【java的排序、求和、求质数、求奇数以及一些简单工具】

工具

取当月的第一天和最后一天

            LocalDate.now().with(TemporalAdjusters.firstDayOfMonth());
            LocalDate.now().with(TemporalAdjusters.lastDayOfMonth());

生成UUID

        UUID.randomUUID().toString();

遍历list里面的对象

        List<budgetDTO> list1 =sysserver.getAllChange();
        List<budgetDTO> list2 =new ArrayList<>();
        for(budgetDTO list3: list1){
            if(list3.getCode()==1){
                continue;
            }
            list2.add(list3;
        }
        return list2;

在常量里面做map的映射关系

private static final Map<String,Integer> tagMap=new HashMap<>();
    {
        tagMap.put("红包",1);
        tagMap.put("活动",2);
    }
 public static void main(String[] args) {

        tagMap.get("红包");
}

list 和arrays 之间的转换

数组转换为list排序

public class TestSort {
    public static void main(String[] args) {
        Integer[] arry1={2,4,5,6,7,2,2,4,6,8,9};
        System.out.println(Arrays.toString(arry1));
        arraus_to_list_sort(arry1);
  }
    public static List<Integer> arraus_to_list_sort(Integer[] list1){
        List<Integer> li=Arrays.stream(list1).sorted().collect(Collectors.toList());
        System.out.println(li);
        return li;
    }
}

     Integer[] arry1={2,4,5,6,7,2,2,4,6,8,9};
        System.out.println(Arrays.toString(arry1));
        System.out.println(Arrays.stream(arry1).sorted().collect(Collectors.toList()));

数组排序

使用Arrays对数组进行排序

public class TestSort {
    public static void main(String[] args) {
      Integer[] arry2={2,4,5,6,7,2,2,4,6,8,9};
        System.out.println(Arrays.toString(arry1));
        arrays_to_arrays_sort(arry2);
  }
    public static Integer[] arrays_to_arrays_sort(Integer [] list2){
        Arrays.sort(list2);
        System.out.println(Arrays.toString(list2));
        return list2;
    }
}

使用Steam对list进行排序

      Integer[] arry1={2,4,5,6,7,2,2,4,6,8,9};
        System.out.println(Arrays.toString(arry1));

        System.out.println(Arrays.toString(Arrays.stream(arry1).sorted().toArray()));

list转换为数组进行排序

public class TestSort {
    public static void main(String[] args) {
        ArrayList<Integer> list1=new ArrayList<>();
        list1.add(1);
        list1.add(133);
        list1.add(12);
        list1.add(134);
        list1.add(5);
        list_to_arrays(list1);
  }
    public static void list_to_arrays(ArrayList<Integer> listoarr){
        Integer [] arrays=listoarr.toArray(new Integer[0]);
        System.out.println(Arrays.toString(arrays));
    }
}

      ArrayList<Integer> list=new ArrayList<>();
        list.add(1);
        list.add(13);
        list.add(15);
        list.add(11);
        list.add(6);
        list.add(9);
        System.out.println(list);
        System.out.println(Arrays.toString(list.stream().sorted().toArray()));

排序的三种方式

1、冒泡排序

  int[] arry1={2,4,5,6,7,2,2,4,6,8,9};

        int temp=0;
        for (int i = 0; i < arry1.length; i++) {
            for(int j=0;j<arry1.length-1-i;j++){
                if(arry1[j]>arry1[j+1]){
                    temp=arry1[j];
                    arry1[j]=arry1[j+1];
                    arry1[j+1]=temp;
                }
            }
        }
        System.out.println(Arrays.toString(arry1));

2、使用sort进行排序

        int[] arry2={2,4,5,6,7,2,2,4,6,8,9};
        Arrays.sort(arry2);
        System.out.println(arry2);

3、选择排序

        int[] arry3={2,4,5,6,7,2,2,4,6,8,9};
        int max=arry3[0];
        for (int i = 1; i < arry3.length; i++) {
            if(arry3[i]>max){
                max=arry3[i];
            }
        }
        System.out.println(max);

2、求和的几种方式

do-while求和

        int sum3=0;
        int i=0;
        do{
            sum3=sum3=i;
            i++;
        }while (i<=10);
        System.out.println(sum3);

while求和

        int sum4=0;
        int j=1;
        while (j<=10){
            sum4+=j;
            j++;
        }
        System.out.println(sum4);

for循环求偶数

 //1-100之間偶數和
        int sum5 = 0;
        for (int a = 1;a <= 100;a++) {
            if (a % 2 == 0) {
                sum5 += a;
            }
        }
        System.out.println("结果是： " + sum5);

for循环求奇数

        for(int b=2;b<=100;b++){
            boolean flag=true;
            for(int d=2;d<b;d++){
                if(b%d==0){
                    flag=false;
                    break;
                }

            }
            if(flag==true){
                System.out.print(b+" ");
            }
        }

for循环求质数

        int count=0;
        ArrayList<Integer> zhishu=new ArrayList<>();
        for(int n=2;n<=100;n++){
            for(int ii=2;ii<n;ii++){
                if (n % ii == 0) {
                    
                    count++;
                }
            }
            if(count==0){
                zhishu.add(n);
                
            }
            count=0;
            System.out.println("1-100之间的质数："+zhishu);
        }