Java学习记录(补充七:一些工具类)

Posted 2020-10-04

Math类
package Box;

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

public class TestBox {
    public static void main(String[] args) {
        //八个包装类
        /*Integer i=5;//在栈里面开辟了一个内存空间(基本数据类型)
        Integer n = new Integer(5);//用new在堆里面开辟了一个内存空间(对象)
        int m = 5;
        
        Byte b    = new Byte("1");
        Boolean bool=new Boolean(true);
        Long lon = new Long(1000);
        Short s    = new Short("5");
        Double d = new Double(100.0);
        Float f = new Float(66.00);
        Character c = new Character(‘c‘);
        */
        //Math类
        System.out.println(Math.ceil(6.9));//向上取整
        System.out.println(Math.floor(6.9));//向下
        System.out.println(Math.round(6.3));//四舍五入
        System.out.println(Math.pow(8, 3));//求次幂
        
        double r = 5.5;
        double s = Math.PI*Math.pow(r, 2);
        System.out.println(s);
        
        //产生随机数
        List<Integer> list = new ArrayList<>(50);
        for (int i = 0; i < 50; i++) {
            list.add(i);
        }
        Collections.shuffle(list);//collections工具类的一个方法shuffle()
        list.forEach(System.out::println);
    }
}

结果图:

 

String类
package Box;

public class StringTest {

    public static void main(String[] args) {
        String s = "  abfd";//String不是基本数据类型,是非常特殊的一个类
        String s1 = new String("assd");
        //trim()去掉字符串"两端"的空格(中间的不能去除)
        System.out.println(s.length());
        System.out.println(s.trim().length());
        
        //substring:截取字符串
        String s2 = "再来一瓶";
        System.out.println(s2.substring(2));
        System.out.println(s2.substring(1, 2));
        
        //indexOf:找到子字符串在当前字符中的首字母的索引,找不到就返回-1
        String s3 = "123ab45ab67";
        System.out.println(s3.indexOf("ab"));
        
        System.out.println(s3.indexOf(97));//97为Ascll码
        
        System.out.println(s3.indexOf("ab", 5));
        //找到字符串在当前字符串中最后出现的索引位置
        String s4 = "123321";
        System.out.println(s4.lastIndexOf("2"));
        
        //replace 替换指定字符
        String s5 = "a b c d   ";
        System.out.println(s5.replace(" ",""));
        
        System.out.println("*****************************************");
        //split 按指定格式分割字符串,返回字符串数组
        String s6 = "a b c d ef   ";
        String []s7 = s6.split(" ");
        for(String str:s7){
            System.out.println(str);
        }
        
        String s8 = "abcdbdfg";
        System.out.println(s8.charAt(1));//超过会报错
    }

}

结果图:

 

StingBuffer 和 StringBuilder(注意两者使用的区别)
package Box;
//String是常量  StringBuffer是变量(频繁的拼接需要用到)
public class StringBufferTest {
        public static void main(String[] args) {
            StringBuffer sb = new StringBuffer("abc");//安全性高
            sb.append("de");
            sb.append("fg");
            String s9 = sb.toString();
            System.out.println(s9);
            
            StringBuilder sb1 = new StringBuilder("abd");//速度快但安全性低(可以
                                                        //    自己加锁)
            sb1.append("de");
            String s10 = sb1.toString();
            System.out.println(s10);
            
        }
}

结果图:

 

Radom类
public class RandomTest {
    public static void main(String[] args) {
        Random random = new Random();
        int n=random.nextInt(10);
        System.out.println(n);
        
        boolean bool=random.nextBoolean();
        System.out.println(bool);
        
        
        for(int i = 0;i<10;i++){
            double n1 = random.nextDouble();
            System.out.println(n1*9+1);
        }
        
        for(int i = 0;i<10;i++){
            boolean b1 = random.nextBoolean();
            System.out.println(b1);
        }
        
        
        
    }
}

结果图: