[Java知识库]Java常用类

常用类

包装类Wrapper的分类

包装类和基本数据的转换

01:

package TryCatchExercise;

public class WrapperDemo {
    public static void main(String[] args) {


        //手动装箱
      int n1 = 100;
        Integer integer = new Integer(n1);
        Integer integer1 = Integer.valueOf(n1);

        //手动拆箱
        int i = integer.intValue();

        //JKD5后，自动装箱
        int n2 = 200;
        Integer integer2 = n2;//底层使用的是 Integer.valueOf(n2)

        //自动拆箱
        int n3 = integer2;//底层仍然使用的是 intValue()方法

    }
}

小练习

包装类型和String类型的相互转换

01:

package TryCatchExercise;

public class WrapperVSString {
    public static void main(String[] args) {
        //包装类(Integer)->String
        Integer i = 100;
        //1.
        String str1 = i+"";

        //2.
        String str2 = i.toString();

        //3.
        String str3 = String.valueOf(i);

        //String -> Integer
        //1.
        String str4 = "12345";
        Integer i1 = Integer.parseInt(str4);

        //2.
        Integer integer = new Integer(str4);
        
        
    }
}

Integer类和Character类的常用方法

小练习

01:

package AbstractDemo01;

public class Test {
    public static void main(String[] args) {
        Integer i = new Integer(1);
        Integer j = new Integer(1);
        System.out.println(i==j);

        //底层Integer.valueOf()
        Integer n = 1;
        Integer m = 1;
        System.out.println(n==m);

        //底层Integer.valueOf()
        Integer x = 128;
        Integer y = 128;
        System.out.println(x==y);

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

//        false
//        true
//        false
    }
}

02：

String类的理解和创建对象

两种创建String对象的区别

字符串的特性

String类的常用方法

01:

package StringDemo01;

import java.util.Locale;

public class StringMethod01 {
    public static void main(String[] args) {
        String str1 = "hello";
        String str2 = "Hello";
        System.out.println(str1.equals(str2));//false


        String username = "join";

        if ("join".equalsIgnoreCase(username))
        {
            System.out.println("join yes");//join yes
        }
        else
        {
            System.out.println("join no");
        }

        String username2 = "JoIn";

        if ("join".equalsIgnoreCase(username2))
        {
            System.out.println("join yes");//join yes
        }
        else
        {
            System.out.println("join no");
        }


        System.out.println("jack".length());//4


        String s1 = "wer@terwe@g";
        int idx = s1.indexOf('@');
        System.out.println(idx);//3

        s1 = "wer@asdasda@ad";
        idx = s1.lastIndexOf('@');
        System.out.println(idx);//11

        String name = "jackioio";
        System.out.println(name.substring(6));//io

        System.out.println(name.substring(0,3));//jac

        String s = "hello";
        System.out.println(s.toUpperCase());//HELLO

        System.out.println(s.toLowerCase());//hello

        String ss1 = "baoyu";
        ss1 = s.concat("jack").concat("jxuad").concat("dasda");
        System.out.println(ss1);//hellojackjxuaddasda

        ss1 = "jack and mike jack";
        ss1 = ss1.replace("jack","tom");
        System.out.println(ss1);//tom and mike tom


        String sss1 = "jack,miek,lll";
        String[] split = sss1.split(",");

        for (String v:split)
        {
            System.out.println(v);
//            jack
//                    miek
//            lll
        }


        String hhh = "hhkad\\asdas\\adas";
        String[] hhhs = hhh.split("\\\\");
        for (String v:hhhs)
        {
            System.out.println(v);

//            hhkad
//                    asdas
//            adas
        }

        String h1 = "dafsfsdf";
        char [] chs = h1.toCharArray();
        for (int i = 0;i< h1.length();i++)
        {
            System.out.println(chs[i]);
        }

        //1.如果长度相同，并且每个字符也相同，返回0
        //2.如果长度相同或者不相同，但是在进行比较时，可以区分大小就返回
//        if (c1 != c2)
//        {
//            return c1-c2;
//        }

        //3.如果前面的部分都相同，就返回str1.len-str2.len




        String a = "jack";
        String b = "apple";

