Android按返回键退出程序的两种方式

package com.example.onemoreexit;

import android.app.Activity;
import android.app.AlertDialog;
import android.content.DialogInterface;
import android.os.Bundle;
import android.view.KeyEvent;
import android.widget.Toast;

public class MainActivity extends Activity {

	@Override
	protected void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		setContentView(R.layout.activity_main);
	}

	
	  // 1.以AlertDialog的方式
	  
	@Override
	public boolean onKeyDown(int keyCode, KeyEvent event) {
		switch (keyCode) {
		case KeyEvent.KEYCODE_BACK:
			AlertDialog.Builder build = new AlertDialog.Builder(this);
			build.setTitle("提示").setMessage("确定退出？");
			build.setPositiveButton("确定", new DialogInterface.OnClickListener() {

				@Override
				public void onClick(DialogInterface dialog, int which) {
					finish();
				}
			});
			build.setNegativeButton("取消", null).show();
			break;
		}
		return super.onKeyDown(keyCode, event);
	}
	 

	// 2.以Toast的方式
/*	private long exitTime = 0;

	@Override
	public boolean onKeyDown(int keyCode, KeyEvent event) {
		if (keyCode == KeyEvent.KEYCODE_BACK && event.getAction() == KeyEvent.ACTION_DOWN) {
			if ((System.currentTimeMillis() - exitTime) > 2000) {
				Toast.makeText(getApplicationContext(), "再按一次退出", Toast.LENGTH_SHORT).show();
				exitTime = System.currentTimeMillis();
			} else {
				finish();
				System.exit(0);
			}
			return true;
		}
		return super.onKeyDown(keyCode, event);
	}*/
}