解释器模式的原理和实现

解释器模式为某个语言定义它的语法（或者叫文法）表示，并定义一个解释器用来处理这个语法。

public interface Expression {
  long interpret();
}
public class NumberExpression implements Expression {
  private long number;
  public NumberExpression(long number) {
    this.number = number;
  }
  public NumberExpression(String number) {
    this.number = Long.parseLong(number);
  }
  @Override
  public long interpret() {
    return this.number;
  }
}
public class AdditionExpression implements Expression {
  private Expression exp1;
  private Expression exp2;
  public AdditionExpression(Expression exp1, Expression exp2) {
    this.exp1 = exp1;
    this.exp2 = exp2;
  }
  @Override
  public long interpret() {
    return exp1.interpret() + exp2.interpret();
  }
}
// SubstractionExpression/MultiplicationExpression/DivisionExpression与AdditionExpression代码结构类似，这里就省略了
public class ExpressionInterpreter {
  private Deque<Expression> numbers = new LinkedList<>();
  public long interpret(String expression) {
    String[] elements = expression.split(" ");
    int length = elements.length;
    for (int i = 0; i < (length+1)/2; ++i) {
      numbers.addLast(new NumberExpression(elements[i]));
    }
    for (int i = (length+1)/2; i < length; ++i) {
      String operator = elements[i];
      boolean isValid = "+".equals(operator) || "-".equals(operator)
              || "*".equals(operator) || "/".equals(operator);
      if (!isValid) {
        throw new RuntimeException("Expression is invalid: " + expression);
      }
      Expression exp1 = numbers.pollFirst();
      Expression exp2 = numbers.pollFirst();
      Expression combinedExp = null;
      if (operator.equals("+")) {
        combinedExp = new AdditionExpression(exp1, exp2);
      } else if (operator.equals("-")) {
        combinedExp = new AdditionExpression(exp1, exp2);
      } else if (operator.equals("*")) {
        combinedExp = new AdditionExpression(exp1, exp2);
      } else if (operator.equals("/")) {
        combinedExp = new AdditionExpression(exp1, exp2);
      }
      long result = combinedExp.interpret();
      numbers.addFirst(new NumberExpression(result));
    }
    if (numbers.size() != 1) {
      throw new RuntimeException("Expression is invalid: " + expression);
    }
    return numbers.pop().interpret();
  }
}

解释器模式实战举例