后缀表达式的计算

1.新建栈类，里面包括基本对象 

public class StackX {
    private int maxSize;
    private int [] stackArray;
    private int top;

    public StackX(int size){
        maxSize=size;
        stackArray=new int[maxSize];
        top=-1;
    }

    public void push(int j){
        stackArray[++top]=j;
    }

    public int pop(){
        return stackArray[top--];
    }
    public int peek(){
        return stackArray[top];
    }

    public boolean isEmpty(){
        return (top==-1);
    }

    public boolean isFull(){
        return (top==maxSize-1);
    }

    public int size(){
        return top+1;
    }

    public int peekN(int n){
        return stackArray[n];
    }

    public void displayStack(String s){
        System.out.print(s);
        System.out.print("Stack (bottom-->top):");
        for(int j=0;j<size();j++){
            System.out.print(peekN(j));
            System.out.print(' ');
        }
        System.out.println("");
    }
}

2.开始计算--》遇到数字压入栈中，遇到操作符就直接把前两个出栈进行计算，后面以此类推

import java.util.Stack;

public class ParsePost {
    private StackX theStack;
    private String input;
    public ParsePost(String s){
        input=s;
    }

    public int doParse(){
        theStack=new StackX(20);
        char ch; int j;
        int num1,num2,interAns;

        for(j=0;j<input.length() ;j++){
            ch=input.charAt(j);
            theStack.displayStack(""+ch+" ");
            if(ch>='0'&&ch<='9')
                theStack.push((int)(ch-'0'));
            else{
                num2=theStack.pop();
                num1=theStack.pop();
                switch (ch){
                    case '+':
                        interAns=num1+num2;
                        break;
                    case '-':
                        interAns=num1-num2;
                        break;
                    case '*':
                        interAns=num1*num2;
                        break;
                    case '/':
                        interAns=num1/num2;
                        break;
                        default:
                            interAns=0;
                }

                theStack.push(interAns);
            }
        }
        interAns=theStack.pop();
        return interAns;
    }
}

3.主类

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

public class PostfixApp {
    public static void main(String[] args) throws IOException {
        String input;
        int output;
        while (true) {
            System.out.println("Enter infix");
            System.out.flush();
            input = getString();
            if (input.equals(""))
                break;

            ParsePost aParser = new ParsePost(input);
            output = aParser.doParse();
            System.out.println("Evaluates to" + output);


        }

    }

    public static String getString() throws IOException {
        InputStreamReader isr = new InputStreamReader(System.in);
        BufferedReader br = new BufferedReader(isr);
        String s = br.readLine();
        return s;

    }
}