java常用的工具类

 包装类
 https://www.cnblogs.com/benjieqiang/p/11305777.html

 Arrays类（数组工具类）
 package day02.com.offcn.test;

 import java.util.Arrays;
 import java.util.List;

 public class 数组 {
     public static void main(String[] args) {
         //1.生成list
         List<Integer> list = Arrays.asList(1,1,1,1);
         Integer a[] = {1,23,34,13,23,12,343};

         //2.根据计算索引处左右对象,返回赋值右边
         Arrays.parallelPrefix(a, (left, right) -> right);

         //3.数组输出
         System.out.println(Arrays.toString(a));

         //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
         Arrays.sort(a, (o1, o2) -> o1-o2);

         //5,数组二分查找
         int b = Arrays.binarySearch(a, 238);

         //6.数组复制,调用本地方法
         //两个数组必须是同一类型,int和integer也不行
         int a1[] = {1,23,34,13,23,12,343};
         int c[] = new int[10];
         System.arraycopy(a1, 0,c, 0 ,a.length);

         //7.数组填充,根据索引
         Arrays.parallelSetAll(a1, index -> (int)(Math.random()*10));

         //8.转换为流对象,在转回数组对象
         a1 = Arrays.stream(a1).toArray();

     }
 }

 BigDecimal类
 public static void main(String[] args) {

         double x = 0.1;
         double y = 0.2;

         BigDecimal b1 = new BigDecimal(x+"");
         BigDecimal b2 = new BigDecimal(y+"");
         BigDecimal b3 = b1.add(b2);

         double result1 = b3.doubleValue();
         System.out.println(result1);

         double result2 = b1.subtract(b2).doubleValue();
         System.out.println(result2);

         double result3 = b1.multiply(b2).doubleValue();
         System.out.println(result3);

         double result4 = b1.divide(b2).doubleValue();
         System.out.println(result4);

 }

  Math类和Random类
 public class Test04 {
    public static void main(String[] args) {

       System.out.println((int)(Math.random()*10));
       //随机生成从20到121之间101个数,
       Arrays.parallelSetAll(f, operand -> (int)(Math.random()*101+20));

       System.out.println(Math.abs(-9));
       System.out.println(Math.cbrt(8));
       System.out.println(Math.max(4, 3));
       System.out.println(Math.min(4, 3));
       System.out.println(Math.round(99.9));
       System.out.println(Math.floor(2.2));
       System.out.println(Math.ceil(3.4));
    }
 }
 Random random = new Random();
 random.nextInt(10)+1;

 System类
 gc()
 exit()
 currentTimeMillis()
 arraycopy(Object src, int srcPos, Object dest,int destPos, int length)
 int a1[] = {2,2,2,33,23,23,43,4,23,2,32,32,32};
 int a2[] = new int[8];
 System.arraycopy(a1, 2, a2, 2, 5);
 [0, 0, 2, 33, 23, 23, 43, 0]