Java学习笔记（9）

Stringjoiner

Add添加的内容只能是字符串

如果要添加的是一个int怎么办呢？

sj.add(arr[i] + "");

+一个”” 空字符串，这样就变成了一个字符串

package exercise;
 
import java.util.StringJoiner;
 
public class exercise11 {
    public static void main(String[] args) {
        int[] arr = {1, 2, 3, 1518, 1115};
        System.out.println(arrToString(arr));
    }
 
    public static String arrToString(int[] arr) {
        StringJoiner sj = new StringJoiner(", ", "[","]");
        String result = "";
        for (int i = 0; i < arr.length; i++) {
            sj.add(arr[i] + "");
        }
        return sj.toString();
    }
}

字符串原理

如果没有变量

JDK8之前

+号其实也有new了个stringbuilder

所以

String s2 = s1 + “b”; 至少会new了两个对象，一个是stringbuilder对象，一个是string字符串对象（是在tostring里面new的）

JDK8做了优化

会预估最终字符串的长度，并创建数组

会扩容，默认是16 查容量（sb.capacity()）

如果存的更多并超出，创建实际的长度

容量是有上限的，最大是int的最大值

package exercise;
 
import java.util.Scanner;
 
public class exercise12 {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        String str;
        while (true) {
            System.out.println("输入字符串：");
            str = sc.next();
            if (checkStr(str)) {
                break;
            }else {
                System.out.println("出错，重新输！");
            }
        }
 
        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < str.length(); i++) {
            sb.append(changeToLuoMa(str.charAt(i) - 48));
        }
        System.out.println(sb);
 
    }
 
    public static boolean checkStr(String str) {
        if (str.length() > 9) {
            return false;
        }
 
        for (int i = 0; i < str.length(); i++) {
            char c = str.charAt(i);
            if (c < '0' || c > '9') {
                return false;
            }
        }
 
        return true;
    }
 
    public static String changeToLuoMa(int number) {
        String[] Luoma = {"", "I ", "II ", "III ", "IV ", "V ", "VI ", "VII ", "VIII ", "IX ", "X "};
        return Luoma[number];
    }
}

toCharArray()

package exercise;
 
public class exercise13 {
    public static void main(String[] args) {
        String strA = "abcde";
        String strB = "cdecb";
 
        boolean check = check(strA, strB);
        if (check) {
            System.out.println("匹配成功！");
        }else {
            System.out.println("匹配失败！");
        }
 
 
    }
 
    public static String rotate(String str) {
        return str.substring(1) + str.charAt(0);
    }
 
    public static boolean check(String strA, String strB){
        for (int i = 0; i < strA.length(); i++) {
            strA = rotate(strA);
            if (strA.equals(strB)) {
                return true;
            }
        }
        return false;
    }
}

package exercise;
 
import java.util.Random;
 
public class exercise15 {
    public static void main(String[] args) {
        char[] resultChar = new char[5];
 
        char[] arr = new char[52];
        for (int i = 0; i < 26; i++) {
            arr[i] = (char) ('a' + i);
            arr[i + 26] = (char) ('A' + i);
        }
 
        Random r = new Random();
        for (int i = 0; i < 4; i++) {
            int index = r.nextInt(52);
            resultChar[i] = arr[index];
        }
 
        int number = r.nextInt(10);
        resultChar[4] = (char) (number + 48);
 
        int index = r.nextInt(5);
        char temp = resultChar[4];
        resultChar[4] = resultChar[index];
        resultChar[index] = temp;
 
        String resultStr = new String(resultChar);
 
        System.out.println(resultStr);
 
 
    }
}

package exercise;
 
public class exercise16 {
    public static void main(String[] args) {
        String num1 = "13";
        String num2 = "57";
 
        //string -> int -> string
        int num1Int = charToInt(num1);
        int num2Int = charToInt(num2);
 
        String result = num1Int * num2Int + "";
 
        System.out.println(result);
    }
 
    public static int charToInt(String num) {
        char[] charArray = num.toCharArray();
        int result = 0;
        for (char c : charArray) {
            int ge = c - 48;
            result = result * 10 + ge;
        }
        return result;
    }
 
 
}

package exercise;
 
public class exercise17 {
    public static void main(String[] args) {
        String str = "  hjel sjv  aesdsfsjkes ";
        char[] charArray = str.toCharArray();
        int count = 0;
        for (int i = charArray.length - 1; i >= 0; i--) {
 
            if (charArray[i] != 32) {
                count++;
            } else if (count != 0) {
                break;
            }
        }
        System.out.println(count);
    }
}

集合

集合不能存基本数据类型，要变成包装类才可以存

Arraylist

集合为空，打印出来会是[]

注意：

增加功能：

Append 是stringbuilder的

Add是stringjoiner和arraylist的

获取长度：

Size()是arraylist的一个方法，所以带一个（）

arraylist获取其中的元素是通过get（索引）得到的，arraylist[索引]是得不到的，因为arraylist里面存的是地址

基本数据类型对应的包装类 写在<>里

返回多个数据，最好用集合返回

package exercise;
 
import java.util.ArrayList;
 
public class exercise2 {
    public static void main(String[] args) {
        ArrayList<Phone> phoneArr = new ArrayList<>();
 
        Phone phone1 = new Phone("xiaomi", 1000);
        Phone phone2 = new Phone("apple", 8000);
        Phone phone3 = new Phone("hammer", 2999);
 
        phoneArr.add(phone1);
        phoneArr.add(phone2);
        phoneArr.add(phone3);
 
        ArrayList<Phone> phoneInfoList = getPhoneInfo(phoneArr);
        for (int i = 0; i < phoneInfoList.size(); i++) {
            Phone phone = phoneInfoList.get(i);
            System.out.println(phone.getBrand() + ", " + phone.getPrice());
        }
    }
 
    public static ArrayList<Phone> getPhoneInfo(ArrayList<Phone> phoneArr) {
        ArrayList<Phone> result = new ArrayList<>();
 
        for (Phone phone : phoneArr) {
            if (phone.getPrice() < 3000) {
                result.add(phone);
            }
        }
        return result;
    }
}