JAVA常用类

JAVA常用类,第1张

JAVA常用类 JAVA常用类 一、String类 1.String的不可变性
 
package Test01;

import org.junit.Test;

public class StringTest1 {
    @Test
    public  void test1(){

        String s1="fan";
        String s2="fan";
        s1="FYX";
        System.out.println (s1);
        System.out.println (s2);
        System.out.println ("**************");
        String s3="fan123";
        String s4="fan123";
        s3+="1116";
        System.out.println (s3);
        System.out.println (s4);
        System.out.println ("**************");
    //注意:replace换完后,必须有一个新的字符串去接收
        String s7 = s5.replace ("fan", "YYY");
        System.out.println (s7);
        System.out.println (s6);
        System.out.println ("**************");
    }
}
2.string 的实例化
   
    @Test
    public void test2(){
   //通过字面量定义的方式:此时的s1和s2的数据javaEE声明在方法区中的字符串常量池中。
        String s1="fan";
        String s2="fan";
   //通过new +构造器的方式:此时的s3和s4保存的地址值,是数据在堆空间中开辟空间以后对
        String s3=new String ("fan");
         String s4=new String ("fan");
        System.out.println (s1==s2);//true
        System.out.println (s1==s3);//false
        System.out.println (s3==s4);//false

        System.out.println ("**********");
        Person p1=new Person ("Tom",18);
       Person p2=new Person ("Tom",20);
        System.out.println (p1.name.equals (p2.name));//true
        System.out.println (p1.name==p2.name);//true


    }
3.string类不同拼接的对比
@Test
public  void test3(){
    String s1="Tom";
    String s2="cat";
    String s3="Tomcat";
    String s4="Tom"+"cat";
    String s5=s1+"cat";
    String s6="Tom"+s2;
    String s7=s1+s2;

    System.out.println (s3==s4);//true
    System.out.println (s3==s5);//false
    System.out.println (s3==s7);//false
    System.out.println (s5==s6);//false
    System.out.println (s5==s7);//false

    String s8 = s5.intern ();///返回值得到的s8使用的常量值中已经存在的"Tomcat"
    System.out.println (s8==s3);//true

}
4.string类面试题
package Test01;

public class StringTest2 {
    String str=new String ("good");
    char[] ch={'t','e','s','t'};

    public  void change(String str,char ch[]){
        str="test ok";
        ch[0]='b';

    }

    public static void main(String[] args) {
        StringTest2 stringTest2=new StringTest2 ();
        stringTest2.change (stringTest2.str,stringTest2.ch);
        System.out.println (stringTest2.ch);//best
        System.out.println (stringTest2.str);//good
    }
}
5.string类的常用方法 常用方法一:
 
package Test01;


import org.junit.Test;

public class StringTest3 {
    @Test
    public  void  test1(){

        String s1="HelloWord";
        System.out.println (s1.length ());//长度
        System.out.println (s1.charAt (5));//返回特定位置元素的信息
        System.out.println (s1.isEmpty ());//判断是否为空
        System.out.println (s1.toLowerCase ());//全部换成小写
        System.out.println (s1.toUpperCase ());//全部换成大写
        System.out.println ("***************");
        String s2="    Hello    word     ";
        System.out.println ("——"+s2+"——");
        System.out.println ("——"+s2.trim ()+"——");//忽略字符串前面和尾部空白
        System.out.println ("***************");
      String s3="helloword";
        System.out.println (s1.equals (s3));  //判断字符串是否相等
        System.out.println (s1.equalsIgnoreCase (s3));//不论大小写,判断字符串是否相等
        System.out.println (s1.concat ("    haha"));//连接其他字符串或者字符
        System.out.println (s3.substring (5));//从特定位置开始输出
        System.out.println (s3.substring (0,5));//从特定位置i到特定位置j


    }
}
常用方法二:
@Test
public  void test2(){
    String s1="HelloWord";
    System.out.println (s1.endsWith ("ord"));//后缀是否是以str结束
    System.out.println (s1.startsWith ("He"));//是否是以str开头
    System.out.println (s1.startsWith ("lo",3));//指定位置i是否是以str开头
    System.out.println (s1.contains ("ord"));//字符串中是否包含str
    System.out.println (s1.indexOf ("lo"));//返回str在字符串中第一次出现的位置
    System.out.println (s1.indexOf ("lo",2));//从i位置开始寻找str第一次出现的位置
    System.out.println ("*************");
    System.out.println (s1.lastIndexOf ("or"));//从末尾反向搜索
    System.out.println (s1.lastIndexOf ("or",7));//从i位置反向搜索
//    注意:indexOf和lastIndexOf方法如果未找到都是返回-1
}
常用方法三:
@Test
public  void  test3(){

