Java Program:

Look through the Language Description and build a list of keywords. Add a HashMap to your Lexer class and initialize all the keywords. Change your lexer so that it checks each string before making the WORD token and creates a token of the appropriate type if the work is a key word. When the exact type of a token is known (like WHILE), you should NOT fill in the value string, the type is enough. For tokens with no exact type (like hello), we still need to fill in the tokens string. Finally, rename WORD to IDENTIFIER.

Similarly, look through the Language Description for the list of punctuation. A hash map is not necessary or helpful for these they need to be added to your state machine. Be particularly careful about the multi-character operators like := or >=. These require a little more complexity in your state machine. See the comment state machine example for an idea on how to implement this.

Strings and characters will require some additions to your state machine. Create STRINGLITERAL and CHARACTERLITERAL token types. These cannot cross line boundaries. Note that we arent going to build in escaping like Java does ( This is a double quote\ that is inside a string or \). Comments, too, require a bit more complexity in your state machine. When a comment starts, you need to accept and ignore everything until the closing comment character. Assume that comments cannot be nested {{this is invalid} and will be a syntax error later}. Remember, though, that comments can span lines, unlike numbers or words or symbols; no token should be output for comments.

Your lexer should throw an exception if it encounters a character that it doesnt expect outside of a comment, string literal or character literal. Create a new exception type that includes a good error message and the token that failed. Ensure that the ToString method prints nicely. An example of this might be:

ThisIsAnIdentifier 123 ! { <- that exclamation is unexpected }

Add line number to your Token class. Keep track of the current line number in your lexer and populate each Tokens line number; this is straightforward because each call to lex() will be one line greater than the last one. The line number should be added to the exception, too, so that users can fix the exceptions.

Finally, indentation. This is not as bad as it seems. For each line, count from the beginning the number of spaces and tabs until you reach a non-space/tab. Each tab OR four spaces is an indentation level. If the indentation level is greater than the last line (keep track of this in the lexer), output one or more INDENT tokens. If the indentation level is less than the last line, output one or more DEDENT tokens (obviously you will need to make new token types). For example:

1 { indent level 0, output NUMBER 1 }

a { indent level 1, output an INDENT token, then IDENTIIFIER a }

b { indent level 2, output an INDENT token, then IDENTIFIER b }

c { indent level 4, output 2 INDENT tokens, then IDENTIFIER c }

2 { indent level 0; output 4 DEDENT tokens, then NUMBER 2 }

Be careful of the two exceptions:

If there are no non-space/tab characters on the line, dont output an INDENT or DEDENT and dont change the stored indentation level.

If we are in the middle of a multi-line comment, indentation is not considered.

Note that end of file, you must output DEDENTs to get back to level 0. Your exception must be called SyntaxErrorException and be in its own file. Unterminated strings or characters are invalid and should throw this exception, along with any invalid symbols.

Below is the lexer file where all the new elements will be added:

Lexer.java

package mypack;

import java.util.ArrayList; import java.util.List;

public class Lexer { private static final int INTEGER_STATE = 1; private static final int DECIMAL_STATE = 2; private static final int IDENTIFIER_STATE = 3; private static final int ERROR_STATE = 4;

private static final char EOF = (char) -1 private static String input; private static int index; private static char currentChar;

public List lex(String inputString) throws Exception { input = inputString; index = 0; currentChar = input.charAt(index); List tokens = new ArrayList(); while (currentChar != EOF) { switch (currentState()) { case INTEGER_STATE: integerState(tokens); break; case DECIMAL_STATE: decimalState(tokens); break; case IDENTIFIER_STATE: identifierState(tokens); break; case ERROR_STATE: throw new Exception("Invalid character: " + currentChar); default: break; } } return tokens; } private static int currentState() { if (Character.isDigit(currentChar)) { return INTEGER_STATE; } else if (Character.isLetter(currentChar)) { return IDENTIFIER_STATE; } else if (currentChar == '.') { return DECIMAL_STATE; } else { return ERROR_STATE; } } private static void integerState(List tokens) { StringBuilder builder = new StringBuilder(); while (Character.isDigit(currentChar)) { builder.append(currentChar); advance(); } tokens.add(new Token(Token.TokenType.NUMBER, builder.toString())); } private static void decimalState(List tokens) { StringBuilder builder = new StringBuilder(); builder.append(currentChar); advance(); while (Character.isDigit(currentChar)) { builder.append(currentChar); advance(); } tokens.add(new Token(Token.TokenType.NUMBER, builder.toString())); } private static void identifierState(List tokens) { StringBuilder builder = new StringBuilder(); while (Character.isLetterOrDigit(currentChar)) { builder.append(currentChar); advance(); } tokens.add(new Token(Token.TokenType.WORD, builder.toString())); } private static void advance() { index++; if (index >= input.length()) { currentChar = EOF; } else { currentChar = input.charAt(index); } } }