Java (File, Scanner, PrintStream)

import java.util.*;
import java.io.*;

public class YazInterpreter {
   public static void main(String[] args) throws FileNotFoundException {
      Scanner console = new Scanner(System.in);
      boolean go = true;
      intro();
      
      while (go) {
         System.out.print("(I)nterpret YazLang program, (V)iew output, (Q)uit? ");
         String ans = console.next();
    
         if (ans.equalsIgnoreCase("I")) {
            interpret(console);
            go = true;
         } else if (ans.equalsIgnoreCase("V")) {
            view(console);
            System.out.println();
            go = true;
         } else if (ans.equalsIgnoreCase("Q")) {
            go = false;
         }
      }
   }
   
   //This method prints out the introduction of this program.
   public static void intro() {
      System.out.println("Welcome to YazInterpreter!");
      System.out.println("You may interpret a YazLang program and output");
      System.out.println("the results to a file or view a previously");
      System.out.println("interpreted YazLang program.");
      System.out.println();
   }
   
   //This method allows user to input files' names.
   public static File fileName(Scanner console) throws FileNotFoundException {
      System.out.print("Input file name: ");
      String inputName = console.nextLine();
      File file = new File(inputName);
      
      while(!file.exists()) {
         System.out.print("File not found. Try again: ");
         inputName = console.next();
         file = new File(inputName);
      }
      return file;
   }
   
   //This method allows people to view the content of YazLang program in the console.
   public static void view(Scanner console) throws FileNotFoundException {
      File inputName = fileName(console);
      Scanner input = new Scanner(new File(inputName.getName()));
      
      while(input.hasNextLine()) {
         String line = input.nextLine();
         System.out.print(line);
      }
      System.out.println();
   }
   
   //This method interprets.
   public static void interpret(Scanner console) throws FileNotFoundException {
      File file = fileName(console);
      
      System.out.print("Output file name: ");
      String outputName = console.next();
      PrintStream out = new PrintStream(outputName);
      Scanner inputFile = new Scanner(file);
      
      while (inputFile.hasNextLine()) {
         String row = inputFile.nextLine(); //修改处，从“String row = inputFile.next();” 改成 “String row = inputFile.nextLine();”
         command(console, row, out);
      }
      System.out.println("YazLang interpreted and output to a file!"); 
      System.out.println();
   }
   
   //This method determine which Command to use.
   public static void command(Scanner console, String row, PrintStream out) 
                      throws FileNotFoundException {
      Scanner input = new Scanner(row);
      String commandtype = input.next();
      if (commandtype.equalsIgnoreCase("convert")) {
         int temp = input.nextInt();
         String unit = input.next();
         String convertResult = convert(temp, unit);
         out.println(convertResult);
      } else if (commandtype.equalsIgnoreCase("repeat")) {
         String repeatResult = repeat(row);
         out.println(repeatResult);
      } else if (commandtype.equalsIgnoreCase("range")) {
         int start = input.nextInt();
         int end = input.nextInt();
         int incre = input.nextInt();
         String rangeResult = range(start, end, incre);
         out.println(rangeResult);
      }
   }
   
   
   //This method prints out the command CONVERT
   public static String convert(int temp, String unit) {
      String convertTemp = "";
      if (unit.equalsIgnoreCase("f")) {
         int temp1 = (int)((temp - 32) / 1.8);
         convertTemp = temp1 + "C";
      } else if (unit.equalsIgnoreCase("c")) {
         int temp1 = (int)(temp * 1.8) + 32;
         convertTemp = temp1 + "F";
      }
      return convertTemp;
   }
   
   //This method prints out the command REPEAT
   public static String repeat(String statement) 
                      throws FileNotFoundException {
      Scanner input = new Scanner(statement);
      String command = input.next();

      String statementNew = "";
      while(input.hasNext()) {
         String word = input.next();
         word = word.replace("_", " ");
         word = word.replaceFirst("\"", "");
         word = word.substring(0, word.length() - 1);

         int num = input.nextInt();
         
         for (int i =1; i <= num; i++) {
         statementNew = statementNew + word;
         }
      }
      return statementNew;
   }
   
   //This method prints out the command RANGE
   public static String range(int start, int end, int incre) {
      String numRange = "";
      while (start < end) {
         numRange = numRange + start + " ";
         start = start + incre;
      }
      return numRange;
   }  
}

上面两张图是没有改成console.nextLine()时的错误

这张是改成console.nextLine()之后的错误。 

看你的截图，跟console.nextLine()不相关，错误提示是Input file name: File not found，这个错误是你输入的文件名oops.txt在当前路径找不到，你需要把oops.txt放到项目路径下
你可以在main方法加上下面这个代码查看当前项目的路径：

        File directory = new File("");
        String courseFile = directory.getCanonicalPath() ;
        System.out.println("当前路径："+courseFile);

然后运行项目开头就会输出项目路径

当前路径：D:\test\testProject
Welcome to YazInterpreter!
You may interpret a YazLang program and output
the results to a file or view a previously
interpreted YazLang program.

(I)nterpret YazLang program, (V)iew output, (Q)uit? i
Input file name: File not found. Try again: 

然后把你的文件oops.txt放入D:\test\testProject路径下，这样就可以解决File not found问题
至于next和nextLine两个方法问题，只要看一下这两个方法的区别就是了

1. next()一定要读取到有效字符后才可以结束输入，对输入有效字符之前遇到的空格键、tab键或enter键等结束符，next()方法会自动将其去掉，只有在输入有效字符之后，next()方法才将其后输入的空格键、tab键或enter键等视为分隔符或结束符。
2.  简单地说，next()查找并返回来自此扫描器的下一个完整标记。完整标记的前后是与分隔模式匹配的输入信息，所以next方法不能得到带空格的字符串。
3. nextline()方法的结束符只是enter键，即nextline()方法返回的是enter键之前的所有字符，它是可以得到带空格的字符串的