Java基础⑧——常用类

String类

不可变字符串

创建

pubilc class test{
    public static void main(String[] args) {
        String a = "hello world";
		String b = new String("hello world");
		System.out.println(b.equals(a));
    }
}

常用方法

public class test{
    public static void main(String[] args) {
        //获取指定下标的字符
        String a = "hello";
        System.out.println(a.charAt(0));//h
        
        //连接字符
        String b = a.concat("world");
        System.out.println(b);//helloworld

        //检测是否包含指定字符串
		System.out.println(a.contains("lo"));//true

		//将字符串转成数组返回
		char[] c = a.toCharArray();
		System.out.println(Array.toString(c));

		//求字符串长度
		System.out.println(a.length());//5

		//输出指定字符串的下标
		System.out.println(a.indexOf("ll"));//2

		//小写转换成大写
		System.out.println(a.toUpperCase());//HELLO
		
		//判断是否以指定字符开始
		System.out.println(a.startsWith("h")); //true
		System.out.println(a.startsWith("hello"));//true

		//去除左右空格
		System.out.println("  h e ".trim()); h e

		//替换字符
		String d = "严禁黄赌毒";
		System.out.println(d.replace("黄","*"));//严禁*赌毒

		//按指定字符串进行分隔，返回字符串数组
		String[] e = d.split("*");
		System.out.println(Arrays.toString(e));
    }
}

可变字符串

StringBuffer——加锁，安全，性能低

StringBuilder——无锁，不安全，性能高

public class test {
    public static void mian(String[] args) {
        //字符串创建
        StringBuilder a = new StringBuilder("hello");
        a.append(" world");
        System.out.println(a);//hello world

        //删除指定开始下标和结束下标字符串，不包括结束下标
        System.out.println(a.delete(1,4));//ho world
    }
}

Math类

public class test {
    public static void main(String[] args) {
        System.out.println(Math.ceil(1.1));  //2.0  向上取整
        System.out.println(Math.ceil(1.9));  //2.0
        
	    System.out.println(Math.floor(1.1)); //1.0 向下取整
        System.out.println(Math.floor(1.9)); //1.0
		
        System.out.println(Math.round(1.4)); //1  四舍五入
        System.out.println(Math.round(1.5)); //2
    }
}

Random类

import java.util.Rndom;
public class test {
    public static void main(String[] args) {
        Random random = new Random();
        for(int i=0;i<8;i++) {
            System.out.print(random.nextInt(10)+"\t");
            //0	 1	 2	 9 	 1 	 0	 2	 9
        }
    }
}

日期类

import java.util.Date;
public class test {
    pubilc static void main(String[] args) {
        Date date = new Date();
        System.out.println(date);
        //Sun Jul 17 12:40:58 GMT+08:00 2022
        
        Date date1 = new Date(date.getTime());
        System.out.println(date1);
        //Sun Jul 17 12:40:58 GMT+08:00 2022
    }
}

日历类

import java.util.Calendar;

//日历类： abstract抽象类     用于求时间的类
public class test {
	public static void main(String[] args) {
		Calendar calendar = Calendar.getInstance();
		
		System.out.println(calendar.get(Calendar.YEAR));  //2022
		System.out.println(calendar.get(Calendar.MONTH)+1); //7
		System.out.println(calendar.get(Calendar.DAY_OF_MONTH)); //17
		
		calendar.set(calendar.YEAR, 2050);  //设置年份
		System.out.println(calendar.get(Calendar.YEAR));  //2050
		
		calendar.add(Calendar.YEAR, 20);  //给年份增加数量
		System.out.println(calendar.get(Calendar.YEAR));  //2070
		
	}
}