android.app.AlertDialog;
import android.app.Application;
import android.content.DialogInterface;
import android.content.Intent;

public class SysApplication extends Application{
private List<Activity> mList = new LinkedList<Activity>();
private static SysApplication instance;

private SysApplication(){
}

public synchronized static SysApplication getInstance(){
if(null == instance){
instance = new SysApplication();
}
return instance;
}

public void addActivity(Activity activity){// add Activity
mList.add(activity);
}

public void exit(){
try{
for(Activity activity : mList){
if (activity != null)  //注意这里判断null
activity.finish();
}
}catch (Exception e) {
e.printStackTrace();
}finally {
System.exit(0);
}
}

public void onLowMemory() {
super.onLowMemory();
System.gc();
}
}

public void onCreate(Bundle savedInstanceState){
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main) ;
SysApplication.getInstance().addActivity(this) ;
}

public void Leavedialog(){
AlertDialog.Builder builder = new Builder(MainActivity.this);
builder.setMessage("确定要退出吗?");
builder.setTitle("提示");
builder.setPositiveButton("确认",
new android.content.DialogInterface.OnClickListener() {
@Override
public void onClick(DialogInterface dialog, int which) {
dialog.dismiss();
SysApplication.getInstance().exit() ;
}
}
);
builder.setNegativeButton("取消",
new android.content.DialogInterface.OnClickListener() {
@Override
public void onClick(DialogInterface dialog, int which) {
dialog.dismiss();
}
}
);
builder.create().show();
}