在使用对象树时我应该避免使用instanceof吗？

发布于 2024-09-29 11:03:46 字数 676 浏览 7 评论 0原文

我有一个代表不同语言构造的类层次结构：

Expression <- NumericLiteral
              UnaryExpression
              BinaryExpression
              IndexingExpression
              IteratedExpression
                   ...

这些类的对象形成复杂的树层次结构，我必须在其上执行各种结构检查，例如，如果节点是 IteratedExpression，则其第一个子节点应该是 IndexingExpression。如果检查只涉及一个级别，我可以使用访问者模式，但在更复杂的情况下，如下例所示，我使用instanceof。

void visit(IteratedExpression node) {
    if (!(node.getChild(0) instanceof IndexingExpression)) {
        // report error
    }
}

这是instanceof的正确使用还是我的设计有缺陷？有哪些替代方案？

由于建议了一些替代方案，我想强调问题的第一部分：

这是instanceof的正确使用还是我的设计有缺陷？

原文

I have a class hierarchy representing different language constructs:

Expression <- NumericLiteral
              UnaryExpression
              BinaryExpression
              IndexingExpression
              IteratedExpression
                   ...

The objects of these classes form complex tree hierarchies on which I have to perform various structural checks, e.g. if the node is an IteratedExpression then its first child should be an IndexingExpression. If the check only involves one level I can use the Visitor pattern, but in more complex cases as in the example below I am using instanceof.

void visit(IteratedExpression node) {
    if (!(node.getChild(0) instanceof IndexingExpression)) {
        // report error
    }
}

Is it a proper use of instanceof or I have a flaw in my design?
What are the alternatives?

Since there were some alternatives suggested I would like to emphasize the first part of the question:

Is it a proper use of instanceof or I have a flaw in my design?

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╄→承喏 2024-10-06 11:03:46

像这样：

class Visitor {
  boolean indexingExpected;
  void startIteratedExpression() {
    indexingExpected = true;
  }
  void doIndexing() {
    indexingExpected = false;
  }
  void someOtherVisit() {
    if (indexingExpected) {
      throw IllegalStateException();
    }
  }
}
clas IteratedExpression {
  private List<Expression> children;
  public void visit(Visitor visitor) {
    visitor.startIteratedExpression();
    for(Expression child : childrenExpression) {
      child.visit(visitor);
    }
    visitor.endIteratedExpression();
  }
}
class IndexingExpression extends Expression {
  public void visit(Visitor visit) {
    visitor.doIndexing();
  }
}

如果您想使用 Visitor，树中有多少层并不重要。

Like this:

class Visitor {
  boolean indexingExpected;
  void startIteratedExpression() {
    indexingExpected = true;
  }
  void doIndexing() {
    indexingExpected = false;
  }
  void someOtherVisit() {
    if (indexingExpected) {
      throw IllegalStateException();
    }
  }
}
clas IteratedExpression {
  private List<Expression> children;
  public void visit(Visitor visitor) {
    visitor.startIteratedExpression();
    for(Expression child : childrenExpression) {
      child.visit(visitor);
    }
    visitor.endIteratedExpression();
  }
}
class IndexingExpression extends Expression {
  public void visit(Visitor visit) {
    visitor.doIndexing();
  }
}

If you want use Visitor it doesn't matter how many levels you have in your tree.

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