        System.out.println(b.compareTo(a));//-9

        a = "jack1";
        b = "jack111";
        System.out.println(b.compareTo(a));//2

        String studentname = "john";
        int age = 10;
        double score = 98.3/3;
        char gender = '男';
        String info = "My name is"+name+"age = "+age+"score = "+score+"gender =  "+gender+"I want to make eyeryone happy";

        String formatStr = "my name is%s age = %d score = %.2f gender = %c ";

        String info2 =   String.format(formatStr,name,age,score,gender);
        System.out.println(info);

        System.out.println(info2);

//        My name isjackioioage = 10score = 32.766666666666666gender =  男I want to make eyeryone happy
//        my name isjackioio age = 10 score = 32.77 gender = 男

    }
}

StringBuffer类基本介绍

String VS StringBuffer

StringBuffer的构造器

01：

package StringBufferDemo01;

public class StringBufferDemo {
    public static void main(String[] args) {


        //1.创建一个大小为16的char[],用于存放字符内容
        StringBuffer stringBuffer = new StringBuffer();

        //2.通过构造器指定char[]大小
        StringBuffer stringBuffer1 = new StringBuffer(100);

        //3.通过给一个String创建StringBuffer,char[]大小就是str.length()+16
        String hello = new String("hello");


        //String  ->  StringBuffer

        String str = "hello tom";
        //1.使用构造器
        //注意:返回的才是StringBuffer对象，对str本身没有影响
        StringBuffer stringBuffer2 = new StringBuffer(str);


        //2.使用append方法
        StringBuffer stringBuffer3 = new StringBuffer();
        StringBuffer append = stringBuffer3.append(str);


        //StringBuffer -> String
        //1.使用StringBuffer的toString方法
        StringBuffer hspedu = new StringBuffer("hspedu");
        String s = hspedu.toString();

        //2.使用构造器
        String s1 = new String(stringBuffer3);



    }
}

StringBuffer类常见方法

01：

package StringBufferMethod;

public class StringBufferMethod {
    public static void main(String[] args) {


        StringBuffer hello = new StringBuffer("hello");

        //增
        hello.append(',');
        hello.append("zsf");
        hello.append("zhaom").append(100).append(true).append(10.5);
        System.out.println(hello);

        //删
        hello.delete(11,14);//[11,14)
        System.out.println(hello);

        //改
        hello.replace(9,11,"jack");//[9,11)
        System.out.println(hello);

        //插
        hello.insert(9,"mike");//插在坐标为9的前面
        System.out.println(hello);

        int length = hello.length();
        System.out.println(length);


    }
}

小练习

01:

02:

package StringBufferMethod;

public class StringBufferExercise01 {
    public static void main(String[] args) {

        String price = "123565467894.59";

        StringBuffer sb = new StringBuffer((price));

//        int i = sb.lastIndexOf(".");
//        sb = sb.insert(i-3,",");
//        System.out.println(sb);

        for (int i = sb.lastIndexOf(".");i-3 >0;i-=3)
        {
            sb = sb.insert(i-3,",");
        }

        System.out.println(sb);


    }
}

StringBuilder类结构剖析

Math类

01：

package StringBufferMethod;

public class MathMethod {
    public static void main(String[] args) {

        //1.abs 绝对值
        int abs = Math.abs(9);
        System.out.println(abs);


        //2.pow 求幂
        double pow = Math.pow(2,4);
        System.out.println(pow);

        //3.ceil 向上取整,返回>=该参数的最小整数
        double ceil = Math.ceil(-3.0001);
        System.out.println(ceil);

        //4.floor 向下取整,返回<=该参数的最大整数
        double floor = Math.floor(-4.999);
        System.out.println(floor);

        //5.round 四舍五入 Math.floor(该参数+0.5)
        long round = Math.round(-5.001);
        System.out.println(round);

        //6.sqrt 求开方
        double sqrt = Math.sqrt(9.0);
        System.out.println(sqrt);


        //7.random 求随机数
        //返回的是0<=x < 1之间的一个随机小数
        for (int i  =0;i<10;i++)
        {
            System.out.println(Math.random());
        }

