java 写的 用优先函数分析 数学表达式的程序

package book.string; public class ExpressionParser { public static final int NONE_TOKEN = 0; public static final int DELIMITER_TOKEN = 1; public static final int VARIABLE_TOKEN = 2; public static final int NUMBER_TOKEN = 3; // 四种错误类型 public static final int SYNTAX_ERROR = 0; public static final int UNBALPARENS_ERROR = 1; public static final int NOEXP_ERROR = 2; public static final int DIVBYZERO_ERROR = 3; public static final String[] ERROR_MESSAGES = { "systax Error ", "Unbalanced parentheses", "no Expression present", "Division by Zero" }; public static final String EOE = "\0";// 结束标志 private String exp; private int expIndex; private String token; private int tokenType; private double vars[] = new double[26]; public double evaluate(String expStr) throws Exception { double result; this.exp = expStr; this.expIndex = 0; this.getToken(); if (this.token.equals(EOE)) { this.handleError(NOEXP_ERROR); } result = this.parseAssign(); if (!this.token.equals(EOE)) { this.handleError(SYNTAX_ERROR); } return result; } public double parseAssign() throws Exception { double result; int varIndex; String oldToken; int oldTokenType; if (this.tokenType == VARIABLE_TOKEN) { oldToken = new String(this.token); oldTokenType = this.tokenType; varIndex = Character.toUpperCase(this.token.charAt(0)) - 'A';// ????????????????????????? this.getToken(); if (!this.token.equals("=")) { this.putBack(); this.token = new String(oldToken); this.tokenType = oldTokenType; } else { this.getToken(); result = this.parseAddOrSub(); this.vars[varIndex] = result; return result; } } return this.parseAddOrSub(); } public double parseAddOrSub() throws Exception { char op; double result; double partialResult; result = this.parseMulOrDiv(); while ((op = this.token.charAt(0)) == '+' || op == '-') { this.getToken(); partialResult = this.parseMulOrDiv(); switch (op) { case '-': result = result - partialResult; break; case '+': result = result + partialResult; break; } } return result; } public double parseMulOrDiv() throws Exception { char op; double result; double partialResult; result = this.parseExponent(); while ((op = this.token.charAt(0)) == '*' || op == '/' || op == '%') { this.getToken(); partialResult = this.parseExponent(); switch (op) { case '*': result = result * partialResult; break; case '/': if (partialResult == 0.0) { this.handleError(DIVBYZERO_ERROR); } result = result / partialResult; break; case '%': if (partialResult == 0.0) { this.handleError(DIVBYZERO_ERROR); } result = result % partialResult; break; } } return result; } public double parseExponent() throws Exception { double result; double partialResult; double ex; int t; result = this.parseUnaryOperator(); if (this.token.equals("^")) { this.getToken(); partialResult = this.parseExponent(); ex = result; if (partialResult == 0.0) { result = 1.0; } else { for (t = (int) partialResult - 1; t > 0; t--) { result = result * ex; } } } return result; } public double parseUnaryOperator() throws Exception { double result; String op; op = ""; if ((this.tokenType == DELIMITER_TOKEN) && this.token.equals("+") || this.token.equals("-")) { op = token; this.getToken(); } result = this.parseBracket(); if (op.equals("-")) { result = -result; } return result; } public double parseBracket() throws Exception { double result; if (this.token.equals("(")) { this.getToken(); result = this.parseAddOrSub(); if (!this.token.equals(")")) { this.handleError(UNBALPARENS_ERROR); } this.getToken(); } else { result = this.parseAtomElement(); } return result; } public double parseAtomElement() throws Exception { double result = 0.0; switch (this.tokenType) { case NUMBER_TOKEN: try { result = Double.parseDouble(this.token); } catch (NumberFormatException exc) { this.handleError(SYNTAX_ERROR); } this.getToken(); break; case VARIABLE_TOKEN: result = this.findVar(token); this.getToken(); break; default: this.handleError(SYNTAX_ERROR); break; } return result; } public double findVar(String vname) throws Exception { if (!Character.isLetter(vname.charAt(0))) { handleError(SYNTAX_ERROR); return 0.0; } return vars[Character.toUpperCase(vname.charAt(0)) - 'A']; } public void putBack() { if (this.token == EOE) { return; } for (int i = 0; i < this.token.length(); i++) { this.expIndex--; } } public void handleError(int errorType) throws Exception { throw new Exception(ERROR_MESSAGES[errorType]); } public void getToken() { this.tokenType = NONE_TOKEN; this.token = ""; if (this.expIndex == this.exp.length()) { this.token = EOE; return; } while (this.expIndex < this.exp.length() && Character.isWhitespace(this.exp.charAt(this.expIndex))) { ++this.expIndex; } if (this.expIndex == this.exp.length()) { this.token = EOE; return; } char currentChar = this.exp.charAt(this.expIndex); if (isDelim(currentChar)) { this.token += currentChar; this.expIndex++; this.tokenType = DELIMITER_TOKEN; } else if (Character.isLetter(currentChar)) { while (!isDelim(currentChar)) { this.token += currentChar; this.expIndex++; if (this.expIndex >= this.exp.length()) { break; } else { currentChar = this.exp.charAt(this.expIndex); } } this.tokenType = VARIABLE_TOKEN; } else if (Character.isDigit(currentChar)) { while (!isDelim(currentChar)) { this.token += currentChar; this.expIndex++; if (this.expIndex >= this.exp.length()) { break; } else { currentChar = this.exp.charAt(this.expIndex); } } this.tokenType = NUMBER_TOKEN; } else { this.token = EOE; return; } } public boolean isDelim(char c) { if ((" +-*/%^=()".indexOf(c) != -1)) return true; return false; } public static void main(String[] srgs) throws Exception { ExpressionParser test = new ExpressionParser(); String exp1 = "(4-2)*5/2-6"; System.out.println(exp1 + "=" + test.evaluate(exp1)); } }