我正在写这个作业，我想我拥有的一切都是我需要的。当我运行它时，它编译然后一旦我输入一些中缀问题来测试它我得到一个EmptyStackException。
import java.util.Scanner;
import java.util.Stack;
//InfixtoPostfix class inherits Stack methods so you can use push(), pop() and peek()
public class InfixtoPostfix extends Stack {
Stack st = new Stack();
public String InToPost(String infixString) {
// declare and initialize postfixString
String postfixString = "";
// now parse the infixString character by character
for (int index = 0; index < infixString.length(); ++index) {
// declares new stack
Stack st = new Stack();
// declares topStack as type char
char topStack;
// splits up the infixString to single chars
char parseString = infixString.charAt(index);
// if the next character is a left brace add it to the stack
if (parseString == '(') {
st.push(parseString);
/*
* if the next character is a right brace set //the top Stack st
* as the variable topStack while the stack is not empty and top
* of the stack is NOT left brace add topStack to end of
* postfixString and pop the stack, then set topStack as the top
* st.
*/
} else if (parseString == ')') {
topStack = (char) st.peek();
while (!st.isEmpty() && (char) st.peek() != '(') {
postfixString = postfixString + topStack;
st.pop();
topStack = (char) st.peek();
}
/*
* runs if the next character is a '+' or '-' if stack is empty
* push the character onto the stack otherwise set topStack as
* the top of st and while st is not empty and topStack is not
* left or right brace pop the stack and add topstack onto the
* postfixString, then push the character onto the stack
*/
} else if (parseString == '+' || parseString == '-') {
if (st.isEmpty()) {
st.push(parseString);
}
else {
topStack = (char) st.peek();
while (!st.isEmpty() || topStack != '(' || topStack != ')') {
st.pop();
postfixString = postfixString + topStack;
}
st.push(parseString);
}
/*
* if the next character is '*' or '/' and if the stack is empty
* push it onto the stack, else set topStack as the top of st
* and while the stack is not empty and topStack does not equal
* '+' or '-' pop the stack and add topStack onto the
* postfixString, then push the character onto the stack
*/
} else if (parseString == '*' || parseString == '/') {
if (st.isEmpty()) {
st.push(parseString);
} else {
topStack = (char) st.peek();
while (!st.isEmpty() && topStack != '+' && topStack != '-') {
st.pop();
postfixString = postfixString + topStack;
}
st.push(parseString);
}
/*
* if the character is not an operator then addit onto the
* postfixString
*/
} else {
postfixString = postfixString + parseString;
}
/*
* while the stack is not empty set topStack as the top of st if
* topStack is not a left brace pop the stack and add topStack onto
* the postfixString
*/
while (st.isEmpty()) {
topStack = (char) st.peek();
if (topStack != '(') {
st.pop();
postfixString = postfixString + topStack;
}
}
}
// returns the postfixString
return postfixString;
}
/*
* complete this code if parseString is left brace, then push it to stack
* else if the character is right brace: a. Declare a variable of type char
* called topStack to store the top of stack you might need to cast the top
* of stack to char using (char) b. Write a while loop. while topStack is
* not left brace AND stack is not empty 1- add topStack to postfixString 2-
* pop the stack 3- set topStack to store the new top of stack exit the
* while then pop the stack else if parseString is '+' or '-' if the stack
* is empty, push parseString to stack else declare topStack, store the top
* stack in topStack variable while the stack is not empty OR topStack is
* not left brace OR topStack is not right brace pop the stack add topStack
* to postfixString exit the while then push parseString to stack else if
* parseString is '*' or '/' if the stack is empty, push parseString to
* stack else declare topStack then store the top stack in topStack variable
* while the stack is not empty AND topStack is not '+' AND topStack is not
* '-' pop the stack add topStack to postfixString exit the while then push
* parseString to stack else add parseString to postfixString
*
* while stack is empty declare topStack variable to store top stack if
* topStack is not left brace then pop the stack and add topStack to
* postfixString exit the while loopreturn postfixString. This is the return
* of InToPost method
*/
public static void main(String[] args) {
InfixtoPostfix mystack = new InfixtoPostfix();
System.out.println("Type in an infix string followed by key");
Scanner scan = new Scanner(System.in);
String str = scan.next();
System.out.println("The Expression you have typed in infix form :\n"
+ str);
System.out.println("The Equivalent Postfix Expression is :\n"
+ mystack.InToPost(str));
}
}
答案 0 :(得分：1)
在以下代码中：
while (st.isEmpty()) {
topStack = (char) st.peek();
if (topStack != '(') {
st.pop();
postfixString = postfixString + topStack;
}
}
您在堆栈isEmpty()时进行迭代。如果您在空堆栈上执行peek()，则会抛出EmptyStackException。
将条件替换为：
while (!st.isEmpty())