Menu
Consider the following 2 method declarations:
1. public abstract <T extends MetaData> List<T> execute();
2. public abstract List<? extends MetaData> execute();
Both seem to return back a list of objects that extend MetaData.
What is the difference between them please?
911
For static generic methods, the type parameter section must appear before the method's return type. The complete syntax for invoking this method would be: Pair<Integer, String> p1 = new Pair<>(1, "apple"); Pair<Integer, String> p2 = new Pair<>(2, "pear"); boolean same = Util. <Integer, String>compare(p1, p2);
If we want the data to be of int type, the T can be replaced with Integer, and similarly for String, Character, Float, or any user-defined type. The declaration of a generic class is almost the same as that of a non-generic class except the class name is followed by a type parameter section.
In a nutshell, generics enable types (classes and interfaces) to be parameters when defining classes, interfaces and methods. Much like the more familiar formal parameters used in method declarations, type parameters provide a way for you to re-use the same code with different inputs.
Generic methods allow type parameters to be used to express dependencies among the types of one or more arguments to a method and/or its return type. If there isn't such a dependency, a generic method should not be used. It is possible to use both generic methods and wildcards in tandem.
In the first case you will allow Java to use type inference and infer the type of T at each call site.
In the second case you will always get a List<? extends MetaData> and so won't be able to assign it to a variable of any narrower type like List<IntegerMetaData>.
135
If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
Donate Us With