        //思考:请写出获取2-7之间的一个随机整数
        //Math.random()*(b-a) 返回的就是0<=数<b-a
        //(1)(int)(a) <= x <= (int)(a+Math.random()*(b-a+1));


        int num1 = Math.max(1,2);
        System.out.println(num1);



    }
}

Arrays类

01:

package AbstractDemo01;

import java.util.Arrays;
import java.util.Comparator;

public class ArraysMethod01 {
    public static void main(String[] args) {
        Integer[] integers = {1,2,3};

        for (int i  =0;i<integers.length;i++)
        {
            System.out.println(integers[i]);
        }

        System.out.println(Arrays.toString(integers));


        Integer arr[] = {1,-1,7,0,89};
        Arrays.sort(arr);
        System.out.println(Arrays.toString(arr));

        Arrays.sort(arr, new Comparator<Integer>() {
            @Override
            public int compare(Integer o1, Integer o2) {
                return o2-o1;
            }
        });

        System.out.println(Arrays.toString(arr));

        
    }
}

自己简单实现一波sort

02：

package AbstractDemo01;

import java.util.Arrays;
import java.util.Comparator;

public class ArraysSortCustom {
    public static void main(String[] args) {
        int[] arr = {1,-1,8,0,20};
        bubble01(arr);
        System.out.println(Arrays.toString(arr));

        bubble02(arr, new Comparator() {
            @Override
            public int compare(Object o1, Object o2) {
                 int o11 = (Integer) o1;
                int o21 = (Integer) o2;

                return o21-o11;

            }
        });

        System.out.println(Arrays.toString(arr));

    }


    public static void bubble01(int [] arr)
    {
        int tmp = 0;
        for (int i = 0;i<arr.length;i++)
        {
            for (int j = 0;j<arr.length-1-i;j++)
            {
                if (arr[j] > arr[j+1])
                {
                    tmp = arr[j+1];
                    arr[j+1] = arr[j];
                    arr[j] = tmp;
                }
            }
        }
    }


    public static void bubble02(int[] arr, Comparator c)
    {
        int tmp = 0;
        for (int i = 0;i<arr.length;i++)
        {
            for (int j = 0;j<arr.length-1-i;j++)
            {
                if (c.compare(arr[j],arr[j+1]) > 0)
                {
                    tmp = arr[j+1];
                    arr[j+1] = arr[j];
                    arr[j] = tmp;
                }
            }
        }
    }

}

03:

import java.util.Arrays;

public class SortDemo01 {
    public static void main(String[] args) {
        //要求有序
        Integer[] arr = {1,2,3,4,5,6,6786,56734,322445};

        //找不到返回 -(low+1);
        int idx = Arrays.binarySearch(arr,1);
        System.out.println(idx);//0

        int idx02 = Arrays.binarySearch(arr,92);
        System.out.println(idx02);//-7

    }
}

04：

import java.util.Arrays;

public class SortDemo01 {
    public static void main(String[] args) {

        //如果拷贝长度>arr.length,就在新数组后面增加null
        //如果拷贝长度<0 就抛出异常NegativeArraySizeException
        //该方法的底层使用的是 System.arraycopy()

        Integer[] arr = {1,2,3,4,5,6,6786,56734,322445};
        System.out.println(Arrays.toString(arr));
        System.out.println("==============================================");

       Integer[] newArray = Arrays.copyOf(arr,arr.length);
        System.out.println(Arrays.toString(newArray));

        System.out.println("==============================================");

        Integer[] newArray01 = Arrays.copyOf(arr,arr.length-1);
        System.out.println(Arrays.toString(newArray01));


        System.out.println("==============================================");

        Integer[] newArray02 = Arrays.copyOf(arr,arr.length+1);
        System.out.println(Arrays.toString(newArray02));

//        [1, 2, 3, 4, 5, 6, 6786, 56734, 322445]
//==============================================
//[1, 2, 3, 4, 5, 6, 6786, 56734, 322445]
//==============================================
//[1, 2, 3, 4, 5, 6, 6786, 56734]
//==============================================
//[1, 2, 3, 4, 5, 6, 6786, 56734, 322445, null]

