Java novice learning 2021-1-27 record daily learning content (if there is any infringement, please contact to delete!!!)

2021-1-27

1. Recursion

Case study

package com.wc.show;

/**
 * @author wc
 * @Date: 2021/01/27/16:29
 */
public class Factorial {
    
    
    public static void main(String[] args) {
    
    
        int result=getJc(5);
        System.out.println(result);
    }
    private static int getJc(int i) {
    
    
        //出口，当参数为多少时，递归结束
        if (i==1){
    
    
            return 1;
        }else{
    
    
            return i*getJc(i-1);
        }
    }
}

2. Quick sort

Code

package com.wc.show;

/**
 * @author wc
 * @Date: 2021/01/27/16:50
 */
public class Quick {
    
    
    public static void main(String[] args) {
    
    
        int[] arr = {
    
    6, 1, 2, 7, 9, 3, 4, 5, 10, 8};
        quiteSort(arr, 0, arr.length - 1);
        for (int i = 0; i < arr.length; i++) {
    
    
            System.out.print(arr[i]+" ");
        }
    }
    private static void quiteSort(int[] arr, int left, int right) {
    
    
        if (left>right){
    
    
            return;
        }
        int left0 = left;
        int right0 = right;
        //循环条件，当左等于右时停止循环
        //基准数默认为第一个
        int baseNumber =arr[left0];
        while (left != right) {
    
    
            //1.从右开始找比基准数小的
            while (arr[right] >= baseNumber && right > left) {
    
    
                right--;
            }
            //2.从左开始找比基准数大的
            while (arr[left] <= baseNumber && right > left) {
    
    
                left++;
            }
            //3.左右数值交换
            int temp = arr[left];
            arr[left] = arr[right];
            arr[right] = temp;
        }
        //基准数归位
        int temp=arr[left];
        arr[left]=arr[left0];
        arr[left0]=temp;
        quiteSort(arr,left0,left-1);
        quiteSort(arr,left+1,right0);
    }
}

3. Arrays class and common methods

package com.wc.show;

import java.util.Arrays;

/**
 * @author wc
 * @Date: 2021/01/28/9:30
 */
public class MyArrays {
    
    
    public static void main(String[] args) {
    
    
        //toString 将数组按照字符串形式返回
        int [] arr={
    
    3,1,2,6,7};
        System.out.println(Arrays.toString(arr));

        //sort 按照数字顺序排列指定数组
        int [] arr1={
    
    3,1,2,6,7};
        Arrays.sort(arr1);
        System.out.println(Arrays.toString(arr1));
        
    }
}

4. Date class (Date has parameters without parameters to construct, get the current time)

package com.wc.show;

import java.util.Date;

/**
 * @author wc
 * @Date: 2021/01/28/9:54
 */
public class DateDemo {
    
    
    public static void main(String[] args) {
    
    
        //空参构造 获取当前系统时间
        Date date=new Date();
        System.out.println(date);

        //有参构造 从时间原点开始（1970.1.1），过了多少时间（指定毫秒值）
        Date date1=new Date(3600L*1000);
        System.out.println(date1);

        //获取当前时间毫秒值
        Date date2=new Date();
        long time = date2.getTime();
        System.out.println(time);

        long l = System.currentTimeMillis();
        System.out.println(l);

        //设置时间
        Date date3=new Date();
        date3.setTime(0L);
        System.out.println(date3);
    }
}

5. SimpleDateFormat class (construction method, formatting, parsing date)

package com.wc.show;

import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;

/**
 * @author wc
 * @Date: 2021/01/28/10:01
 */
public class SimpleDateFormatDemo {
    
    
    public static void main(String[] args) throws ParseException {
    
    
        //SimpleDateFormat 构造方法
        //空参，默认格式
        //SimpleDateFormat sdf = new SimpleDateFormat();
        //有参，自定义格式
        //SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyy年MM月dd日 HH:mm:ss");
        Date date = new Date();
        SimpleDateFormat sdf = new SimpleDateFormat("yyyy年MM月dd日 HH:mm:ss");
        //格式转换 date->string  format
        String s = sdf.format(date);
        System.out.println(s);
        //格式转换 string->date  parse
        String s1 = "2021年01月28日 10:16:35";
        SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyy年MM月dd日 HH:mm:ss");
        Date parse = simpleDateFormat.parse(s1);
        System.out.println(parse);
    }
}

Case study

package com.wc.show;

import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;

/**
 * @author wc
 * @Date: 2021/01/28/10:41
 */
public class SimpleDateFormatDemo1 {
    
    
    public static void main(String[] args) throws ParseException {
    
    
        String startTime="2020年11月11号 0:0:0";
        String endTime="2020年11月11号 0:10:0";

        String jia="2020年11月11号 0:03:47";
        String pi="2020年11月11号 0:10:11";

