类实现

import java.util.Scanner;

public class Expression {
	public Expression(){}
	public Expression(String str){
		this.str = str + "#";
	}
	public int Compute(){
		int[] OPND = new int[100];//运算对象栈
		char[] OPTR = new char[100];//运算符栈
		OPTR[0] = '#';
		int top1 = -1;
		int top2 = 0;//当前符号栈内存放着'#'
		int i,k,x,y,z = 0;
		char op;
		Scanner scan = new Scanner(str);
		for(i = 0; i<str.length();){
			if(str.charAt(i) >= 48 && str.charAt(i) <= 57){
				OPND[++top1] = str.charAt(i++) - 48;//数字直接入栈
			}
			else{
				k = this.Comp(str.charAt(i), OPTR[top2]);
				if(k == 1){
					OPTR[++top2] = str.charAt(i++);
				}
				else if(k == -1){
					y = OPND[top1--];
					x = OPND[top1--];
					op = OPTR[top2--];
					switch(op){
					case '+':z = x + y;break;
					case '-':z = x - y;break;
					case '*':z = x * y;break;
					case '/':z = x / y;break;
					}
					OPND[++top1] = z;
				}
				else if(k == 0){
					top2--;
					i++;
				}
			}
		}
		return OPND[top1];
	}
	private int Comp(char str1,char str2){//1 str1优先 0 优先级相等 -1 str2优先
		switch(str1){
			case '+':
			case '-':
				if(str2 == '(' || str2 == '#')
					return 1;
				else 
					return -1;
			case '*':
			case '/':
				if(str2 == '*' || str2 == '/') 
					return -1;
				else
					return 1;
			case '(':
				return 1;
			case ')':
				if(str2 == '(') 
					return 0;
				else
					return -1;
			case '#':
				if(str2 == '#')
					return 0;
				else
					return -1;
			default:
				break;
		}
		return 2;
	}
	private String str;
}
	

主方法的调用

import java.util.Scanner;

public class Shujvjiegou {

	public static void main(String[] args) {
		Scanner scan = new Scanner(System.in);
		System.out.println("请输入一个表达式");
		String str = scan.next();
		Expression ex = new Expression(str);
		System.out.println("表达式的值是:" + ex.Compute());
		scan.close();
	}

}

效果预览