Java基础(八) -- Math/Arrays/System/大数据/日期类

Math

Arrays

• toString

  • 将数组转为字符串
  • 示例

    int[] arr = {1,2,3};
    System.out.println(Arrays.toString(arr));

• sort

  • 排序数组
  • 示例

        public static void main(String[] args) {
            Integer[] arr = {1,2,3,4,5};
            Arrays.sort(arr, new Comparator() {
                @Override
                public int compare(Object o1, Object o2) {
                    Integer i1 = (Integer) o1;
                    Integer i2 = (Integer) o2;
                    return i2 - i1;
                }
            });
            System.out.println(Arrays.toString(arr));
        }
        ```   
    <!-- more -->
    * binarySearch
      + 二叉查找
      + 示例
        ```java
            Integer[] arr = {1,2,3,4,5};
            // 数组必须是有序的 小于0表示没找到
            System.out.println(Arrays.binarySearch(arr,4));
        ```   
    
    * copyOf/fill/equals/asList
      + 复制/填充/比较/转为list
      + 示例
      ```java
      public static void main(String[] args) {
            Integer[] arr = {1,2,3,4,5};
            // 拷贝 length 个元素到arr2中
            Integer[] arr2 = Arrays.copyOf(arr,arr.length);
            System.out.println(Arrays.toString(arr2));
    
            Integer[] nums = {1,2,3};
            // 使用10替换原数组所有值
            Arrays.fill(nums,10);
            System.out.println(Arrays.toString(nums));
    
            Integer[] arr1 = {1,2,3,5,4};
            // 比较两个数组是否一致,完全一致包括顺序
            System.out.println(Arrays.equals(arr,arr1));
    
            // 将数组转为list
            List<Integer> list = Arrays.asList(arr);
            System.out.println(list);
        }

System

public static void main(String[] args) {
    // 获取当前毫秒时间戳
    System.out.println(System.currentTimeMillis());

    Integer[] arr = {1,2,3,4};
    Integer[] arr2 = new Integer[3];
    // 复制数组
    System.arraycopy(arr,0,arr2,1,2);
    System.out.println(Arrays.toString(arr2));

    // 主动触发垃圾回收
    System.gc();

    // 退出
    System.exit(0);
}

大数处理

public static void main(String[] args) {
        BigInteger bigInteger = new BigInteger("32234234121231232234234212");
        System.out.println(bigInteger);
        // 加
        bigInteger = bigInteger.add(new BigInteger("20"));
        System.out.println(bigInteger);
        // 减
        bigInteger = bigInteger.subtract(new BigInteger("20"));
        System.out.println(bigInteger);
        // 乘
        bigInteger = bigInteger.multiply(new BigInteger("20"));
        System.out.println(bigInteger);
        // 除
        bigInteger = bigInteger.divide(new BigInteger("20"));
        System.out.println(bigInteger);

        // 大精度小数
        BigDecimal bigDecimal = new BigDecimal("1231023.12312312312");
        // 不指定精度操作时可能会出现无限循环所以抛出异常
        // RoundingMode.CEILING 保留和分子相同的精度
        bigDecimal = bigDecimal.divide(new BigDecimal("11.22"), RoundingMode.CEILING);
        System.out.println(bigDecimal);
    }

日期类

• java.util.Date

  package com.gphper.date;
  
  import java.text.ParseException;
  import java.text.SimpleDateFormat;
  import java.util.Date;
  
  public class DateClass {
      public static void main(String[] args) throws ParseException {
          Date date = new Date();
          System.out.println(date);
          SimpleDateFormat dateFormat = new SimpleDateFormat("yyyy年MM月dd日 hh:mm:ss E");
          System.out.println(dateFormat.format(date));
  
          //根据毫秒数计算时间
          Date date1 = new Date(953453453);
          System.out.println(date1);
  
          // 将字符串转为时间
          String s  = "2023年06月01日 10:12:22 星期四";
          Date pdate = dateFormat.parse(s);
          System.out.println(pdate);
      }
  }

• Calendar

  import java.util.Calendar;
  
  public class DateClass {
      public static void main(String[] args) {
          // 获取年月日信息需要单独获取
          Calendar instance = Calendar.getInstance();
          // 获取年份
          System.out.println(instance.get(Calendar.YEAR));
          // 获取月份
          System.out.println(instance.get(Calendar.MONTH) + 1);
          System.out.println(instance.get(Calendar.HOUR));
          System.out.println(instance.get(Calendar.HOUR_OF_DAY));
      }
  }

• LocalDate / locaTime / LocalDateTime

   import java.time.Instant;
  import java.time.LocalDateTime;
  import java.time.format.DateTimeFormatter;
  import java.util.Date;
  
  public class DateClass {
      public static void main(String[] args) {
  
          LocalDateTime now = LocalDateTime.now();
          System.out.println(now);
          System.out.println(now.getMinute());
          // 格式化时间格式
          DateTimeFormatter dateTimeFormatter = DateTimeFormatter.ofPattern("yyyy年MM月dd日 hh:mm:ss E");
          System.out.println(now.format(dateTimeFormatter));
  
          // 获取当前时间戳
          Instant now1 = Instant.now();
          System.out.println(now1);
          Date date = Date.from(now1);
          System.out.println(date);
          // Date 转 Instance
          now1 = date.toInstant();
          // 获取一天后的日期
          LocalDateTime m = now.plusDays(1);
          System.out.println(m.format(dateTimeFormatter));
          // 获取100天前
          LocalDateTime m1 = now.minusDays(100);
          System.out.println(m1.format(dateTimeFormatter));
      }
  }