java基础-常用类-day11

目录

• 1. 包装类
• 2. Integer类
• 3.util.date
• 4. java.sql.Date
• 5. SimpleDateFormat
• 6. Calendar
• 7. Math
• 8. String 字符串
• 9.StringBuilder

1. 包装类

基础数据类型 进行封装 产生一个新的类 （属性 方法 构造器）

基本数据类型 包装类 继承关系

byte Byte Number-->Object

short Short Number-->Object

int Integer Number-->Object

long Long Number-->Object

float Float Number-->Object

double Double Number-->Object

char Character Object

boolean Boolean Object

为什么要有封装类

java是面型对象语言 操作类对象方便

可以放引用数据类型

数组 int[] String[] Student[]

集合 只能装引用数据类型

2. Integer类

public class Test {
    public static void main(String[] args) {
        System.out.println(Integer.MAX_VALUE);
        System.out.println(Integer.MIN_VALUE);

        System.out.println("物极必反");
        System.out.println(Integer.MAX_VALUE+1);
        System.out.println(Integer.MIN_VALUE-1);
    }
}

Integer构造器

public class Test {
    public static void main(String[] args) {
        System.out.println(Integer.MAX_VALUE);
        System.out.println(Integer.MIN_VALUE);

        System.out.println("物极必反");
        System.out.println(Integer.MAX_VALUE+1);
        System.out.println(Integer.MIN_VALUE-1);

        Integer i1 = new Integer(12);
        System.out.println(i1);
    }
}

入参为String的构造器

    public Integer(String s) throws NumberFormatException {
        this.value = parseInt(s, 10);
    }

包装类的 装箱 与拆箱

装箱：int 自动转 Integer

拆箱： Integer --> int

如何快速使用快捷键 变量接收

new 类()

alt+enter

再按一次enter

打断点阅读源码

    public static Integer valueOf(int i) {
        if (i >= IntegerCache.low && i <= IntegerCache.high)
            return IntegerCache.cache[i + (-IntegerCache.low)];
        return new Integer(i);
    }

-128--127之间会去cache缓存里面取 之外的才会new对象

compareTo方法

    public static int compare(int x, int y) {
        return (x < y) ? -1 : ((x == y) ? 0 : 1);
    }

x<y -1

x==y 0

x>y 1

public class Test04 {
    public static void main(String[] args) {
        //compareTo
        Integer integer = new Integer(6);
        Integer integer2 = new Integer(12);

        System.out.println(integer.compareTo(integer2));
    }
}

package com.msb2;

/**
 * @Auther: jack.chen
 * @Date: 2023/9/17 - 09 - 17 - 16:54
 * @Description: com.msb2
 * @version: 1.0
 */
public class Test04 {
    public static void main(String[] args) {
        //compareTo
        Integer integer = new Integer(6);
        Integer integer2 = new Integer(12);

        System.out.println(integer.compareTo(integer2));

        Integer integer3 = new Integer(12);
        Integer integer4 = new Integer(12);
        System.out.println(integer3 == integer4);//对地址的比较 两个不同的对象 返回false
        System.out.println(integer3.equals(integer4));//对封装的value的比较 true

        //自动装箱 int-->Integer
        //装箱 int自动转Integer对象可以具备很多方法
        //只有装箱后才有 equal... 才具有很多类的很多方法 供调用
        Integer i5 = 4;
        Integer i6 = 4;
        System.out.println(i5 == i6);//这里为啥返回true 在 -128--127之间 指向的是缓存里面的同一个对象
        System.out.println(i5.equals(i6)); //true

        Integer i7 = 130;
        Integer i8 = 130;
        System.out.println(i7 == i8);//这里为啥返回false  // 不在 -128--127之间
        System.out.println(i7.equals(i8)); //true

    }
}

Integer --> int 拆箱 只保留里面有用的value 其他的都不要扒掉 仅仅暴露本原

        Integer i9 = 130; //装箱
        int i10 = i9.intValue(); //方式1

        int i11 = Integer.parseInt("120");//方式2

        System.out.println(i9.toString());

3.util.date

public class Test {

    public static void main(String[] args) {
        Date date = new Date();
        System.out.println(date);
        System.out.println(date.toString());
        System.out.println(date.toGMTString());
        System.out.println(date.toLocaleString());

        System.out.println(date.getYear());//1200开始
        System.out.println(date.getMonth());//开始月为0

        System.out.println(date.getTime());// 1970-1-1 00:00:00 到现在的时间 ms为单位
        System.out.println(System.currentTimeMillis());//系统时间

        // 常用的还是System.currentTimeMillis()
        // 计算代码的运行时间
        long startTime = System.currentTimeMillis();
        for (int i = 0; i < 100000; i++) {
            System.out.println(i);
        }
        long endTime = System.currentTimeMillis();
        System.out.println(endTime-startTime);

    }
}

4. java.sql.Date

package com.msb;

import java.sql.Date;

