简易版的计算器

就不多说了，很简单，直接上代码。（注意在整个运算中没有涉及浮点数）

public class StackTest3 {


	private static Stack s1=new Stack();  //符号栈
	private static Stack s2=new Stack();  //数字栈
	
	public static void main(String[] args) {
		Scanner in=new Scanner(System.in);
		
		String s=in.next();
		int by=0;
		for (int i = 0; i < s.length(); i++) {
			char ch=s.charAt(i);   //当前字符
			int flag = priority(ch);
			if(s2.isEmpty()==true&&(flag==1||flag==0)) {
				if(flag==1) {
					System.out.println("错误");
					return;
				}
				s2.push(0);
			}
			if(flag==-1) {
				if(by>0) {
					Integer pop1 = s2.pop();
					
					StringBuffer v1=new StringBuffer(String.valueOf(pop1));
					v1.append(String.valueOf(ch));
					int num = Integer.parseInt(v1.toString());
					s2.push(num);
					by++;
				}else {
					s2.push(Integer.valueOf(String.valueOf(ch)));
					by++;
				}
			}else if(flag==1) {
				by=0;
				if(s1.isEmpty()==true||s1.peek()=='(') {
					s1.push(ch);
				}else {
					Character peek = s1.peek();
					if(priority(peek)==1) {
						//当前的优先级小于或等于栈顶的优先级
						Integer pop1 = s2.pop();   //数据栈中的第一个 元素
						Integer pop2 = s2.pop();  //数据栈中的第二个元素
						
						Character pop = s1.pop();  //字符栈中的栈顶元素
						
						Integer num = yunsuan(pop2,pop1,pop);
						if(num==null) {
							System.out.println("错误");
							return;
						}
						s1.push(ch);
						s2.push(num);
					}else if(priority(peek)==0) {
						s1.push(ch);
					}
				}
			}else if(flag==0) {   //当前是 + 或-
				by=0;
				if(s1.isEmpty()==true||s1.peek()=='(') {
					s1.push(ch);
				}else {
					Character peek = s1.peek();
					if(priority(peek)==1||priority(peek)==0) {
						//当前的优先级小于或等于栈顶的优先级
						Integer pop1 = s2.pop();   //数据栈中的第一个 元素
						Integer pop2 = s2.pop();  //数据栈中的第二个元素
						
						Character pop = s1.pop();  //字符栈中的栈顶元素
						
						Integer num = yunsuan(pop2,pop1,pop);
						if(num==null) {
							System.out.println("错误");
							return;
						}
						s1.push(ch);
						s2.push(num);
					}
				}
			}else if(flag==2) {				//左括号
				by=0;
				s1.push(ch);
			}else if(flag==3) {				//右括号
				by=0;
				while(s1.peek()!='(') {
					Integer pop1 = s2.pop();   //数据栈中的第一个 元素
					Integer pop2 = s2.pop();  //数据栈中的第二个元素
					Character pop = s1.pop();  //字符栈中的栈顶元素
					
					Integer num = yunsuan(pop2,pop1,pop);
					if(num==null) {
						System.out.println("错误");
						return;
					}
					s2.push(num);
				}
				s1.pop();
			}
		}
		while(s1.isEmpty()!=true) {
			Integer pop1 = s2.pop();
			Integer pop2 = s2.pop();
			Character pop = s1.pop();
			Integer num = yunsuan(pop2,pop1,pop);	
			if(num==null) {
				System.out.println("错误");
				return;
			}
			s2.push(num);
		}
		System.out.println(s2.peek());
	}
	
	public static int priority(char oper) {
		if(oper=='*'||oper=='/') {
			return 1;
		}else if(oper=='+'||oper=='-') {
			return 0;
		}else if(oper=='(') {
			return 2;
		}else if(oper==')') {
			return 3;
		}else {
			return -1;
		}
	}
	
	public static Integer yunsuan(Integer x,Integer y,Character pop) {
		if(pop=='+') {
			return x+y;
		}else if(pop=='-') {
			return x-y;
		}else if(pop=='*') {
			return x*y;
		}else if(pop=='/') {
			if(y==0) {
				return null;
			}
			return x/y;
		}
		return null;
	}
}