Java: <...> (less than/greater than) syntax

The <...> syntax allows you to write generic classes and methods that can handle multiple different types. This means, for instance, that you don’t have to write one list class that can store integers, one list class that can store strings, and so on. Instead you can write one list class that’s generic and can be used for any type.

ArrayList does precisely this. It is parametrized on type E

…which means that you can give it a type argument as follows:

ArrayList<String> strList;

In this case strList will only contain objects of type String and something like strList.add(5) will fail to compile.

Looking at the documentation, we see that the ArrayList.get(int) method returns something of type E:

This means that strList.get() will return a String.

Diamond Syntax: new ArrayList<>()

If the type is left out, as below, the type is inferred:

List<String> l1 = new ArrayList<>();
List<String> l2 = new ArrayList<String>(); // Equivalent

Details

  • ArrayList (without any type argument) is called the raw type and should not be used if it can be avoided.

  • ArrayList<Object> is not a super type of ArrayList<String>. In technical terms: generic classes are invariant (as opposed to arrays that are covariant).

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