1     @Override
 2     public boolean onKeyDown(int keyCode, KeyEvent event) {
 3         // TODO Auto-generated method stub
 4         //点击了后退键
 5         if(keyCode == KeyEvent.KEYCODE_BACK)
 6         {
 7         //创建对话框构建器对象
 8             AlertDialog.Builder builder = new AlertDialog.Builder(this);
 9             builder.setTitle("温馨提示")//对话框标题
10             .setMessage("确定要退出么?")//对话框内容
11             .setIcon(R.drawable.ic_launcher)//对话框图标
12             .setCancelable(false) //点击对话框之外的部分是否取消对话框 
13             .setPositiveButton("确定", new OnClickListener() {
14                 
15                 @Override
16                 public void onClick(DialogInterface dialog, int which) {
17                     // TODO Auto-generated method stub
18                     finish();//结束当前Activity
19                 }
20             })
21             .setNegativeButton("取消", new OnClickListener() {
22                 
23                 @Override
24                 public void onClick(DialogInterface dialog, int which) {
25                     // TODO Auto-generated method stub
26                     dialog.dismiss();//取消对话框
27                 }
28             });
29             //生成对话框
30             Dialog dialog = builder.create();
31                         //显示对话框
32             dialog.show();
33         }
34         
35         return super.onKeyDown(keyCode, event);
36     }     

 1 public class MainActivity extends Activity {
 2     long timefirst = 0; //一定要是long类型的
 3     long timesecond = 0;
 4     boolean isKeyBack = false;//当为false ，为应用程序第一次按后退键，第一次按键之后 永为true ， 
 5     @Override
 6     protected void onCreate(Bundle savedInstanceState) {
 7         super.onCreate(savedInstanceState);
 8         setContentView(R.layout.activity_main);
 9     }
10 
11     @Override
12     public boolean onKeyDown(int keyCode, KeyEvent event) {
13         // TODO Auto-generated method stub
14 
15         if(keyCode == KeyEvent.KEYCODE_BACK)//如果点击了撤退键
16         {
17             if(isKeyBack==true) //如果isKeyBack==true 说明第二次点击后退键
18             {
19                 timesecond = System.currentTimeMillis();//记录第二次按键的时间
20                 System.out.println("2"+timesecond);
21 
22                 if(Math.abs(timesecond-timefirst)<2000)//说明两次按键间隔小于2秒，满足退出条件，退出Activity
23                 {
24                     finish();
25                 }
26                 else 
27                 {
28                     timefirst  = System.currentTimeMillis();//记录第一次按键的时间
29                     Toast.makeText(this, "再按一次退出程序", 1).show();
30                     //不满足条件，则设置此次为第一次按键，因为isKeyBack为true
31                                          //屏蔽后退键的撤销功能
32                     return true;
33                 }
34             }
35             else if(isKeyBack==false) //如果isKeyBack为false 说明第一次点击后退键
36             {
37                 timefirst  = System.currentTimeMillis();//记录第一次按键的时间
38                 Toast.makeText(this, "再按一次退出程序", 1).show();
39                 System.out.println("1"+timefirst);
40                 isKeyBack = true;
41                                 //屏蔽后退键的撤销功能
42                 return true;
43                 
44             }
45         }
46         
47         return super.onKeyDown(keyCode, event);
48     }    

 1 public class MainActivity extends Activity {
 2     long timefirst = 0; //一定要是long类型的
 3     long timesecond = 0;
 4     boolean isKeyBack = false;
 5     @Override
 6     protected void onCreate(Bundle savedInstanceState) {
 7         super.onCreate(savedInstanceState);
 8         setContentView(R.layout.activity_main);
 9     }
10     
11     @Override
12     public boolean onKeyDown(int keyCode, KeyEvent event) {
13         // TODO Auto-generated method stub
14         if(keyCode==KeyEvent.KEYCODE_BACK)
15         {    
16             timesecond = System.currentTimeMillis();  //记录当前按键时间，相当于上一次为第二次按键
17             if(timesecond-timefirst<2000) //如果两次按键时间小于2000毫秒,结束应用程序
18             {
19                 finish(); 
20             }
21             else    
22             {    
23                 Toast.makeText(this, "再按一次退出程序", 1).show();
24                 timefirst = timesecond; //如果不能退出，则当前按键时间设为第一次，下一次为第二次
25                 return true;            //屏蔽后退键的效果 return false;也可以 ，只要不让方法执行super.onKeyDown(keyCode, event);即可

 26  } 27  } 28 return super.onKeyDown(keyCode, event); 29 }

 1 @Override 
 2     public boolean onKeyDown(int keyCode, KeyEvent event) {
 3 
 4         switch (keyCode){
 5 
 6 
 7             case KeyEvent.KEYCODE_BACK:
 8 
 9                 if(isExit){
10 
11                     return super.onKeyDown(keyCode, event);
12                 }else {
13 
14                     Toast.makeText(this, "再按一下退出程序", Toast.LENGTH_SHORT).show();
15                     isExit = true;
16 
17                     new Thread(new Runnable() {
18 
19                         @Override
20                         public void run() {
21 
22                             try {
23 
24                                 Thread.sleep(2000);
25 
26                                 isExit = false;
27 
28                             } catch (InterruptedException e) {
29                                 e.printStackTrace();
30                             }
31 
32                         }
33 
34                     }).start();
35                     return false;
36                 }
37 
38             default:
39                 return super.onKeyDown(keyCode, event);
40 
41         }
42     }