如何建立上面的对话框和提示信息

 1 @Override
 2     protected void onCreate(Bundle savedInstanceState) {
 3         super.onCreate(savedInstanceState);
 4         setContentView(R.layout.activity_main);
 5
 6         btn = (Button)findViewById(R.id.btn);
 7         builder = new AlertDialog.Builder(this);
 8         builder.setTitle("提示框");
 9         builder.setIcon(R.drawable.ic_launcher);
10         builder.setMessage("确定删除吗");
11         builder.setNegativeButton("取消", new DialogInterface.OnClickListener(){
12             @Override
13             public void onClick(DialogInterface arg0, int arg1) {
14                 Toast.makeText(MainActivity.this, "取消了删除", 1).show();
15             }
16
17         });
18         builder.setPositiveButton("确定", new DialogInterface.OnClickListener(){
19             @Override
20             public void onClick(DialogInterface arg0, int arg1) {
21                 Toast.makeText(MainActivity.this, "已经删除", 1).show();
22             }
23
24         });
25         btn.setOnClickListener(new OnClickListener() {
26             @Override
27             public void onClick(View arg0) {
28                 builder.show();
29             }
30         });
31     }

为什么要用AlertDialog.Builder创建对话框????