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Just came across a place where I'd like to use generics and I'm not sure how to make it work the way I want.
I have a method in my data layer that does a query and returns a list of objects. Here's the signature.
public List getList(Class cls, Map query)
This is what I'd like the calling code to look like.
List<Whatever> list = getList(WhateverImpl.class, query);
I'd like to make it so that I don't have to cast this to a List<Whatever> coming out, which leads me to this.
public <T> List<T> getList(Class<T> cls, Map query)
But now I have the problem that what I get out is always the concrete List<WhateverImpl> passed in whereas I'd like it to be the Whatever interface. I tried to use the super keyword but couldn't figure it out. Any generics gurus out there know how this can be done?
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The question mark (?) is known as the wildcard in generic programming. It represents an unknown type. The wildcard can be used in a variety of situations such as the type of a parameter, field, or local variable; sometimes as a return type.
In generic code, the question mark (?), called the wildcard, represents an unknown type. The wildcard can be used in a variety of situations: as the type of a parameter, field, or local variable; sometimes as a return type (though it is better programming practice to be more specific).
We can use the Java Wildcard as a local variable, parameter, field or as a return type. But, when the generic class is instantiated or when a generic method is called, we can't use wildcards. The wildcard is useful to remove the incompatibility between different instantiations of a generic type.
A Generic class can have muliple type parameters.
You need to define the method like this:
public <B, C extends B> List<B> getList(final Class<C> cls, final Map<?, ?> query)
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