使用substring（）和charAt（）方法的Java程序[closed]

Question

我尝试使用charAt和子字符串，但被卡在子字符串部分，输出结果应该交换第一个和最后一个字符并打印出来。

import java.util.*;

public class Q1 {

    public static void main(String[] args) {
        while(true) {
            char choice = getChoice();
            switch(choice) {
                case 'a': printDetails(); break; //exit switch
                case 'b': getThreeDecPrintLargestAndSmallest(); break;
                case 'c': printNumberRange(); break;
                case 'd': isTriangle(); break;
                case 'e': isPrime(); break;
                case 'f': getAndProcessTenInterger(); break;
                case 'g': getStringAndSwap(); break;
                case 'q': break;
                default : System.out.println("Invalid choice:");
            }
            if(choice=='q') {
                break; // exit while true
            }
        }
        System.out.println("Quit");
    }

    private static void getStringAndSwap() { //g
        try (Scanner kb = new Scanner(System.in)) {
            String stringAndSwap = "";
            
            while (true) {
                System.out.println("Enter a String up to Max of 50 Characters:");
                stringAndSwap =kb.nextLine();
                if  (stringAndSwap.length() >50) {
                    System.out.println("String is too long");
                }else {
                    break;
                }
            }
            char beginning = stringAndSwap.charAt(0);
            char end = stringAndSwap.charAt(-1);
            
            stringAndSwap.substring(beginning, end);
            stringAndSwap.substring(end, beginning);
            
                return System.out.println(stringAndSwap);
            
            System.out.println(stringAndSwap);
    }
}

Answer 1

正如注释中指出的，带有参数-1的方法charAt不会返回字符串的最后一个字符，但它会抛出一个StringIndexOutOfBoundsException。要检索字符串的最后一个字符，您需要依赖long（）方法并编写如下内容：

myString.charAt(myString.length() - 1);

此外，String类表示不可变的对象，因此如果需要操作String，则需要使用StringBuilder类。您的交换方法应该如下所示：

private static void getStringAndSwap() {
    try (Scanner kb = new Scanner(System.in)) {
        String stringAndSwap = "";

        while (true) {
            System.out.println("Enter a String up to Max of 50 Characters:");
            stringAndSwap = kb.nextLine();
            if (stringAndSwap.length() > 50) {
                System.out.println("String is too long");
            } else {
                break;
            }
        }

        //Creating a StringBuilder based on the String read
        StringBuilder strBld = new StringBuilder(stringAndSwap);

        //Saving the first character of the string in a temp var
        char beginning = strBld.charAt(0);

        //Setting the last character of the string in place of the first character
        strBld.setCharAt(0, strBld.charAt(strBld.length() - 1));

        //Setting the last character of the string with the first character previously saved
        strBld.setCharAt(strBld.length() - 1, beginning);

        
        //Printing the result 
        System.out.printf("Swapped string is %s%n", strBld);
    }
}