        SimpleDateFormat sdf=new SimpleDateFormat("yyyy年MM月dd号 HH:mm:ss");
        //String->date  date 调用getTime方法,获取毫秒值
        long time1 = sdf.parse(startTime).getTime();
        long time2 = sdf.parse(endTime).getTime();
        long time3 = sdf.parse(jia).getTime();
        long time4 = sdf.parse(pi).getTime();

        if (time3>=time1&&time3<=time2){
    
    
            System.out.println("成功");
        }else {
    
    
            System.out.println("失败");
        }
        if (time4>=time1&&time4<=time2){
    
    
            System.out.println("成功");
        }else {
    
    
            System.out.println("失败");
        }
    }
}

6. JDk8 time tool class (LocalDateTime construction method, common method, formatting and analysis)

LocalDateTime creation method

package com.wc.show;

import java.time.LocalDate;
import java.time.LocalDateTime;

/**
 * @author wc
 * @Date: 2021/01/28/11:58
 */
public class Local {
    
    
    public static void main(String[] args) {
    
    
        //LocalDateTime 构造方法 获取当前时间
        LocalDateTime now = LocalDateTime.now();
        System.out.println(now);

        LocalDateTime localDateTime = LocalDateTime.of(2020, 11, 11, 11, 11, 11);
        System.out.println(localDateTime);
    }
}

LocalDateTime acquisition method

LocalDateTime conversion method

LocalDateTime formatting and conversion method

package com.wc.show;

import java.time.LocalDate;
import java.time.LocalDateTime;
import java.time.LocalTime;
import java.time.format.DateTimeFormatter;

/**
 * @author wc
 * @Date: 2021/01/28/11:58
 */
public class Local {
    
    
    public static void main(String[] args) {
    
    
        LocalDateTime localDateTime1 = LocalDateTime.of(2020, 11, 11, 11, 11, 11);
        System.out.println(localDateTime1);
        //LocalDateTime格式化成字符串
        DateTimeFormatter pattern = DateTimeFormatter.ofPattern("yyyy年MM月dd日 HH:mm:ss");
        String s = localDateTime1.format(pattern);
        System.out.println(s);
        //把一个日期字符串解析成LocalDateTime格式
        String date="2020年11月11日 11:11:11";
        DateTimeFormatter pattern1 = DateTimeFormatter.ofPattern("yyyy年MM月dd日 HH:mm:ss");
        LocalDateTime parse = LocalDateTime.parse(date,pattern1);
        System.out.println(parse);
    }
}

LocalDateTime increase or decrease time


LocalDateTime modification method

7. JDk8 time summary

8. Abnormal

9.throws和throw

10. Exception handling method

package com.wc.show;

import java.util.Scanner;

/**
 * @author wc
 * @Date: 2021/01/28/17:16
 */
public class ExceptionUse {
    
    
    public static void main(String[] args) {
    
    
        //1.如果try中没出现问题,怎么执行？
                //会把try中所有代码执行，不会执行catch里面代码
        //2.如果try中出现问题，try下面代码还会执行吗？
                //直接跳转到对应的catch中，try下面代码不执行
                //当catch里面的语句全部执行完毕，表示整个体系执行完毕，继续执行下面的代码
        //3.如果try中出现问题的问题没有被捕获，怎么运行？
                //那么try...catch相当于没有，也就是没处理
        //默认交给虚拟机处理
        //4.如果同事出现多个异常，怎么执行？
                //出现多个异常，就多些几个catch
                //注意点：如果异常之间存在父子关系，父类要写在下面
        try {
    
    
            Scanner sc = new Scanner(System.in);
            System.out.println("请输入你的年龄：");
            String line = sc.nextLine();
            int age = Integer.parseInt(line);
            System.out.println(age);
            System.out.println(2/0);
        } catch (NumberFormatException e) {
    
    
            System.out.println("格式化异常错误");
        }catch (ArithmeticException e){
    
    
            System.out.println("数学异常");
        }
        System.out.println("测试");
    }
}

11. Throwable member method

Summary

case

package com.wc.scanner;

import java.util.Scanner;

/**
 * @author wc
 * @Date: 2021/01/28/19:49
 */
public class StudentScanner {
    
    
    public static void main(String[] args) {
    
    
        Student student = new Student();
        Scanner scanner = new Scanner(System.in);
        System.out.println("请输入姓名");
        String name = scanner.nextLine();
        student.setName(name);
        while (true) {
    
    
            System.out.println("请输入年龄");
            String ageArr = scanner.nextLine();
            try {
    
    
                int age = Integer.parseInt(ageArr);
                student.setAge(age);
                break;               
            } catch (NumberFormatException e) {
    
    
                System.out.println("请输入一个整数");
                continue;
            } catch (RuntimeException e) {
    
    
                System.out.println("请输入范围内的数");
                continue;
            }
        }
        System.out.println(student);
    }
}

12. Custom exception