正如大多数人所知，这类任务令人困惑。经过长时间的搜索编写Java代码来解决这个问题，我收到了以下问题：
Exception in thread "main" java.util.EmptyStackException
at java.util.Stack.peek(Unknown Source)
at java.util.Stack.pop(Unknown Source)
at FirstProTry.parseRPN(FirstProTry.java:128)
at FirstProTry.main(FirstProTry.java:192)
代码是：
import java.util.*;
import java.io.IOException;
import java.util.Stack;
public class FirstProTry {
boolean debug;
final String operands = "^*+-/" ;
double result;
private Stack1 theStack;
private String input;
private String output = "";
final static int PRECEDENCE_PLUS= 1;
final static int PRECEDENCE_MINUS= 1;
final static int PRECEDENCE_MULTIPLIY= 2;
final static int PRECEDENCE_DIVIDE= 2;
final static int PRECEDENCE_EXPONENT= 3;
final static int PRECEDENCE_PARANTHESIS= 4;
public FirstProTry(boolean db){
debug = db;
}
public FirstProTry(String in) {
input = in;
int stackSize = input.length();
theStack = new Stack1(stackSize);
}
public String doTrans() {
for (int j = 0; j < input.length(); j++) {
char ch = input.charAt(j);
switch (ch) {
case '+':
case '-':
gotOper(ch, 1);
break;
case '*':
case '/':
gotOper(ch, 2);
break;
case '(':
theStack.push(ch);
break;
case ')':
gotParen(ch);
break;
default:
output = output + ch;
break;
}
}
while (!theStack.isEmpty()) {
output = output + theStack.pop();
}
//System.out.println(output);
return output;
}
public void gotOper(char opThis, int prec1) {
while (!theStack.isEmpty()) {
char opTop = theStack.pop();
if (opTop == '(') {
theStack.push(opTop);
break;
}
else {
int prec2;
if (opTop == '+' || opTop == '-')
prec2 = 1;
else
prec2 = 2;
if (prec2 < prec1) {
theStack.push(opTop);
break;
}
else
output = output + opTop;
}
}
theStack.push(opThis);
}
public void gotParen(char ch){
while (!theStack.isEmpty()) {
char chx = theStack.pop();
if (chx == '(')
break;
else
output = output + chx;
}
}
//The following is the calculation part
public void parseRPN(String input){
String rpnStr = input;
String[] tokens = rpnStr.split("\\s+");//remove all white space
//Stack numberStack =new Stack(0);
Stack<Double> numberStack =new Stack<Double>();
boolean bAllowParenthesis = false;
for( String token : tokens)
{
if(token.equals("-")==false && isNumber(token ))
{
double d = Double.parseDouble( token ) ;
numberStack.push(d ) ;
}
else if( isOperand( token , bAllowParenthesis ) )
{
if( numberStack.size() < 2 )
{
System.out.println("Invalid Syntax, operator " + token + " must be preceeded by at least two operands");
return;
}
double num1 = numberStack.pop();
double num2 = numberStack.pop() ;
double result = this.calculate( num2 , num1 , token ) ;
numberStack.push( result);
}
else if( token.trim().length() > 0 )
System.out.println( token + " is invalid, only use numbers or operators " );
}
result= numberStack.pop();
}
double getResult() {
return result;
}
public boolean isNumber(String s){
String master="-0123456789.";
s = s.trim();
for( int i = 0;i < s.length() ;i++)
{
String lttr = s.substring(i, i+1);
if(master.indexOf( lttr) == -1)
return false;
}
return true ;
}
public boolean isOperand(String s, boolean allowParanethesis){
s = s.trim();
if (s.length() != 1 )
return false;
if (allowParanethesis && ( s.equals("(") || s.equals(")") ) )
return true;
else return operands.indexOf( s ) != -1 ;
}
private Double calculate(double num1, double num2, String op ) {
if( op.equals("+"))
return num1 + num2;
else if( op.equals("-"))
return num1 - num2;
else if( op.equals("*"))
return num1 * num2;
else if( op.equals("^"))
return Math.pow(num1 , num2 );
else if( op.equals("/") )
{
if(num2 ==0 )
throw new ArithmeticException("Division by zero!");
return num1 / num2;
}
else
{
System.out.println(op + " is not a supported operand") ;
return null;
}
}
public static void main(String[] args) throws IOException {
boolean debug = false;
FirstProTry rp = new FirstProTry(debug );
Scanner scan = new Scanner (System.in);
String output;
System.out.print("Enter an expression to be evaluated: ");
String input = scan.next();
FirstProTry theTrans = new FirstProTry(input);
output = theTrans.doTrans();
System.out.println("The expression in a postfix form: " + output + '\n');
String asPostfix_str = output.toString().replaceAll(",", " " ) ;
rp.parseRPN( asPostfix_str ) ;
System.out.println("The value of the postfix expression is: " + rp.getResult() ) ;
}
class Stack1 {
private int maxSize;
private char[] stackArray;
private int top;
public Stack1(int max) {
maxSize = max;
stackArray = new char[maxSize];
top = -1;
}
public void push(char j) {
stackArray[++top] = j;
}
public char pop() {
return stackArray[top--];
}
public char peek() {
return stackArray[top];
}
public boolean isEmpty() {
return (top == -1);
}
}
}
我尽力解决问题，但我无法解决问题。你能帮助我吗？
答案 0 :(得分：1)
在这一行中，如果堆栈上没有令牌会发生什么？
result= numberStack.pop();
提示：堆栈是空的，因此没有任何东西可以弹出......
似乎永远不会调用push，所以请在调试循环之前检查为什么会出现这种情况。