CODEWITHSUNDEEP
javareflection
Davio
Davio

Reputation: 4737

Compare Java Type to Class

I'm having a little trouble putting this simple example together.

With reflection, I can get a type of a certain declared field. But how do I compare this to a known class?

What I'm trying to do is something like:

Type t = myField.getType();
if (t.equals(MyOwnClass.class)) {
    // Now I know myField is of type MyOwnClass
}

Upvotes: 4

Views: 2255

Answers (2)

user719662
user719662

Reputation:

Essentially, you already found the correct answer, with only a bit of commenting needed. The other answer is just wrong.

Firstly, getType() returns a Class<?> reference, with a Class class implementing a Type interface; using a Type variable for a isAssignableFrom() call is wrong for two reasons:

  • Type doesn't have isAssignableFrom() method
  • Class#isAssignableFrom() requires a Class<?> parameter, not a Type parameter

Simply put, you probably shouldn't compare a Class/Type this way, because you're hiding the relevant typing and requiring unnecessary casts. On the other hand, you just upcasted the Class<?> reference to Type reference, and since equals() called on Class (even casted to Type) actually calls Object#equals(), it simply calls

public boolean equals(Object obj) {
    return (this == obj);
}

straight from Object.java, as Class.java doesn't override it in any way.

Secondly, you could do what you already done, but is one of the proper ways:

if ( myField.getType() == MyOwnClass.class ) {
    // Now I know myField is of type MyOwnClass
} // note that this requires a single ClassLoader to be used! Two ClassLoaders could in theory load one class with two separate Class instances

alternatively, you could

if ( myField.getType().getCanonicalName().equals( MyOwnClass.class.getCanonicalName() ) {
    // Now I know myField is of type MyOwnClass
} // note that this uses a name comparison, not a class comparison

or even

if ( myField.getType().isAssignableFrom( MyOwnClass.class )) {
  // Now I know MyOwnClass objects can be cast to myField.getType() safely
} // note: they are either equal or MyOwnClass is a subclass of myField.getType()

tl;dr Using equals() on a Class object is unnecessary here, as two equal classes have to be == anyway, as is casting to Type - still, the answer you provided in your question is a working solution indeed.

Upvotes: 1

Marius
Marius

Reputation: 3161

You could use the function 

isAssignableFrom()

to determine this. Note, that also inherited classes are allowed.

Type t = myField.getType();
if (t.isAssignableFrom(MyOwnClass.class)) {
    // Now I know myField is of type MyOwnClass
}

Upvotes: 0

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