当输入空白行时退出java程序的输入

Question

我已经用Java编写了一个RPN计算器，但是当用户输入空白行而不是公式时，我正在努力让我的代码退出。程序需要循环，直到输入空行。

以下是我的源代码。

$`import java.util.ArrayList; 
import java.util.Scanner; 
import java.io.*; 
import java.io.FileNotFoundException; 

public class Calculator { 
private static Scanner input; 
public static int invalidlines = 0; 
public static int validlines = 0; 
public static ArrayList<String> validList = new ArrayList<String>(); 

public void FileNotFoundException(){ 
System.out.println("Please enter a valid expression!"); 

} 

public static boolean isInt(String userinput) { 
    try { 
     Integer.parseInt(userinput); // Try to parse. Makes sure that the values entered are actual numbers 
     return true; // Boolean value to show if the equation entered is valid or not 
    } 
    catch (NumberFormatException e) { 
     System.out.println("Please enter a valid expression!"); 
     return false; 
    } 
} 
    public static boolean isValidLine(String line) { 
     line = line.trim(); 
    if (line.length() <= 4) { // Trims the lines down to 4 and ensures there is no spaces being included 
      return false; 
     } 
    else 
     { 
     String[] calcarray = new String[3]; 
     calcarray = line.split(" "); 
     String operators = new String("[+\\-\\*\\/]"); // Validator using regular expressions to check the operator used 

    if (isInt(calcarray[0].toString()) && isInt(calcarray[1].toString()) && calcarray[2].matches(operators)) { // Checks that the operator in the string matches the ones in the regular expression 
     return true; 
} 
    else 
{ 
     return false; 
} 
} 
} 
public static void main(String[] args) throws IOException { 
input = new Scanner(System.in); 
String keyboardInput = new String(); 
String currentLine = new String(); 
Scanner kbScan = new Scanner(System.in); 
System.out.println("Please press the letter F for file input or K for keyboard input"); 
     String inputString = new String(input.nextLine()); 
    int answer = 0; 
    while (true){ 
    if (inputString.equals("K") || inputString.equals("k")) { 
     System.out.println("Please enter an equation"); 
     keyboardInput = kbScan.nextLine(); 
    } 
    if (isValidLine(keyboardInput)) { 
     String[] equation = new String[3]; // We know that this is only going to contain 3 to be valid 
     equation = keyboardInput.split(" "); // split this up, as it's stored with the spaces. 
    if (inputString.equals("") || inputString.equals("")); 
    { 
     System.exit(0); 
    } 
    int num1 = Integer.parseInt(equation[0]); 
    int num2 = Integer.parseInt(equation[1]); 
    switch(equation[2]) { // This case switch checks the third position of the string to decide which operator is being used. It then works out the answer and breaks to the next instruction 

    case("+"): 
     answer = num1 + num2; 
    break; 
    case("-"): 
     answer = num1 - num2; 
    break; 
    case("/"): 
     answer = num1/num2; 
    break; 
    case("*"): 
     answer = num1 * num2; 
    break; 
} 
     System.out.println("Your post fix expression: " + equation[0] + " " + equation[1] + " " + equation[2]); 
     System.out.println("Your calculation: " + equation[0] + " " + equation[2] + " " + equation[1] + " = " + answer); 
} 
    else{ 
     System.out.println("The equation you entered is invalid"); 
} 
    if (inputString.equals("f") || inputString.equals("F")) { 
     try{ 
     //Open the file 
      System.out.println("Enter File Name: "); 
      FileInputStream fstream = new FileInputStream(input.nextLine()); // make a input stream 
      BufferedReader br = new BufferedReader(new InputStreamReader(fstream)); // pass input stream to a buffered reader for manipulation 
      String strLine; // create string vars 
     //loop to read the file line by line 
     while ((strLine = br.readLine()) != null) { // Whilst the buffered readers read line method is not null, read and validate it. 
      currentLine = strLine; 
     if(isValidLine(currentLine)) 
     { 
      validList.add(currentLine); 
      validlines++; 
      String[] filearray = new String[3]; 
      filearray = currentLine.split(" ");    
     int val1 = Integer.parseInt(filearray[0]); 
     int val2 = Integer.parseInt(filearray[1]); 
      System.out.println("Your expression is: " + filearray[0] + " " + filearray[1] + " " + filearray[2]); 
     switch(filearray[2]) { 
     case("+"): 
       answer = val1 + val2; 
      break; 
     case("-"): 
       answer = val1 - val2; 
      break; 
     case("/"): 
       answer = val1/val2; 
      break; 
     case("*"): 
       answer = val1 * val2; 
      break; 
      } 
      System.out.println("Your calculation is " + filearray[0] + " " + filearray[2] + " " + filearray[1] + " = " + answer); 
      } 
     } 
     } 
      catch (FileNotFoundException e) 
      { 
       System.out.println("Please Enter a valid file name"); 
      } 
     } 
} 
} 
} 

Answer 1

只需使用以下代码来检查您正在接受用户输入的变量，无论它是否为空？

我假设你正在参加一个名为inputString变量，则代码应该是─

if(inputString.isEmpty()) 
{ 
    System.exit(0); 
} 

Answer 2

也许你应该使用break而不是System.exit(0)。

此外，我一读到输入就会检查空输入。