解析算术表达式

下面是用栈结构解析算术表达式并计算的方法，只适用于操作数是个位数的（如果需要处理多位数，则修改后缀表达式的存储方式即可）。

由中缀表达式获得后缀表达式

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

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

    public void push(char c){
        stackArray[++top]=c;
    }
    public char pop(){
        return stackArray[top--];
    }

    public char peek() {
        return stackArray[top];
    }

    public int size(){
        return top+1;
    }
    public boolean isEmpty(){
        return top==-1;
    }
    public char peekN(int index){
        return stackArray[index];
    }
    public void displayStack(String s){
        System.out.print(s);
        System.out.print(" bottom --> top :");
        for (int i=0;i<size();i++){
            System.out.print(peekN(i)+" ");
        }
        System.out.println();
    }

    public static void main(String[] args) throws IOException {
        String out;
        while (true){
            String s=getString();
            if (s=="")
                break;
            InToPost inToPost=new InToPost(s);
            out= inToPost.doTrans();
            System.out.println(out);
        }
    }
    public static String getString() throws IOException {
        InputStreamReader inputStreamReader=new InputStreamReader(System.in);
        BufferedReader br=new BufferedReader(inputStreamReader);
        String s= br.readLine();
        return s;
    }
}

class InToPost{
    private StackX theStack;
    private String in;
    private String out="";

    public InToPost(String in){
        this.in=in;
        int stackSize=in.length();
        theStack=new StackX(stackSize);
    }

    public String doTrans(){
        for (int i=0; i<in.length();i++){
            char c=in.charAt(i);
            this.theStack.displayStack("For "+c+" ");

            switch (c){
                case '+':
                case '-':
                    getOper(c,1);
                    break;
                case '*':
                case '/':
                    getOper(c,2);
                    break;
                case '(':
                    theStack.push(c);
                    break;
                case ')':
                    getParen(c);
                    break;
                default:
                    out+=c;
            }
        }
        while (!theStack.isEmpty()){
            theStack.displayStack("while ");
            out+=theStack.pop();
        }
        theStack.displayStack(" end ");
        return out;
    }

    private void getParen(char c) {
        while (!this.theStack.isEmpty()){
            char cc=theStack.pop();
            if(cc=='(')
                break;
            else {
                out+=cc;
            }
        }
    }

    public void getOper(char c,int prec){
        while (!this.theStack.isEmpty()){
            char top=this.theStack.pop();
            if (top=='('){
                theStack.push(top);
                break;
            }else {
                int prec2;
                if (top =='+' ||top =='-'){
                    prec2=1;
                }else {
                    prec2=2;
                }
                if (prec2<prec){
                    theStack.push(top);
                    break;
                }else {
                    out+=top;
                }
            }
        }
        theStack.push(c);
    }
}

计算后缀表达式：

class ParsePost{
    private StackO theStack;
    private String input;
    ParsePost(String input){
        this.input=input;
    }
    public int doParse(){
        theStack=new StackO(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;
    }
}
class StackO {
    private int maxSize;
    private int[] stackArray;
    private int top;

    public StackO(int maxSize){
        this.maxSize=maxSize;
        this.stackArray=new int[maxSize];
        this.top=-1;
    }

    public void push(int c){
        stackArray[++top]=c;
    }
    public int pop(){
        return stackArray[top--];
    }

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

    public int size(){
        return top+1;
    }
    public boolean isEmpty(){
        return top==-1;
    }
    public int peekN(int index){
        return stackArray[index];
    }
    public void displayStack(String s){
        System.out.print(s);
        System.out.print(" bottom --> top :");
        for (int i=0;i<size();i++){
            System.out.print(peekN(i)+" ");
        }
        System.out.println();
    }

    public static void main(String[] args) throws IOException {
        String out;
        while (true){
            System.out.println("Enter postfix: ");
            System.out.flush();
            String s=getString();
            if (s=="")
                break;
            InToPost inToPost=new InToPost(s);
            out= inToPost.doTrans();
            ParsePost parsePost=new ParsePost(out);
            int result= parsePost.doParse();
            System.out.println(out);
            System.out.println(result);
            System.out.println("------------");
        }
    }
    public static String getString() throws IOException {
        InputStreamReader inputStreamReader=new InputStreamReader(System.in);
        BufferedReader br=new BufferedReader(inputStreamReader);
        String s= br.readLine();
        return s;
    }
}