解释器模式

给定一个语言，定义它的文法的一种表示，并定义一个解释器，这个解释器使用该表示来解释语言中的句子

实例（简易加减法解释器，单个数字不大于10）

public interface Expression {
    int operation();
}

//解释数字
public class VarExpression implements Expression {
    private int  value;
    public VarExpression(int value){
        this.value  = value;
    }
    @Override
    public int operation() {
        return value;
    }
}

//解释减法
public class SubExpression implements Expression {
    private int left;
    private int right;
    public SubExpression(int left, int right){
        this.left =  left;
        this.right = right;
    }
    @Override
    public int operation() {
        return left - right;
    }
}

//解释加法
public class AddExpression implements Expression {
    private int left;
    private int right;
    public AddExpression(int left,int right){
        this.left =  left;
        this.right = right;
    }
    @Override
    public int operation() {
        return left + right;
    }
}

public class Calculator {
    private int left;
    private int right;

    public int operation(List<String> list) {
        Stack<Integer> stack = new Stack<>();
        for (int i = 0; i < list.size(); i++) {
            switch (list.get(i)) {
                case "+":
                    left = stack.pop();
                    right = Integer.valueOf(list.get(++i));
                    stack.push(new AddExpression(left, right).operation());
                    break;
                case "-":
                    left = stack.pop();
                    right = Integer.valueOf(list.get(++i));
                    stack.push(new SubExpression(left, right).operation());
                    break;
                default:
                    stack.push(Integer.valueOf(list.get(i)));
            }
        }
        return stack.pop();
    }
}

//测试
public class Client {

    public static void main(String[] args) {
        Calculator calculator = new Calculator();
        while (true) {
            List<String> list = new ArrayList<>();
            System.out.println("输入计算公式");
            Scanner input = new Scanner(System.in);
            String str = input.nextLine();
            String var;
            for (char c : str.toCharArray()) {
                if (c != '+' && c != '-') {
                    System.out.print(c + "=");
                    var = input.nextLine();
                    list.add(var);
                } else {
                    list.add(String.valueOf(c));
                }
            }
            System.out.println(str + "=" + calculator.operation(list));
        }
    }
}

结果

 通常不在重要模块使用，避免过于复杂难以维护