处理数据库中的null值问题 

  1 public class Test {
  2 
  3     public static void nullPoint1() {
  4         String test = null;
  5         try {
  6             if (test.equals(null)) {
  7                 System.out.println("test is null !");
  8             } 
  9         } catch (NullPointerException e) {
 10             System.out.println("test.equals(null): " + e);
 11         }
 12     }
 13 
 14     public static void nullPoint2() {
 15         String test = null;
 16         try {
 17             if (test.length() == 0) {
 18                 System.out.println("test is null !");
 19             } 
 20         } catch (NullPointerException e) {
 21             System.out.println("test.length() == 0: " + e);
 22         }
 23     }
 24 
 25     public static void nullPoint3() {
 26         String test = null;
 27         try {
 28             if (test.isEmpty()) {
 29                 System.out.println("test is null !");
 30             } 
 31         } catch (NullPointerException e) {
 32             System.out.println("test.isEmpty(): " + e);
 33         }
 34     }
 35 
 36     public static void nullPoint4() {
 37         String test = null;
 38         try {
 39             if (test.equals("")) {
 40                 System.out.println("test is null !");
 41             } 
 42         } catch (NullPointerException e) {
 43             System.out.println("test.equals(\"\"): " + e);
 44         }
 45     }
 46 
 47     public static void error() {
 48         String test = null;
 49         if ("".equals(test)) {
 50             System.out.println("test = null, \"\".equals(test): true");
 51         } else {
 52             System.out.println("test = null, \"\".equals(test): false");
 53         }
 54     }
 55 
 56     public static void error2() {
 57         String test = null;
 58         if (test == "") {
 59             System.out.println("test = null, test == \"\": true");
 60         } else {
 61             System.out.println("test = null, test == \"\": false");
 62         }
 63     }
 64 
 65     public static void right() {
 66         String test = null;
 67         if (test == null) {
 68             System.out.println("test = null, test == null: true");  
 69         }
 70     }
 71 
 72     public static void right2() {
 73         String test = "not null";
 74         if (test.equals(null)) {
 75             System.out.println("test is null");
 76         }
 77         if (test.length() <= 0) {
 78             System.out.println("test is null");
 79         }
 80         if (test.isEmpty()) {
 81             System.out.println("test is null");
 82         }
 83         if (test == null) {
 84             System.out.println("test is null");
 85         }
 86         if (test != null && !test.isEmpty() && test.length() > 0 && !test.equals(null)) {
 87             System.out.println("test : " + test);
 88         }
 89     }
 90 
 91     public static void right3() {
 92         String test = "";
 93         if (test.equals("") && test == "") {
 94             System.out.println("test = \"\"");
 95         }
 96     }
 97 
 98     public static void main(String[] args) {
 99         Test.nullPoint1();
100         Test.nullPoint2();
101         Test.nullPoint3();
102         Test.nullPoint4();
103         Test.error();
104         Test.error2();
105         Test.right();
106         Test.right2();
107         Test.right3();
108     }   
109 }

运行结果为：
test.equals(null): java.lang.NullPointerException
test.length() == 0: java.lang.NullPointerException
test.isEmpty(): java.lang.NullPointerException
test.equals(“”): java.lang.NullPointerException
test = null, “”.equals(test): false
test = null, test == “”: false
test = null, test == null: true
test : not null
test = “”

可以看到当字符串为null时，用equals()、length()、isEmpty() 都会抛出异常；而用与 “” 进行比较时，会出现判断失误。
所以可以使用 “if (test == null)” 来判断，简单直接。
注意当字符串为 “” 时，与null不同，不是空指针，得与”“比较。字符串为空

 

这个方法也适用于处理数据库中的null值问题
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作者：sys_
来源：CSDN
原文：https://blog.csdn.net/qq_34732088/article/details/79571938
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posted @ 2019-07-04 11:13  迷你熊爱你  阅读(1066)  评论(0)  编辑  收藏  举报