    }
}

05:

import java.util.Arrays;
import java.util.List;

public class SortDemo01 {
    public static void main(String[] args) {

      Integer[] arr = new Integer[]{9,3,2};
      Arrays.fill(arr,99);
        System.out.println(Arrays.toString(arr));
        //[99, 99, 99]

        Integer[] arr01 = {1,2,3,124,2,423,4,23};
        Integer[] arr02 = {1,3,123,12421,41,3,14};

        boolean equals = Arrays.equals(arr01,arr02);
        System.out.println(equals);//false

        Integer[] arr03 = {1,2,3};
        Integer[] arr04 = {1,2,3};

        boolean equals01 = Arrays.equals(arr03,arr04);
        System.out.println(equals01);//true

        List<Integer> asList = Arrays.asList(2,3,4,5,6,8);
        System.out.println(asList);
        //[2, 3, 4, 5, 6, 8]
        
        
        //asList 运行类型 java.util.Arrays#ArrayList,是Arrays类的静态内部类
        
        //private static class ArrayList<E> extends AbstractList<E> implements RandomAccess,java.io.Serializable
        
 System.out.println(asList.getClass());//class java.util.Arrays$ArrayList
    }
}

小练习

01:

package AbstractDemo01;


import java.util.Arrays;
import java.util.Comparator;

public class ArrayExercise {
    public static void main(String[] args) {
        Book[] books = new Book[4];
        books[0] = new Book("red dream",100);
        books[1] = new Book("gold new ",90);
        books[2] = new Book("youth composition 20",5);
        books[3] = new Book("java from  enter door to give up",300);

        //price 从大到小
        Arrays.sort(books, new Comparator<Book>() {
            @Override
            public int compare(Book o1, Book o2) {
               double priceVal = o2.getPrice()-o1.getPrice();
               if (priceVal > 0)
               {
                   return 1;
               }
               else if (priceVal < 0)
               {
                   return -1;
               }
               else return 0;
            }
        });

        System.out.println(Arrays.toString(books));


        Arrays.sort(books, new Comparator<Book>() {
            @Override
            public int compare(Book o1, Book o2) {
                double priceVal = o2.getPrice()-o1.getPrice();
                if (priceVal > 0)
                {
                    return -1;
                }
                else if (priceVal < 0)
                {
                    return 1;
                }
                else return 0;
            }
        });

        System.out.println(Arrays.toString(books));


        Arrays.sort(books, new Comparator<Book>() {
            @Override
            public int compare(Book o1, Book o2) {
                return o2.getName().length()-o1.getName().length();
            }
        });

        System.out.println(Arrays.toString(books));

    }
}


class Book
{
    private String name;
    private double price;

    public Book(String name, double price) {
        this.name = name;
        this.price = price;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public double getPrice() {
        return price;
    }

    public void setPrice(double price) {
        this.price = price;
    }

    @Override
    public String toString() {
        return "Book{" +
                "name='" + name + '\'' +
                ", price=" + price +
                '}';
    }
}

System类

01：

package AbstractDemo01;

public class SystemTestDemo {
    public static void main(String[] args) {
        System.out.println("ok1");

        //1.exit(0)表示程序退出
        //2.0表示一个状态，正常状态
        System.exit(0);
        System.out.println("ok2");
        
        //ok1
    }
}

02:

package AbstractDemo01;

import java.util.Arrays;

public class SystemTestDemo {
    public static void main(String[] args) {

        int[] src = {1,2,3};
        int[] dest = new int[3];



        //参数:
        //src – the source array.
        //srcPos – starting position in the source array.
        //dest – the destination array.
        //destPos – starting position in the destination data.
        //length – the number of array elements to be copied.

        System.arraycopy(src,0,dest,0,src.length);

        System.out.println(Arrays.toString(dest));
        //[1, 2, 3]

    }
}

BigInteger和BigDecimal类

01:

package AbstractDemo01;

import java.math.BigInteger;

public class BigIntegerDemo {
    public static void main(String[] args) {
        //long l = 456789556756734657894678768l;//ERROR 整数过大!!!

