如何从java文本文件中读取输入

Question

我对java有点陌生，我的作业中有一个词法分析器程序，该程序运行良好，我唯一的问题是我的输入需要来自文本文件

public class LexicalAnalyzer {

public static void main(String[] args) {

   Scanner reader = new Scanner(System.in);
   System.out.println("Input strings: ");
   String str = reader.nextLine();

   for (int i = 0; i < str.length(); i++)
   {
       switch (str.charAt(i))
       {
           case '+':
                System.out.println("Token: +   Type: ADDITION");
                break;
           case '-':
                System.out.println("Token: -   Type: SUBTRACTION");
                break;
           case '*':
                System.out.println("Token: *   Type: MULTIPLICATION");
                break;
           case '/':
                System.out.println("Token: /   Type: DIVISION");
                break;
           case '(':
                System.out.println("Token: (   Type: LEFT PARENTHESIS");
                break;
           case ')':
                System.out.println("Token: )   Type: RIGHT PARENTHESIS");
                break;
           case '=':
                System.out.println("Token: =   Type: EQUALS");
                break;
           case ';':
                System.out.println("Token: ;   Type: END");
                break;
           case '0':
                System.out.println("Token: 0   Type: NUM");
                break;
           case '1':
                System.out.println("Token: 1   Type: NUM");
                break;
           case '2':
                System.out.println("Token: 2   Type: NUM");
                break;
           case '3':
                System.out.println("Token: 3   Type: NUM");
                break;
           case '4':
                System.out.println("Token: 4   Type: NUM");
                break;
           case '5':
                System.out.println("Token: 5   Type: NUM");
                break;
           case '6':
                System.out.println("Token: 6   Type: NUM");
                break;
           case '7':
                System.out.println("Token: 7   Type: NUM");
                break;
           case '8':
                System.out.println("Token: 8   Type: NUM");
                break;
           case '9':
                System.out.println("Token: 9   Type: NUM");
                break;   
       }
   }      

关于如何从文本文件中读取输入，有什么想法吗