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]