String s1="易烊千玺超级无敌巨巨巨巨无霸帅";
    System.out.println (s1.replace ("帅","棒"));
    System.out.println (s1.replaceAll ("巨",""));
    System.out.println (s1.replaceFirst ("易烊千玺","霸王花"));
    System.out.println ("************************");
    

}
6.string类与包装类的转换
//string 与基本数据类型、包装类之间的转换。|
//String -->基本数据类型、包装类:调用包装类的静态方法: parseXxx( str)
// 基本数据类型、包装类-->string:调用string重载的vaLue0f(xxx)
public class StringTest4 {
   @Test
   public void test(){
       String s1="123";
       //String -->基本数据类型、包装类:调用包装类的静态方法: parseXxx( str)
       int i = Integer.parseInt (s1);
       System.out.println (i);
       System.out.println ("*************");
       // 基本数据类型、包装类-->string:调用string重载的vaLue0f(xxx)

       String s = String.valueOf (i);//写法一
       System.out.println (s);

       String s2=""+i;//写法二
       System.out.println (s2);
   }
}
7.string类与char[]的转换
//string类与char[]的转换
//string --> char[]:调用string的toCharArray()
// char[] --> string:调用string的构造器
@Test
public  void test2(){

    String s1="fan1116";
    char[] chars = s1.toCharArray ();
    for(int i=0;i 
8.string与byte[]之间的转换 
@Test
public  void test3() {
  String s1="fan霸王花";
    byte[] bytes = s1.getBytes ();
    System.out.println (Arrays.toString (bytes));
    System.out.println ("**************");
       String s2=new String (bytes);
    System.out.println (s2);
}
9.练习题:
 
10.StringBuffer和StringBuilder类 
 
11.StringBuffer的常用方法 
 
@Test
    public  void test1(){
        StringBuffer s1=new StringBuffer ("易烊千玺超级无敌巨无霸帅!");
        System.out.println (s1);
        System.out.println ( s1.append ("哈哈哈哈"));;
        System.out.println (s1.replace (12,18,"啊对对对"));;
        System.out.println (s1.reverse ());
        s1.reverse ();
        System.out.println (s1.length ());
        System.out.println ( s1.substring (0,12));
        System.out.println (s1.charAt (11));


    }
}
12.三者效率比较

对比string,StringBuffer.StringBuilder三者的效率
–>从高到低排列: StringBuilder > stringBuffer > string

13.System类中获取时间的方法

//System类中的currentTimeMillis()
package Test03;

import org.junit.Test;

import java.util.Date;

public class TimeTest {

@Test
public  void  test2(){
    //构造器一: Date():创建一个对应当前时间的Date对象,>toString():显示当前的年、月、日、时、分、秒
 Date date=new Date();
    System.out.println (date.toString ());//Fri Dec 03 16:25:20 CST 2021
    System.out.println (date.getTime ());//1638519995369
 //构造器二:创建指定毫秒数的Date对象
   Date date1=new Date (1638519995369L);
    System.out.println (date1.toString ());//Fri Dec 03 16:25:20 CST 2021
    System.out.println (date1.getTime ());//1638519995369

}
    //1.System类中的currentTimeMillis()
@Test
    public  void test1(){
    //返回当前时间与1970年1月1日0时e分日秒之间以毫秒为单位的时间差。
    long time = System.currentTimeMillis ();
    System.out.println (time );
}

}

二、时间类
 
1. System类中currentTimeMillis() 
    //1.System类中的currentTimeMillis()
@Test
    public  void test1(){
    //返回当前时间与1970年1月1日0时e分日秒之间以毫秒为单位的时间差。
    long time = System.currentTimeMillis ();
    System.out.println (time );
}
2.Date类
@Test
public  void  test2(){
    //构造器一: Date():创建一个对应当前时间的Date对象,>toString():显示当前的年、月、日、时、分、秒
 Date date=new Date();
    System.out.println (date.toString ());//Fri Dec 03 16:25:20 CST 2021
    System.out.println (date.getTime ());//1638519995369
 //构造器二:创建指定毫秒数的Date对象
   Date date1=new Date (1638519995369L);
    System.out.println (date1.toString ());//Fri Dec 03 16:25:20 CST 2021
    System.out.println (date1.getTime ());//1638519995369

}
3.SimpleDateFormat类
package other;

import org.junit.Test;

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

public class SimpleTime {

    @Test
    public  void testSimpleDate() throws ParseException {
        //格式化format:
        //按照指定的方式进行格式化和解析:调用带参构造器
        SimpleDateFormat sdf = new SimpleDateFormat ("yyyy-MM-DD hh:mm:ss");
        Date date=new Date ();
        String s = sdf.format (date );
        System.out.println (s);

