java常用的工具类

 1 包装类
 2 https://www.cnblogs.com/benjieqiang/p/11305777.html
 3 
 4 Arrays类（数组工具类）
 5 package day02.com.offcn.test;
 6 
 7 import java.util.Arrays;
 8 import java.util.List;
 9 
10 public class 数组 {
11     public static void main(String[] args) {
12         //1.生成list
13         List<Integer> list = Arrays.asList(1,1,1,1);
14         Integer a[] = {1,23,34,13,23,12,343};
15 
16         //2.根据计算索引处左右对象,返回赋值右边
17         Arrays.parallelPrefix(a, (left, right) -> right);
18 
19         //3.数组输出
20         System.out.println(Arrays.toString(a));
21 
22         //4.数组自定义排序,int类型不能实现compatetor接口,因为没有int类型的父类
23         Arrays.sort(a, (o1, o2) -> o1-o2);
24 
25         //5,数组二分查找
26         int b = Arrays.binarySearch(a, 238);
27 
28         //6.数组复制,调用本地方法
29         //两个数组必须是同一类型,int和integer也不行
30         int a1[] = {1,23,34,13,23,12,343};
31         int c[] = new int[10];
32         System.arraycopy(a1, 0,c, 0 ,a.length);
33 
34         //7.数组填充,根据索引
35         Arrays.parallelSetAll(a1, index -> (int)(Math.random()*10));
36 
37         //8.转换为流对象,在转回数组对象
38         a1 = Arrays.stream(a1).toArray();
39 
40     }
41 }
42 
43 
44 BigDecimal类
45 public static void main(String[] args) {
46 
47         double x = 0.1;
48         double y = 0.2;
49 
50         BigDecimal b1 = new BigDecimal(x+"");
51         BigDecimal b2 = new BigDecimal(y+"");
52         BigDecimal b3 = b1.add(b2);
53 
54         double result1 = b3.doubleValue();
55         System.out.println(result1);
56 
57         double result2 = b1.subtract(b2).doubleValue();
58         System.out.println(result2);
59 
60         double result3 = b1.multiply(b2).doubleValue();
61         System.out.println(result3);
62 
63         double result4 = b1.divide(b2).doubleValue();
64         System.out.println(result4);
65 
66 }
67 
68  Math类和Random类
69 public class Test04 {
70    public static void main(String[] args) {
71        
72       System.out.println((int)(Math.random()*10));
73       //随机生成从20到121之间101个数,
74       Arrays.parallelSetAll(f, operand -> (int)(Math.random()*101+20));
75       
76       System.out.println(Math.abs(-9));
77       System.out.println(Math.cbrt(8));
78       System.out.println(Math.max(4, 3));
79       System.out.println(Math.min(4, 3));
80       System.out.println(Math.round(99.9));
81       System.out.println(Math.floor(2.2));
82       System.out.println(Math.ceil(3.4));
83    }
84 }
85 Random random = new Random();
86 random.nextInt(10)+1;
87 
88 System类
89 gc()
90 exit()
91 currentTimeMillis()
92 arraycopy(Object src, int srcPos, Object dest,int destPos, int length)
93 int a1[] = {2,2,2,33,23,23,43,4,23,2,32,32,32};
94 int a2[] = new int[8];
95 System.arraycopy(a1, 2, a2, 2, 5);
96 [0, 0, 2, 33, 23, 23, 43, 0]