一些Java知识点

 1 import java.util.ArrayList;
 2 
 3 public class Main {
 4     
 5     public static void main(String[] args) {
 6         StringBuffer buffer = new StringBuffer("Hello World.");
 7         
 8         StringBuffer buffer_other = buffer;
 9         
10         buffer.append("Me too.");
11         
12         if (buffer == buffer_other) {
13             System.out.println("OK.");
14         }
15         
16         
17         //buffer.append(null); 报错
18         String str = null;
19         System.out.println(buffer.append(str));
20         //输出：Hello World.Me too.null 当字符串为null时，StringBuffer的append()会把null 转化为字符串null加在结尾
21         
22         ArrayList<String> list = new ArrayList<>();
23         list.add(str);
24         System.out.println(list.get(0));//输出为null
25         list.add(null);
26         System.out.println(list.get(1));//输出为null
27         
28         /********************************/
29         String string = null;
30         String string2 = null;
31         ArrayList<String> arrayList = null;
32         if (string == string2) {
33             System.out.println("Y");//输出Y
34         }
35         if (string == null) {
36             System.out.println("Y");//输出Y
37         }
38         if (null == null) {
39             System.out.println("Y");//输出Y
40         }
41         //string == arrayList  是语法错误
42         
43         //if (string.equals(string2)) {//NullPointerException
44         //    System.out.println("Y");
45         //}
46         
47     }
48 }