解释器模式

给定一个语言之后，解释器模式可以定义出其文法的一种表示，并同时提供一个解释器。客户端可以使用这个解释器来解释这个语言中的句子。比如一个运算表达式，数字是终结符表达式，运算符号是非终结表达式。
示例如下：

public class Expression {
    public static void main(String[] a){
        Expression expression=new Expression();
        Context context=expression.new Context("34+2+9");
        System.out.println(context.expression());
    }

    interface IOperation{
        int expression();
    }

    class NumOperation implements IOperation{
        private int num;

        public NumOperation(int num) {
            this.num = num;
        }

        @Override
        public int expression() {
            return num;
        }
    }

    class PlusOperation implements IOperation{
        private IOperation start,end;

        public PlusOperation(IOperation start, IOperation end) {
            this.start = start;
            this.end = end;
        }

        @Override
        public int expression() {
            return start.expression()+end.expression();
        }
    }

    class Context implements IOperation{
        private Stack<NumOperation> operations=new Stack<>();
        private String source;

        public Context(String source) {
            this.source = source;
            init();
        }

        private void init(){
            String[] strings=source.split("\\+");
            for(int i=0;i<strings.length;i++){
                if(operations.empty()){
                    operations.add(new NumOperation(Integer.valueOf(strings[i])));
                }else{
                    operations.add(new NumOperation(new PlusOperation(operations.pop(),new NumOperation(Integer.valueOf(strings[i]))).expression()));
                }
            }
        }

        @Override
        public int expression() {
            return operations.pop().expression();
        }
    }
}