/**
 * @Auther: jack.chen
 * @Date: 2023/9/17 - 09 - 17 - 17:20
 * @Description: com.msb
 * @version: 1.0
 */
public class Test02 {
    public static void main(String[] args) {
        // java.sql.Date
        Date d = new Date(1592055964263L);

        System.out.println(d);
        /*
        1. java.sql.Date extends java.util.Date
        2. 输出
            sql.Date 年月日
            util.Date 年月日 时分秒

        */
        java.util.Date d2 = new java.util.Date();
        System.out.println(d2.toLocaleString());

        //转换 向下 转型 util.Date--sql.Date 父类转子类
        java.util.Date d3 = new Date(1592055964263L);// 多态 父类指向子类的引用
        Date d4 = (Date)d3;  // Date 为java.sql.Date;

        /*
        Animal a = new Dog();
        Dog d = (Dog) a;
        * */
        Date d5 = new Date(d3.getTime());// d3 为util.Date对象

        Date d6 = new Date(1592055964263L);// sql.Date
        java.util.Date d7 = d6;

        Date d9 = Date.valueOf("2023-09-17");
        System.out.println(d9);
    }
}

5. SimpleDateFormat

String --> java.util.Date

package com.msb;

/**
 * @Auther: jack.chen
 * @Date: 2023/9/17 - 09 - 17 - 21:36
 * @Description: com.msb
 * @version: 1.0
 */
public class Test03 {

    public static void main(String[] args) {
        // java.sql.Date d = java.sql.Date.valueOf("2023-09-17 19:01:01");
//         java.sql.Date d = java.sql.Date.valueOf("2023/09/17");
        java.sql.Date d = java.sql.Date.valueOf("2023-09-17");
        java.util.Date d2 = d;

        System.out.println(d2.toString());
    }
}

注：上面的代码有局限性 只能转 yyyy-MM-dd 这样的形式的日期

学习新的类 SimpleDateFormat

见名知意 用于日期的简单转换

package com.msb;

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

/**
 * @Auther: jack.chen
 * @Date: 2023/9/17 - 09 - 17 - 21:43
 * @Description: com.msb
 * @version: 1.0
 */
public class Test04 {

    public static void main(String[] args) {
        // SimpleDateFormat 继承 DateFormat-抽象类
        SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");

        //String --> Date
        Date d = null;
        try {
            d = simpleDateFormat.parse("2023-10-01 10:00:00");
        } catch (ParseException e) {
            e.printStackTrace();
        }
        System.out.println(d);

        // Date-->String
        String s = simpleDateFormat.format(new Date());
        System.out.println(s);

        Date d2 = new Date();
        System.out.println(d2.toString());
        System.out.println(d2.toLocaleString());
        System.out.println(d2.toGMTString());

    }
}

6. Calendar

package com.msb;

import java.util.Calendar;
import java.util.GregorianCalendar;

/**
 * @Auther: jack.chen
 * @Date: 2023/9/17 - 09 - 17 - 21:49
 * @Description: com.msb
 * @version: 1.0
 */
public class Test05 {

    public static void main(String[] args) {
        // Calendar 是一个抽象类 不能实例化对象
        // new Calendar()
        // GregorianCalendar 是Calendar的子类 extends Calendar
        Calendar cal = new GregorianCalendar();
        Calendar cal2 = Calendar.getInstance();

        System.out.println(cal.get(Calendar.YEAR));//年  2023
        System.out.println(cal.get(Calendar.MONTH));// 月  8 (从0开始)
        System.out.println(cal.get(Calendar.DATE));// 日  17
        System.out.println(cal.get(Calendar.DAY_OF_WEEK));//星期中的第几天   1
        System.out.println(cal.get(Calendar.DAY_OF_MONTH));// 月里面的第几天  17
        System.out.println(cal.getActualMaximum(Calendar.DATE));//日期的最大天数 30
        System.out.println(cal.getActualMinimum(Calendar.DATE));//日期的最小天数  1

        // set
        cal.set(Calendar.YEAR, 1900);
        cal.set(Calendar.MONTH, 2);
        cal.set(Calendar.DATE, 20);
        System.out.println(cal);

        // String转Calendar
        java.sql.Date d = java.sql.Date.valueOf("2023-09-17");
        cal.setTime(d);
        System.out.println(cal);

    }
}

练习：输入日志 按格式 输出日历

package com.msb;

import java.util.Calendar;
import java.util.Scanner;

/**
 * @Auther: jack.chen
 * @Date: 2023/9/17 - 09 - 17 - 22:03
 * @Description: com.msb
 * @version: 1.0
 */
public class Test06 {

    public static void main(String[] args) {
        Calendar cal = Calendar.getInstance();

        Scanner sc = new Scanner(System.in);
        System.out.println("请输入日期，格式:2019-3-7");
        String strDate = sc.next();
//        java.sql.Date d = java.sql.Date.valueOf("2023-09-16");
        java.sql.Date d = java.sql.Date.valueOf(strDate);
        cal.setTime(d);