        BigInteger bigInteger = new BigInteger("456789556756734657894678768");

        BigInteger bigInteger1 = new BigInteger("100");
        System.out.println(bigInteger);//456789556756734657894678768


        BigInteger add = bigInteger.add(bigInteger1);
        System.out.println(add);

        BigInteger subtract = bigInteger.subtract(bigInteger1);
        System.out.println(subtract);

        BigInteger multiply = bigInteger.multiply(bigInteger1);
        System.out.println(multiply);

        BigInteger divide = bigInteger.divide(bigInteger1);
        System.out.println(divide);
    }
}

02:

package BigDemo;

import java.math.BigDecimal;

public class BigDemo {
    public static void main(String[] args) {

        double d = 893.2134524351235424364325235;
        System.out.println(d);//893.2134524351235

        BigDecimal bigDecimal = new BigDecimal("893.2134524351235424364325235");
        System.out.println(bigDecimal);//893.2134524351235424364325235

        BigDecimal bigDecimal1 = new BigDecimal("1.1");
        System.out.println(bigDecimal.add(bigDecimal1));

        System.out.println(bigDecimal.subtract(bigDecimal1));

        System.out.println(bigDecimal.multiply(bigDecimal1));

        //System.out.println(bigDecimal.divide(bigDecimal1));
        //除不尽，抛出异常
        //Exception in thread "main" java.lang.ArithmeticException: Non-terminating decimal expansion; no exact representable decimal result.
        //	at java.base/java.math.BigDecimal.divide(BigDecimal.java:1715)
        //	at BigDemo.BigDemo.main(BigDemo.java:21)


        //如果有无限循环小数，就会保留分子的精度
        System.out.println(bigDecimal.divide(bigDecimal1,BigDecimal.ROUND_CEILING));
    }
}

日期类

Date类

01：

package DateDemo01;

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

public class Date01 {
    public static void main(String[] args) throws ParseException {
        Date d1 = new Date();
        System.out.println(d1);//Sat Sep 25 10:56:55 CST 2021

        SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyy年MM月dd日 hh:mm:ss E");
        String format = simpleDateFormat.format(d1);
        System.out.println(format);//2021年09月25日 10:56:55 周六

        Date date = new Date(9234567);//通过指定毫秒数得到时间
        System.out.println(date);//获取某个时间对应的毫秒数
        //Thu Jan 01 10:33:54 CST 1970

        String s = "1996年01月01日 10:20:30 星期一";

        Date parse = simpleDateFormat.parse(s);//Mon Jan 01 10:20:30 CST 1996

        System.out.println(parse);

        System.out.println(simpleDateFormat.format(parse));//1996年01月01日 10:20:30 周一
        
        

    }
}

Calendar类

第三代日期

01:

package DateDemo01;

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

public class LocalDateDemo01 {
    public static void main(String[] args) {
        LocalDateTime now = LocalDateTime.now();
        System.out.println(now);

        System.out.println(now.getYear());//年
        System.out.println(now.getMonth());//月
        System.out.println(now.getMonthValue());//月

        System.out.println(now.getDayOfMonth());//日
        System.out.println(now.getHour());//时
        System.out.println(now.getMinute());//分
        System.out.println(now.getSecond());//秒

        LocalDate now1 = LocalDate.now();//可以获取年月日
        System.out.println(now1.getYear());

        LocalTime now2 = LocalTime.now();//可以获取时分秒
        System.out.println(now.getSecond());


    }
}

DateTimeFormatter

01:

package DateDemo01;

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

public class LocalDateDemo02 {
    public static void main(String[] args) {
        LocalDateTime ldt = LocalDateTime.now();
        System.out.println(ldt);

        DateTimeFormatter dateTimeFormatter = DateTimeFormatter.ofPattern("yyyy年MM月dd日 HH小时mm分钟ss秒");
        String format = dateTimeFormatter.format(ldt);
        System.out.println("date = "+format);

        
    }
}

Instant时间戳

01:

package DateDemo01;


import java.time.Instant;
import java.util.Date;

public class InstantDemo01 {
    public static void main(String[] args) {

        Instant now = Instant.now();
        System.out.println(now);

        Date date = Date.from(now);
        Instant instant = date.toInstant();
    }
}

01:

package DateDemo01;