        //解析parse:
        // 格式化的逆过程: 字符串-->日期
        Date date2 = sdf.parse (s);
        System.out.println (date2);
    }
    
      //练习:字符串"2028-09-08"转换为java.sqL.Date
    @Test
    public  void test() throws ParseException {
        String birth="2019-11-16";
        SimpleDateFormat sdf = new SimpleDateFormat ("yyyy-MM-dd");
        //解析:
        Date date = sdf.parse (birth);
        //格式化:
        java.sql.Date birthDate = new java.sql.Date (date.getTime ());
        System.out.println (birthDate);
    }
    
        //练习二: 
    // "三天打渔两天晒网"    1990-01-01 开始打渔,问在XXXX-XX-XX时是打鱼还是晒网?
    //举例:2019-11-16 ? 是打鱼还是晒网
    //总天数% 5 == 1,2,3 ∶打渔
    // 总天数% 5 == 4,0   :晒网
    // 总天数的计算?  -->计算出来的是总天数
    //方式: ( date2.getTime() - date1.getTime())/(1000 * 60* 60* 24) +1
 @Test
    public  void test2() throws ParseException {
        String start="1990-01-01";
        String end="2019-11-16";
     SimpleDateFormat sdf = new SimpleDateFormat ("yyyy-MM-dd");
     Date date1 = sdf.parse (start);
     Date date2 = sdf.parse (end);
     long time =( (date2.getTime () - date1.getTime ()) / (1000 * 60 * 60 * 24) )+ 1;
     long l = time % 5;
      //     System.out.println (l);
      if(l==1||l==2||l==3){
          System.out.println (end+"在打渔");
      }else if(l==0||l==4){
          System.out.println ("end+在晒网");
      }
 }
}
4.Calendar(日历)类
//Calendar (抽象的)日历类---->jdk8.0以前
   @Test
   public  void CalendarTest(){
       //1.实例化 -->常用方法二
       ///方式一:创建其子类(GregorianCalendar)的对象
       // 方式二:调用其静态方法getInstance()
       Calendar calendar = Calendar.getInstance ();

       //2.常用方法  

       // get()
       int day  = calendar.get (Calendar.DAY_OF_MONTH);
       System.out.println (day);
       // set()

       calendar.set (DAY_OF_MONTH,22);
        day = calendar.get (DAY_OF_MONTH);
       System.out.println (day);

       // add()
       calendar.set (DAY_OF_MONTH,3);
        day = calendar.get (DAY_OF_MONTH);
       System.out.println (day);

       // getTime()      日历类-->Date
       Date date = calendar.getTime ();
       System.out.println (date);

       // setTime()       Date -->日历类
       Date date1 = new Date ();
       calendar.setTime (date1);
      day= calendar.get (Calendar.DAY_OF_MONTH);
       System.out.println (day);
   }
5.LocalDateTime 的使用
//LocalDate. LocalTime、LocalDateTime 的使用
//LocalDate. LocalTime使用同LocalDateTime一样
    @Test
    public void LocalDateTime(){
        //now() 获取现在是 日期+地点
        LocalDateTime localDateTime = LocalDateTime.now ();
        System.out.println (localDateTime);

        //of()   设置 日期+时间
        LocalDateTime localDateTime1 = LocalDateTime.of (2019, 11, 16, 11, 28);
        System.out.println (localDateTime);
        System.out.println (localDateTime1);
        System.out.println ("************************");

        //getXxx    获得.....  没有偏移量
        System.out.println (localDateTime.getMonthValue ());
        System.out.println (localDateTime.getDayOfMonth ());
        System.out.println ("************************");

        //withXxx    修改日期、时间------>不可变性
        LocalDateTime localDateTime2 = localDateTime.withDayOfMonth (11);
        System.out.println (localDateTime);
        System.out.println (localDateTime2);
        LocalDateTime localDateTime3 = localDateTime.withHour (5);
        System.out.println (localDateTime);
        System.out.println (localDateTime3);
        System.out.println ("************************");

        //plusXxx    给xx增加多少------>不可变性
        LocalDateTime localDateTime4 = localDateTime.plusDays (5);
        System.out.println (localDateTime);
        System.out.println (localDateTime4);

        //minusXxx    给Xxx减少多少 ------>不可变性
        LocalDateTime localDateTime5 = localDateTime.minusMinutes (20);
        System.out.println (localDateTime);
        System.out.println (localDateTime5);