        System.out.println("日\t一\t二\t三\t四\t五\t六");
        int maxDate = cal.getActualMaximum(Calendar.DATE);
        int nowDay = cal.get(Calendar.DATE);

        // 把一号对应 星期的第几天求出来
        cal.set(Calendar.DATE, 1);
        int weekNum = cal.get(cal.DAY_OF_WEEK);
//        System.out.println("ss:"+weekNum);
        int num = weekNum-1;
        for (int i = 0; i < num; i++) {
            System.out.print("\t");
        }
        int count = 0;
        count = count + num;
        for (int i = 1; i < maxDate+1; i++) {
            if (count % 7 ==0){
                System.out.print("\n");
            }

            if (i == nowDay){
                System.out.print(i+"*"+"\t");
            }else{
                System.out.print(i+"\t");
            }
            count ++;

        }
    }
}

7. Math

package com.msb;

/**
 * @Auther: jack.chen
 * @Date: 2023/9/18 - 09 - 18 - 20:26
 * @Description: com.msb
 * @version: 1.0
 */
public class Test07 {
    public static void main(String[] args) {
        System.out.println(Math.PI);
        System.out.println(Math.random());// [0.0, 1.0)
        System.out.println(Math.abs(-12));
        System.out.println(Math.ceil(9.1));
        System.out.println(Math.floor(9.9));
        System.out.println(Math.round(3.5));
        System.out.println(Math.max(2, 6));
        System.out.println(Math.min(2, 6));
    }
}

Math.Random

package com.msb;

import java.util.Random;

/**
 * @Auther: jack.chen
 * @Date: 2023/9/18 - 09 - 18 - 20:30
 * @Description: com.msb
 * @version: 1.0
 */
public class Test08 {
    public static void main(String[] args) {
        System.out.println(Math.random());

        Random r1 = new Random(System.currentTimeMillis());
        int i = r1.nextInt();
        System.out.println(i);

        Random r2 = new Random();
        System.out.println(r2.nextInt(10));
        System.out.println(r2.nextDouble()); // 0--1之间
    }
}

8. String 字符串

value是用于存放字符串的数组

package com.msb;

/**
 * @Auther: jack.chen
 * @Date: 2023/9/18 - 09 - 18 - 20:34
 * @Description: com.msb
 * @version: 1.0
 */
public class TestString01 {
    public static void main(String[] args) {
        String s = "abc";

    }
}

基本接口

package com.msb;

import java.util.Arrays;

/**
 * @Auther: jack.chen
 * @Date: 2023/9/18 - 09 - 18 - 20:34
 * @Description: com.msb
 * @version: 1.0
 */
public class TestString01 {
    public static void main(String[] args) {
        String s = "abc";
        System.out.println(s);

        String s1 = new String();
        String s2 = new String("abc");
        String s3 = new String(new char[]{'a','b','c'});

        System.out.println(s1.isEmpty());
        System.out.println(s2.charAt(0));

        System.out.println(s1.equals(s2));
        System.out.println(s1.compareTo(s2));

        String s10 = "abcdefhijk";

        System.out.println(s10.substring(3));//从第三个开始截取到最后 包括第三个
        System.out.println(s10.substring(3, 6));
        System.out.println(s10.concat("pppppp"));
        System.out.println(s10.replace('a', 'z'));

        String s11 = "a-b-c-d-e-f";
        String[] s11s = s11.split("-");
        System.out.println(Arrays.toString(s11s));

        String s12 = "abc";
        System.out.println(s12.toUpperCase());

        String s14 = "     123    ";
        System.out.println(s14.trim());

        String s15 = "Abc";
        System.out.println(s15.toString());

        System.out.println(String.valueOf(false));

    }
}

9.StringBuilder

package com.msb;

/**
 * @Auther: jack.chen
 * @Date: 2023/9/18 - 09 - 18 - 20:51
 * @Description: com.msb
 * @version: 1.0
 */
public class StringBuilderTest {
    public static void main(String[] args) {
        StringBuilder sb = new StringBuilder("nihaojavawodeshijie");

        sb.append("这是梦想");
        System.out.println(sb);

        sb.delete(3, 6);
        System.out.println(sb);

        sb.deleteCharAt(0);
        System.out.println(sb);

        StringBuilder s2 = new StringBuilder("23445980947");
        s2.insert(3, ",");
        System.out.println(s2);

        StringBuilder s3 = new StringBuilder("$2你好吗5980947");
        s3.replace(3, 5, "我好累");
        System.out.println(s3);

        s3.setCharAt(3, '!');
        System.out.println(s3);

        StringBuilder sb3 = new StringBuilder("asfda");
        for (int i = 0; i < sb3.length(); i++) {
            System.out.print(sb3.charAt(i)+"\t");
        }

        System.out.println();

        String str = sb3.substring(2, 4);//得到新的string
        System.out.println(str);
        System.out.println(sb3);

    }

}

String 不可变

StringBuilder StringBuffer 可变 append insert resvert setCharAt 改变内容 可以 使用toString

StringBuilder 效率高 线程不安全

StringBuffer 效率低 线程安全