java - 扫描仪从文件而不是控制台读取

Question

我试图通过使用命令“java PostFix < in.dat”从文件中获取表达式来计算后缀表达式。现在，当我想从用户(即控制台)获取变量的值时，它恰好从文件中读取并给出输入不匹配异常(因为它从文件而不是从控制台读取字符串值)。以下是我的代码的摘录。如何从控制台而不是文件获取输入？

public class PostFix{ 

    String operators[] = new String[]{"+","-","*","/","_","!","#","^","<","<=",">",">=","==","!=","&&","||","$"};
    ArrayList<String> variables = new ArrayList<String>();
    ArrayList<Integer> variable_value = new ArrayList<Integer>();

public static void main(String[] args) {
    // TODO Auto-generated method stub

    PostFix pf = new PostFix();
    pf.getVariables();

}

public void getVariables(){

    Scanner sc = new Scanner(System.in);
    String line = sc.nextLine();
    String tokens[] = line.split(" ");

    while(sc.hasNextLine()){
        System.out.println("in geVariables()");
        for(int i = 0; i < tokens.length; i++){

            if(!Arrays.asList(operators).contains(tokens[i])){

                try{
                Integer.parseInt(tokens[i]);    
                }
                catch(Exception e){
                    //e.printStackTrace();

                    variables.add(tokens[i]);
                    System.out.println("Please enter the value of "+tokens[i]);
                    variable_value.add(sc.nextInt());
                }

            }

        }

    }
    sc.close();     
    }   

}

错误:

Please enter the value of a
Exception in thread "main" java.util.InputMismatchException
at java.util.Scanner.throwFor(Unknown Source)
at java.util.Scanner.next(Unknown Source)
at java.util.Scanner.nextInt(Unknown Source)
at java.util.Scanner.nextInt(Unknown Source)
at PostFix.getVariables(PostFix.java:51)
at PostFix.main(PostFix.java:16)

in.dat 文件的内容:

2 a + b _ * ! c c ^ 15 / # - $
value x + value y - != x y <= && $

Answer 1

java PostFix < in.dat

意味着所有标准输入都将从in.dat文件中获取。

您的 Java 代码不知道此重定向。它只是从System.in(即标准输入)读取，这通常意味着键盘。

因此，解决方案就是简单地让您的代码运行而无需任何重定向

java PostFix