java的字符串表达式执行 java 字符串表达式求值

public class Test {
 
 public static void main(String[] args) {
 
  SimpleCalculator s=new SimpleCalculator();
 String methord="80*(1+0.5)"; //test
  double d=s.evaluate(methord );
  System.out.println(d);
 }
}
 
 
 
 
import java.util.Scanner;
import java.util.Stack;
public class SimpleCalculator {
 
 
 /**
  * Evaluate an arithmetic expression, and return the result as a double.
  *
  * @param input
  * the expression to evaluate.
  * @return the evaluated result.
  */
 public double evaluate(String input) {
  initialize();
  this.scanner = new Scanner(input);
  this.scanner
    .useDelimiter("\\s+|(?=[.0-9])(?<![.0-9])|(?![.0-9])(?<=[.0-9])|(?![.0-9])(?<![.0-9])");
  Token currentToken = nextToken();
  Token t = null;
  while (null != currentToken) {
   switch (currentToken.getKind()) {
   case NUMBER:
    // Simply push number tokens onto the evaluation stack.
    this.eval.push(currentToken.getValue());
    break;
   case LPAREN:
    // Simply push left parenthesis tokens onto the operator stack.
    this.ops.push(currentToken);
    break;
   case RPAREN:
    // Until a left parenthesis pops off the operator stack, keep
    // poping operators and execute them.
    // If the stack becomes empty without a matching left
    // parenthesis,
    // the expression must have syntax errors.
    for (t = this.ops.pop(); TokenKind.LPAREN != t.getKind(); t = this.ops
      .pop()) {
     if (ops.empty())
      throw new Error("Syntax Error: unmatched parenthesis");
     doOperation(t);
    }
    break;
   default:
    // For binary arithmetic operators, keep poping operators whose
    // binding power
    // is less or equal to the current token's and execute them;
    // after that push
    // the current token onto the operator stack.
    if (!ops.empty()) {
     for (t = this.ops.pop(); currentToken.getKind()
       .getBindingPower() < t.getKind().getBindingPower(); t = this.ops
       .pop()) {
      doOperation(t);
      if (this.ops.empty()) {
       t = null;
       break;
      }
     }
    }
    if (null != t)
     ops.push(t);
    ops.push(currentToken);
    break;
   }
   // reinitialize
   currentToken = nextToken();
  }
  // execute remaining operators on stack
  while (!ops.empty()) {
   t = this.ops.pop();
   doOperation(t);
  }
  // the result is on the top of evaluation stack,
  // pop it off and return the result.
  return this.eval.pop();
 }
 /*
  * Initialize the evaluation and operator stacks.
  */
 private void initialize() {
  if (null == this.eval)
   this.eval = new Stack<Double>();
  if (null == this.ops)
   this.ops = new Stack<Token>();
  this.eval.clear();
  this.ops.clear();
 }
 /*
  * Return the next token from the input expression. The token returned will
  * be associated with its numeric value, if and only if the token is a
  * number.
  */
 private Token nextToken() {
  Token t = null;
  if (this.scanner.hasNextDouble()) {
   t = new Token(TokenKind.NUMBER, this.scanner.nextDouble());
  } else if (this.scanner.hasNext()) {
   String s = this.scanner.next("[-+*/()]");
   if ("+".equals(s)) {
    t = new Token(TokenKind.ADD);
   } else if ("-".equals(s)) {
    t = new Token(TokenKind.SUBTRACT);
   } else if ("*".equals(s)) {
    t = new Token(TokenKind.MULTIPLY);
   } else if ("/".equals(s)) {
    t = new Token(TokenKind.DIVIDE);
   } else if ("(".equals(s)) {
    t = new Token(TokenKind.LPAREN);
   } else if (")".equals(s)) {
    t = new Token(TokenKind.RPAREN);
   }
  }
  return t;
 }
 /*
  * Execute a binary arithmetic operation. Pop the top two values off the
  * evaluation stack, do the operation, and then push the result back onto
  * the evaluation stack.
  */
 private void doOperation(Token t) {
  double y = this.eval.pop();
  double x = this.eval.pop();
  double temp = t.getKind().doOperation(x, y);
  this.eval.push(temp);
 }
 /*
  * Tokenizer for the input expression.
  */
 private Scanner scanner;
 /*
  * Evaluation stack.
  */
 private Stack<Double> eval;
 /*
  * Operator stack, for converting infix expression to postfix expression.
  */
 private Stack<Token> ops;
 public static void main(String[] args) {
  if (args.length < 1) {
   System.err.println("Usage: java SimpleCalculator <expression>");
   System.exit(1);
  }
  SimpleCalculator calc = new SimpleCalculator();
  double result = calc.evaluate(args[0]);
  System.out.println(result);
 }
}
enum TokenKind {
 // operators
 ADD(1) {
  public double doOperation(double x, double y) {
   return x + y;
  }
 },
 SUBTRACT(2) {
  public double doOperation(double x, double y) {
   return x - y;
  }
 },
 MULTIPLY(3) {
  public double doOperation(double x, double y) {
   return x * y;
  }
 },
 DIVIDE(4) {
  public double doOperation(double x, double y) {
   return x / y;
  }
 },
 // punctuation
 LPAREN(0), RPAREN(0),
 // number
 NUMBER(0);
 TokenKind(int bindingPower) {
  this.bindingPower = bindingPower;
 }
 public int getBindingPower() {
  return this.bindingPower;
 }
 public double doOperation(double x, double y) {
  return Double.NaN; // dummy, operation not supported
 }
 private int bindingPower;
}
class Token {
 public Token(TokenKind kind) {
  this(kind, Double.NaN);
 }
 public Token(TokenKind kind, double value) {
  this.kind = kind;
  this.value = value;
 }
 public TokenKind getKind() {
  return this.kind;
 }
 public double getValue() {
  return this.value;
 }
 private TokenKind kind;
 private double value;
}