如何檢查用戶輸入的字符串是否包含文件擴展名

Question

因此，在此程序中，用戶能夠創建/指定目錄並創建文件，因為該文件尚不存在。 當用戶輸入文件名時，他們也需要輸入文件擴展名。 我遇到的問題是如何使代碼在用戶輸入的字符串末尾查找文件擴展名。 我竟然檢查了一下“。” 但我被困在程序可以看到文件是否有.json或.txt或“。”之后的任何內容的方式上。

TL; DR如何在用戶輸入的字符串末尾檢查文件擴展名（< - 不是單詞）

請注意，下面的代碼還沒有完成，我堅持的條件在里面有評論。

package filecreator.coolversion;

import java.io.File;
import java.io.IOException;
import java.util.*;

public class FileCreatorCoolversion {

public static Scanner sc = new Scanner(System.in);
public static boolean success = false;
public static String filename;
public static String filedir;
public static File file;
public static File dir;

public static void main(String[] args) throws IOException {

    System.out.println("********************************");
    System.out.println("* Welcome to File Creator 2.0! *");
    System.out.println("********************************");
    System.out.println(" ");

    while(!success) {

        System.out.println("Would you like to create a file? Y/N?");
        String usrans = sc.nextLine();

        if(usrans.equalsIgnoreCase("y")) {

            System.out.println("Proceeding with file creation...");
            break;

        } else if(usrans.equalsIgnoreCase("n")) {

            System.out.println("Exiting Program...");
            System.exit(0);

        } else if(!usrans.equalsIgnoreCase("y") || !usrans.equalsIgnoreCase("n")) {

            System.out.println("That is not a valid answer! Please try again!");
            System.out.println(" ");
        }
    }

    while(!success) {

        System.out.println(" ");
        System.out.println("Please enter a valid filename:");
        filename = sc.nextLine();

        if(filename.isEmpty()) {

            System.out.println("Please enter a file name!");
            break;

        } else if(filename.contains("/") || filename.contains(":") || 
                  filename.contains("*") || filename.contains("?") ||
                  filename.contains("<") || filename.contains(">") ||
                  filename.contains("|") || filename.contains("\"") ||
                  filename.contains("\\")) {

            System.out.println("Please do not include / \\ : * ? \" < > |");

        } else if(!filename.contains(".")) {

            System.out.println("Please add a apropriate file extensions");

        } else if (filename.) {

            //HERE IS WHERE IM STUCK

        } else {

            System.out.println(" ");
            System.out.println("File name \"" + filename + "\" chosen");
            break;
        }
    }

        System.out.println(" ");
        System.out.println("Where would you like to have your file saved?");
        System.out.println("Please enter a valid directory");

    while(!success) {

        filedir = sc.nextLine();

        if(!filename.contains(":")) {

            System.out.println(" ");
            System.out.println("Please enter a valid directory!");

        } else if(!filename.contains("\\")) {

            System.out.println(" ");
            System.out.println("Please enter a valid directory!");

        } else {

            System.out.println("File directory \"" + filedir + "\" chosen");
            break;
        }
    }

    System.out.println(" ");
    System.out.println("Creating file...");


}

}

Answer 1

您可能想要做的就是檢查的lastIndex . 您在之前的條件中檢查過，然后查看之后是否有任何字符串：

String extension = filename.substring(filename.lastIndexOf(".") + 1);
//Checks if there is any extension after the last . in your input
if (!extension.isEmpty()) {
    System.out.print("This is the extension - " + extension);
}

Answer 2

找到文件擴展名（如果有）：

static String getExtension(String str){
    int begin = str.lastIndexOf(".");
    if(begin == -1)
        return null;
    String extension = str.substring(begin + 1);
    return extension;
}   

檢查擴展名是否有效：

static boolean isExtensionValid(String extension){
    String validExtensions [] = new String[]{
        "json", "txt"
    };
    for(String str : validExtensions )
        if(str.equals(extension))
            return true;

    return false;
}

測試整個事情：

public static void main (String[] args){
    String val = "Kappa.txt";
    String extension = getExtension(val);

    if(extension != null){
        System.out.println(extension);
        System.out.println(isExtensionValid(extension));
    } else {
    //Handle no extension found here
    }
}

Answer 3

如果你想在之后得到任何擴展. 你可以在這里使用String split一些例子

String exts[] = filename.split("\\.");
int extIndex = exts.length;\\find array length incase user inputed more than one dot

System.out.println(exts[extIndex - 1]); \\ here you get the extension