four06% cat -n Scan.java
     1	/* Scan.java: scanner implementation.
     2	 * Recognizes C-type comments
     3	 * Based on a finite-state automaton.
     4	 */
     5	import java.io.*;
     6	public class Scan {
     7	   Reader in; // internal file name for input stream
     8	   boolean fileOpen = false; // is the file open yet?
     9	   String fileName; // name of input file, if present
    10	
    11	   // Scan(): constructor providing no input file name
    12	   public Scan() {
    13	      fileName = "";
    14	   }
    15	
    16	   // Scan(String ): constructor providing input file name
    17	   public Scan(String f) {
    18	      fileName = f;
    19	   }
    20	
    21	   // getComment: recognize and return the next comment
    22	   public String getComment()  throws IOException {
    23	      int state = 0; // state for the finite automaton
    24	      char ch = 0; // single-character buffer
    25	      String s = "/*";
    26	      while (state != 4) {
    27	         if (ch == 65535) { //end-of-file
    28	            System.out.println("Th-th-th-that's all folks");
    29	            System.exit(0);
    30	         }
    31	         switch (state) {
    32	         case 0: ch = getNextChar();
    33	              if (ch == '/') state = 1;
    34	              else state = 0; break;
    35	         case 1: ch = getNextChar();
    36	              if (ch == '*') state = 2;
    37	              else state = 0; break;
    38	         case 2: ch = getNextChar();
    39	              s += ch;
    40	              if (ch == '*') state = 3;
    41	              else state = 2; break;
    42	         case 3: ch = getNextChar();
    43	              s += ch;
    44	              if (ch == '*') state = 3;
    45	              else if (ch == '/') state = 4;
    46	              else state = 2; break;
    47	         case 4: return s;
    48	         } // end of switch
    49	      } // end of while
    50	      return s;
    51	   }
    52	
    53	   // getNextChar: fetches next char.  Also opens input file
    54	   private char getNextChar() throws IOException {
    55	      if (!fileOpen) {
    56	         fileOpen = true;
    57	         if (fileName == "")
    58	            in = new InputStreamReader(System.in);
    59	         else
    60	            in = new FileReader(fileName);
    61	      }
    62	      return (char)in.read();
    63	   }
    64	}
four06% cat -n ScanTest.java
     1	// ScanTest: Test a scanner for C-style comments
     2	import java.io.*;
     3	public class ScanTest {
     4	   public static void main(String[] args)throws IOException{
     5	      Scan scanner; // the scanner
     6	      String s;
     7	      // pass an input file name if present on command line
     8	      if (args.length > 0)
     9	         scanner = new Scan(args[0]);
    10	      else
    11	         scanner = new Scan();
    12	      while (true) {
    13	         // fetch and print the next comment
    14	         // end-of-file is detected inside Scan.java
    15	         s = scanner.getComment();
    16	         System.out.println("Next comment: "+"\""+s + "\"");
    17	      }
    18	   }
    19	}
four06% cat -n comment.source
     1	 x = 47 / 13 * 12; /**/
     2	 /***/ /****/ /*/ ?? * / */
     3	 /* abc; */    /* over
     4	  two lines */ JUNK! 3.14159; /*quit*/
four06% java ScanTest        (the file comment.source paster here)
 x = 47 / 13 * 12; /**/
 /***/ /****/ /*/ ?? * / */
 /* abc; */    /* over
  two lines */ JUNK! 3.14159; /*quit*/
Next comment: "/**/"
Next comment: "/***/"
Next comment: "/****/"
Next comment: "/*/ ?? * / */"
Next comment: "/* abc; */"
Next comment: "/* over
  two lines */"
Next comment: "/*quit*/"            (control-D typed here)
Th-th-th-that's all folks
four06% java ScanTest < comment.source   (using redirected input)
Next comment: "/**/"
Next comment: "/***/"
Next comment: "/****/"
Next comment: "/*/ ?? * / */"
Next comment: "/* abc; */"
Next comment: "/* over
  two lines */"
Next comment: "/*quit*/"
Th-th-th-that's all folks
four06% java ScanTest comment.source    (using named source file)
Next comment: "/**/"
Next comment: "/***/"
Next comment: "/****/"
Next comment: "/*/ ?? * / */"
Next comment: "/* abc; */"
Next comment: "/* over
  two lines */"
Next comment: "/*quit*/"
Th-th-th-that's all folks