    }
三、JAVA比较器
   



1.compare 接口
 
package Test03;

import org.junit.Test;

import java.util.Arrays;

public class JavaCom {
 
@Test
    public  void ComparableTest(){
    String []arr={"DD","AA","FF","CC","BB","EE"};
    Arrays.sort (arr);
    System.out.println (Arrays.toString (arr));
}

@Test
    public  void SelfComarableTest(){
    Goods []arr1=new Goods[5];
    arr1[0]=new Goods (23,"Lenovo");
    arr1[1]=new Goods (53,"dell");
    arr1[2]=new Goods (32,"HuaWei");
    arr1[3]=new Goods (12,"LuoJi");
    arr1[4]=new Goods (32,"Mi");
    Arrays.sort (arr1);
    System.out.println (Arrays.toString (arr1));
}

}
package Test03;

public class Goods implements Comparable{
    private  int  price;
    String name;

    public Goods(int price, String name) {
        this.price = price;
        this.name = name;
    }

    public int getPrice() {
        return price;
    }

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

    public String getName() {
        return name;
    }

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

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

    //重写输出的方式
    //按照商品的价格从低到高进行排序
    @Override
    public int compareTo(Object o) {
        //判断是否是个商品
        if(o instanceof Goods){
            Goods goods=(Goods)o;

           //进行价格的比较
            //方式一:
            if(this.price>((Goods) o).price){
                return  1;
            }else if(this.price<((Goods) o).price){
                return  -1;
            }else
                return this.name.compareTo (goods.name);
//           //方式二:
//     return    Double.compare (this.price,goods.price);
        }
           //如果不是个商品,则抛出异常
        throw  new RuntimeException ("传入的数据类型不一致!");
    }
}
2.Comparator
@Test
public  void ComparatorTest(){
    String []arr={"DD","AA","FF","CC","BB","EE"};
  Arrays.sort (arr, new Comparator () {
      @Override
      public int compare(String o1, String o2) {
          if(o1 instanceof String && o2 instanceof String){
              String s1=(String)o1;
              String s2=(String)o2;
              //按照从大到小的顺序排列
             return  -s1.compareTo (s2);
          }
          throw  new RuntimeException ("输入的数据类型有错误!");
      }
  });
    System.out.println (Arrays.toString (arr));
}

@Test
    public void ComparatorTest2(){
    Goods []arr1=new Goods[5];
    arr1[0]=new Goods (23,"Lenovo");
    arr1[1]=new Goods (53,"dell");
    arr1[2]=new Goods (32,"HuaWei");
    arr1[3]=new Goods (12,"LuoJi");
    arr1[4]=new Goods (32,"Mi");
    Arrays.sort (arr1, new Comparator () {
        @Override
        public int compare(Goods o1, Goods o2) {
            if(o1 instanceof  Goods && o2 instanceof  Goods){
                Goods g1=(Goods)o1;
                Goods g2=(Goods)o2;
    //指明商品比较大小的方式:按照产品名称从低到高排序,再按照价格从高到低排序
       if(g1.getName ().equals (g2.getName ())){
       return -Double.compare (g1.getPrice (),g2.getPrice ());
       }else
           return g1.getName ().compareTo (g2.getName ());
            }
           throw new RuntimeException ("传入的数据类型有误");
        }
    });
    System.out.println (Arrays.toString (arr1));

}
四、其他类 1.instant类的使用
@Test
public  void instantTest(){
    //now  输出本初子午线的时间
    Instant instant = Instant.now ();
    System.out.println (instant);

    //atOffset设置偏移量
    OffsetDateTime offsetDateTime = instant.atOffset (ZoneOffset.ofHours (8));
    System.out.println (offsetDateTime);

    //toEpochMilli 获取自1970年1月1日0时0分日秒(UTC)开始的毫秒数
    long l = instant.toEpochMilli ();
    System.out.println (l);

    //通过给定的毫秒数,获取Instant实例             -->Date(Long millis)
    Instant instant1 = Instant.ofEpochMilli (1641393066234L);
    System.out.println (instant1);
}
2.DateTimeFormatter
//类似于SimpleDateFormat类
@Test
public  void DateTimeFormatterTest(){
    //自定义的格式   ofPattern("yy-MM-dd hh:mm:ss")

    DateTimeFormatter formatter = DateTimeFormatter.ofPattern ("yy-MM-dd hh:mm:ss");
    //格式化:
    String s = formatter.format (LocalDateTime.now ());
    System.out.println (s);//22-01-22 08:22:58
    //解析:
    TemporalAccessor parse = formatter.parse ("22-01-22 08:22:58");
    System.out.println (parse);//{MinuteOfHour=22, SecondOfMinute=58, MicroOfSecond=0, NanoOfSecond=0, MilliOfSecond=0, HourOfAmPm=8},ISO resolved to 2022-01-22
}

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