Java常用类（十五）

Java常用类（十五）

String

package com.importc;

public class stringDemo01 {
    public static void main(String[] args) {
        String s1 = "Hello";
        String s2 = "HELLO";
        //比较两个字符串是否相同
        System.out.println(s1.equals(s2)); //false
        //忽略大小写比较
        System.out.println(s1.equalsIgnoreCase(s2)); //true
        //是否包含字符串
        System.out.println(s1.contains("ll")); //true
        //开头，结尾，下标
        System.out.println(s1.indexOf("He")); //0
        System.out.println(s1.lastIndexOf("l")); //3
        System.out.println(s1.startsWith("He")); // true
        System.out.println(s1.endsWith("lo")); //true
        //提取字符串
        System.out.println(s1.substring(2)); //llo
        System.out.println(s1.substring(2, 4)); //ll 闭开
        //去除字符串空白
        System.out.println(" Hello\n".trim());
        //判断是空还是空白字符串
        System.out.println(" ".isEmpty()); //false
        System.out.println("".isEmpty()); //true
        //替换字符串
        System.out.println(s1.replace("l", "~")); //He~~o
        //分割字符串
        String[] ss = "A,B,C,D".split(",");
        for (String s: ss) {
            System.out.print(s+" "); //A B C D
        }
        //拼接字符串 视情况使用StringJoiner
        String arr[] = {"A", "B", "C"};
        String ass2 = String.join("*", arr);
        System.out.println(ass2); //A*B*C
    }
}

StringBuilder

package com.importc;

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

        StringBuilder sb = new StringBuilder(1024);

        for (int i = 0; i < 100; i++) {
            sb.append(i);
            sb.append("-");
        }
        String s = sb.toString();

        System.out.println(s.substring(0, sb.length()-1));

    }
}

StringBuilder还可以进行链式操作，如下：

package com.importc;

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

        StringBuilder sb = new StringBuilder(1024);

        sb.append("Good").append(" Morning").append(" Bob").insert(0, "Oh ");

        System.out.println(sb.toString()); // Oh Good Morning Bob

    }
}

可以进行链式操作的关键在于，StringBuilder中方法结束都会返回this当前实例，所以可以接着调用自身的方法。

我们可以仿照这个用法，链式执行一个人一天的作息，如下：

ThisPerson

package com.importc;

public class ThisPerson {
    public ThisPerson eat() {
        System.out.print("吃饭-->");
        return this;
    }

    public ThisPerson sleep() {
        System.out.print("睡觉-->");
        return this;
    }

    public ThisPerson study() {
        System.out.print("学习");
        return this;
    }
}

Application

package com.importc;

public class Application {
    public static void main(String[] args) {
      
        ThisPerson tp = new ThisPerson();
        tp.eat().sleep().study(); // 吃饭-->睡觉-->学习
      
    }
}

BigDecimal

package com.importc;

import com.oop.demo05.B;

import java.math.BigDecimal;
import java.math.RoundingMode;

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

        BigDecimal bd = new BigDecimal("1234.5678");
        System.out.println(bd.multiply(bd)); //1524157.65279684

        BigDecimal d1 = new BigDecimal("123.45");
        BigDecimal d2 = new BigDecimal("123.4500");
        BigDecimal d3 = new BigDecimal("1234500");
        System.out.println(d1.scale()); //输出小数位数 2
        System.out.println(d2.scale()); //4
        System.out.println(d3.scale()); //0

        BigDecimal d4 = d3.stripTrailingZeros();
        System.out.println(d4.scale()); //-2

        //BigDecima在做加减乘时精度不会丢失，但是在做除法时，可能存在除不尽的情况
        BigDecimal d5 = bd.divide(d1, 10, RoundingMode.HALF_UP); //保留十位小数并四舍五入
        //BigDecimal d6 = bd.divide(d1); //会报错ArithmeticException
        System.out.println(d5);

        //比较两个BigDecimal
        System.out.println(d1.equals(d2)); //false 大小相同但是小数位数不同
        System.out.println(d1.compareTo(d2)); //0表示等于 负数表示小于 正数表示大于

    }
}

Math&Random

package com.importc;

import java.util.Random;

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

        //绝对值
        System.out.println(Math.abs(-6)); //6
        //最大值 最小值
        System.out.println(Math.max(1, 2)); //2
        System.out.println(Math.min(3, 4)); //3
        //计算x的y次方
        System.out.println(Math.pow(2, 3)); //8.0
        //数学常量 PI E
        System.out.println(Math.PI);
        System.out.println(Math.E);

        //生成随机数(伪)
        System.out.println(Math.random()); //[0,1)

        Random r = new Random();
        System.out.println(r.nextInt());
        System.out.println(r.nextBoolean());
        System.out.println(r.nextFloat());
        System.out.println(r.nextDouble());

    }
}