Java 输入字母并从大写改为小写，反之亦然？

因此，我必须编写一个程序来执行以下操作：

每次输入1个字母，然后按enter键（“a”）

按enter键将字母更改为大写或小写（“a”）

它输出按enter键的结果（结果为a）

继续上述操作，直到用户输入结束程序的“.”

我不知道为什么我的程序不起作用。它一直在

字母.isUpperCase

和

字母.isLowerCase

上显示“无法解析符号”。如果你们能解决这个问题，如果你们发现任何问题，请告诉我！提前谢谢

import java.io.*;
class Letter {
    public static void main(String[] args) throws IOException {

        InputStreamReader inStream = new InputStreamReader (System.in);
        BufferedReader mVHS = new BufferedReader (inStream);

        String inData; //Store the input data in a String    
        int length;//Assign the length to the int data type
        String letter, result, result2; //Store the sentence in the String type

        //Enter a letter
        System.out.println("Type a letter one at a time");
        System.out.println("Press enter when a letter is entered");
        System.out.println("When done typing all the letters, press . to end the program");
        System.out.println("Enter a letter:");
        String userInput = mVHS.readLine();
        letter = userInput;

        result = letter.toUpperCase();
        result2 = letter.toLowerCase();

        if (letter.isUpperCase(letter.charAt(0)))
            System.out.println(result);
        else if (letter.isLowerCase(letter.charAt(0)))
            System.out.println(result2); 

        //while loop if sentence does not end with a "."
        while(letter.length() == 1){
        {
            if (letter.endsWith("."))
                System.out.println("Thank you come again."); //What the monitor outputs in this condition
            else  
                System.out.println("Enter another letter:");//What the monitor outputs in this condition
            userInput = mVHS.readLine();
            letter = userInput;
        }
        }
    }
}

---

String

上没有名为

isUpperCase

或

isLowerCase

的方法。所以，从

 letter.isUpperCase(letter.charAt(0))

到

---

检查以下代码

import java.io.*;

班级信件{ 公共静态void main（字符串[]args）引发IOException{

    InputStreamReader inStream = new InputStreamReader (System.in);
    BufferedReader mVHS = new BufferedReader (inStream);


    String letter; //Store the sentence in the String type

    //Enter a letter
    System.out.println("Type a letter one at a time");
    System.out.println("Press enter when a letter is entered");
    System.out.println("When done typing all the letters, press . to end the program");
    System.out.println("Enter a letter:");
    String userInput = mVHS.readLine();
    letter = toggleCase(userInput);
       System.out.println(letter);

    //while loop if sentence does not end with a "."
    while(letter.length() == 1){
    {
        if (letter.endsWith("."))
            System.out.println("Thank you come again."); //What the monitor outputs in this condition
        else  
            System.out.println("Enter another letter:");//What the monitor outputs in this condition
        userInput = mVHS.readLine();

        letter = toggleCase(userInput);
        System.out.println(letter);
    }
    }
}

private static String toggleCase(String userInput) {
    String letter;
    String upperString;
    letter = userInput;

    upperString = letter.toUpperCase();


    if (letter.equals(upperString)){
        return (letter.toLowerCase());
    }
    else{
        return (upperString);
    }

}

---

}

String

没有

isLowerCase（char）

方法。

Character

有。你确定你正在制作这些程序并在stackoverflow寻找解决方案吗？你是真的吗？使用一些IDE，比如eclipse。当我使用它时，它会一直使用我的首字母

    InputStreamReader inStream = new InputStreamReader (System.in);
    BufferedReader mVHS = new BufferedReader (inStream);


    String letter; //Store the sentence in the String type

    //Enter a letter
    System.out.println("Type a letter one at a time");
    System.out.println("Press enter when a letter is entered");
    System.out.println("When done typing all the letters, press . to end the program");
    System.out.println("Enter a letter:");
    String userInput = mVHS.readLine();
    letter = toggleCase(userInput);
       System.out.println(letter);

    //while loop if sentence does not end with a "."
    while(letter.length() == 1){
    {
        if (letter.endsWith("."))
            System.out.println("Thank you come again."); //What the monitor outputs in this condition
        else  
            System.out.println("Enter another letter:");//What the monitor outputs in this condition
        userInput = mVHS.readLine();

        letter = toggleCase(userInput);
        System.out.println(letter);
    }
    }
}

private static String toggleCase(String userInput) {
    String letter;
    String upperString;
    letter = userInput;

    upperString = letter.toUpperCase();


    if (letter.equals(upperString)){
        return (letter.toLowerCase());
    }
    else{
        return (upperString);
    }

}