is passing class type necessary?

Question

protected <T> void myMethod(T obj, Class<?> type)

I have seen this method signature and I don't understand why the type of the first argument explicitly passed as an argument. In fact I can get the type from obj.getClass();

protected <T> void myMethod(T obj) {
     Class type = obj.getClass();
}

Is this just a design choice? Is there anything beneficial for doing the first method?

What does the method do? What is its javadoc? Your question implies that the type is the type of the first argument. But maybe it's not. — JB Nizet, Feb 11 '15 at 20:45
Assuming type (second argument) is the type of the first argument — codereviewanskquestions, Feb 11 '15 at 20:49
Add more code, I suppose the `type` parameter has nothing to do with the `obj` parameter — Francesco Palmiotto, Feb 11 '15 at 20:50
Beware that `obj` may be an instance of a subclass `S` of `T`, in which case `obj.getClass()` will give you `S`, not `T`. — Aasmund Eldhuset, Feb 11 '15 at 20:51
"In fact I can get the type from obj.getClass()" No, you cannot in general. `obj` might be `null` or be a subclass of `T`. — newacct, Feb 11 '15 at 23:01

score 0 · Answer 1 · answered Feb 11 '15 at 20:48

The second implementation is actually the better one. In the first one, you might pass inconsistent parameters. E.g. you could pass an object of class A and pass integer as the second parameter. Therefore your code would be more vulnerable concerning typing errors.

Adam · Accepted Answer · 2015-02-11T20:59:58.890

0

One case I can think of

Instantiating a primitive array of a given type T, you cannot do this from the type parameter, you need the actual class.

Example

private static <T> void foo() {
    T[] bar = new T[1];  // <== not allowed
}

With a class parameter

private static <T> void bar(Class<T> clazz) {
    T[] bar = (T[]) Array.newInstance(clazz, 1);
}

edited Feb 11 '15 at 20:59

answered Feb 11 '15 at 20:49

Adam

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is passing class type necessary?

2 Answers2