Does Java have an “is kind of class” test method

问题

I have a baseclass, Statement, which several other classes inherit from, named IfStatement, WhereStatement, etc... What is the best way to perform a test in an if statement to determine which sort of Statement class an instance is derived from?

回答1:

if (obj.getClass().isInstance(Statement.class)) {
   doStuffWithStatements((Statement) obj));
}

The nice thing about this technique (as opposed to the "instanceof" keyword) is that you can pass the test-class around as an object. But, yeah, other than that, it's identical to "instanceof".

NOTE: I've deliberately avoided editorializing about whether or not type-instance-checking is the right thing to do. Yeah, in most cases, it's better to use polymorphism. But that's not what the OP asked, and I'm just answering his question.

回答2:

if(object instanceof WhereStatement) {
   WhereStatement where = (WhereStatement) object;
   doSomething(where);
}

Note that code like this usually means that your base class is missing a polymorphic method. i.e. doSomething() should be a method of Statement, possibly abstract, that is overridden by sub-classes.

回答3:

The answer to your question is instanceof.

However, keep in mind that if your code needs instanceof, it's a sign that something is not right with your design. There are some cases when instanceof is justified, but they are rather exceptions. Usually, if your subclasses need to behave differently, you have to use polymorphism instead of if()s.

回答4:

isAssignableFrom(java.lang.Class) from class is your answer. http://download.oracle.com/javase/6/docs/api/java/lang/Class.html#isAssignableFrom(java.lang.Class)

回答5:

Try this:

if (Statement.class.isInstance(obj)) {
    doStuffWithStatements((Statement) obj));
}

since Class.isInstance() method takes an object instance as a parameter.

回答6:

This is not the way to do things in an object-oriented way, it's a throwback to the old code/data dichotomy. Now that's not necessarily a bad thing (if you know what you're doing) but it should be left to the non-object-oriented languages like C.

With proper design, you don't need that sort of behavior. Instead of the construct:

if (obj.getClass().isInstance(Statement.class)) {
    doStuffWithStatements((Statement) obj));
}

(apologies to benjismith for 'stealing' his code), you should really be making the object itself responsible for its own activities thus:

obj.doStuff();

Then each different obj class will have its own definition for doStuff. That is the right way to do it.

来源：https://stackoverflow.com/questions/928531/does-java-have-an-is-kind-of-class-test-method

标签

java

reflection

inheritance

evaluation