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Diamond operator ArrayList

What's the difference between

ArrayList<Integer> list = new ArrayList<>();

and

ArrayList<Integer> list = new ArrayList();

Is the diamond operator when I create a new ArrayList?

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3 answers


The first statement is clear. The second generates a compilation warning.

The alphabetic iperator was introduced in java 1.7. Since java 1.5 you had to write

ArrayList<Integer> list = new ArrayList<Integer>();

i.e. declare a generic type on both the left and right side of the assignment. Since typical types can be more complex than simple Integer

, it would be very difficult to copy the same definition twice, so they added this cool function to the compiler: you just have to say "This class is generic, use the generic type from the left side of the assignment "using a marker <>

.

Last comment. Please avoid using concrete classes on the left side of assignments and method definitions. The following is much better:



List<Integer> list = new ArrayList<>();

or even

Collection<Integer> list = new ArrayList<>();

or often even

Iterable<Integer> list = new ArrayList<>();
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A rather advanced topic for Java is called Generics, and that's the explanation for all <> characters.

In essence, you should always write

ArrayList<Integer> list = new ArrayList<>();

Because this is shorthand for

ArrayList<Integer> list = new ArrayList<Integer>();

And you need both type arguments (bits) so that the compiler will give you a warning otherwise.



Record:

ArrayList<Integer> list = new ArrayList();

Creates an ArrayList that can contain any type, and when you try to convert it to ArrayList<Integer>

, the compiler will issue a warning.

EDIT:

Here's a detailed guide on which generics are for and how they work. I'm not sure if you are at this level to understand them fully, but worth a try.

https://docs.oracle.com/javase/tutorial/java/generics/

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First, it is a parameterized shared object ArrayList

that is referenced by a parameterized shared reference.

The second is an unparameterized non-shared object ArrayList

that is referenced by a parameterized shared reference.

No, although diamond operators are not needed when you create ArrayList

, it is highly recommended to avoid ClassCastException

or some others RuntimeException

later.

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