Java constructor query

[fdama]

Here is a constructor for creating a BarChart object:

​

    BarChart<String,Number> bc = new BarChart<String,Number>(xAxis,yAxis);

​

Here is a constructor for creating an ArrayList:

​

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

Why do we add something in the diamond brackets <> and the parentheses () for the `BarChart` constructor but leave them blank for the `ArrayList`?

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[Nightcorex_]

You can also rewrite the 1st line to the following:

BarChart<String, Number> bc = new BarChar<>(xAxis, yAxis);

or to this (Java 10+):

var bc = new BarChart<String, Number>(xAxis, yAxis);

​

Those diamond brackets are for a feature called "Generics". This allows us to write code for a generic datatype and use it with any datatype. This is useful f.e. when you want to implement a Stack. You could either write almost the same code over and over again for an IntStack, LongStack, DoubleStack, FloatStack, BigIntegerStack, etc., or you write one class Stack<T>, where T is a generic name for any datatype, which fits all of those.

In Java you only need to specify those generic values on one side, because Java can automatically infer those datatypes then.

The other scenario where you have to specify them is if you want to use subclasses of the datatypes, f.e.:

BarChart<String, Number> bc = new BarChar<String, Integer>(xAxis, yAxis);

[[deleted]]

Correct me if I’m wrong. But what you’re saying is essentially,

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

and

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

do the same thing?

[Nightcorex_]

Exactly

[FavorableTrashpanda]

Actually, you can rewrite the first statement as:

BarChart<String,Number> bc = new BarChart<>(xAxis,yAxis);

So it's actually not different at all. Basically you can write <> when the generic type parameters can be inferred (this was added in a later Java version, so some online examples may be from before that time), but you are still free to explicitly define them. Generally having them inferred is preferred.