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

public class DateDemo {
    public static void main(String[] args) {
        LocalDateTime now = LocalDateTime.now();

        DateTimeFormatter dateTimeFormatter = DateTimeFormatter.ofPattern("yyyy年MM月dd日 HH小时mm分钟ss秒");


        LocalDateTime plus = now.plusDays(890);//890天后
        System.out.println(dateTimeFormatter.format(plus));

        LocalDateTime localDateTime = now.minusMinutes(3456);//3456分钟前
        System.out.println(dateTimeFormatter.format(localDateTime));
    }
}

小练习

01：

package HomeWorkD01;

public class HomeWork01 {
    public static void main(String[] args) {

        String str = "abcdef";
        try {
            str = reverse(str, 1, 4);
        } catch (Exception e) {
            System.out.println(e.getMessage());
            return;
        }
        System.out.println(str);

    }


    public static String reverse(String str,int start,int end)
    {

        if (!(str!=null && start >= 0 && end > start && end <str.length()))
        {
            throw new RuntimeException("参数不正确");
        }

        char[] chars = str.toCharArray();
        char temp = ' ';
        for (int i = start,j = end;i<j;i++,j--)
        {
            temp = chars[i];
            chars[i] = chars[j];
            chars[j] = temp;
        }

        return new String(chars);
    }

}

02:

package HomeWorkD01;

public class HomeWork02 {
    public static void main(String[] args) {


        String name = "tom";
        String pwd = "123456";
        String email = "tom@shouhu.com";

        try {
            userRegister(name,pwd,email);
            System.out.println("OK");
        } catch (Exception e) {

            System.out.println(e.getMessage());
        }


    }

    public static void userRegister(String name,String pwd,String email)
    {
        
        if (!(name!=null && pwd !=null && email !=null))
        {
            throw new RuntimeException("Don't NULL");
        }
        
        int userLength = name.length();
        if (!(userLength >= 2 && userLength <= 4))
        {
            throw new RuntimeException("username length should be between 2 to 4");

        }

        if (!(pwd.length()==6 && isDigital(pwd)))
        {
            throw new RuntimeException("Your  password length should be equals to 6");
        }

        int i = email.indexOf('@');
        int j = email.indexOf('.');
        if ((i > 0 && j > i))
        {
            throw new RuntimeException("Error");
        }
    }


    public static boolean isDigital(String str)
    {
        char[] chars = str.toCharArray();
        for (int i = 0;i<chars.length;i++)
        {
            if (chars[i] < '0' || chars[i] > '9')
            {
                return false;
            }
        }
        return true;
    }

}

03:

package HomeWorkD01;

import java.util.Locale;

public class HomeWork03 {
    public static void main(String[] args) {


        String name = "Han shun Ping";
        printName(name);
    }

    public static void printName(String str)
    {
        if (str==null)
        {
            System.out.println("str don't null");
            return;
        }

        String[] names = str.split(" ");
        if (names.length!=3)
        {
            System.out.println("input format is error");
            return ;
        }

        String format = String.format("%s,%s.%c", names[2], names[0], names[1].toUpperCase().charAt(0));
        System.out.println(format);


    }
}

04:

package HomeWorkD01;

public class HomeWork04 {
    public static void main(String[] args) {
        String str = "absaf 678787 asda HUKJANJKDjkj";
        countStr(str);
    }

    public static void countStr(String str)
    {
        if (str==null)
        {
            System.out.println("input error");
            return ;
        }

        int numCnt = 0;
        int lowerCnt = 0;
        int upperCnt = 0;
        int otherCnt = 0;
        int strlen = str.length();
        for (int i = 0;i<strlen;i++)
        {
            if (str.charAt(i) >= '0' && str.charAt(i) <='9')
            {
                numCnt++;
            }
            else if (str.charAt(i)>='a' && str.charAt(i) <='z')
            {
                lowerCnt++;
            }
            else if (str.charAt(i) >= 'A' && str.charAt(i) <='Z')
            {
                upperCnt++;
            }
            else
            {
                otherCnt++;
            }
        }

        System.out.println(numCnt+" "+lowerCnt+" "+upperCnt);
    }
}