Generic type as parameter in Java Method

Question

Do you think it is possible to create something similar to this?

private ArrayList increaseSizeArray(ArrayList array_test, GenericClass) {
    array_test.add(new GenericObject()); // instance of GenericClass
    return array_test;
}

Solution 1

Yes, you can.

private static <T> List<T> pushBack(List<T> list, Class<T> typeKey) throws Exception {
    list.add(typeKey.getConstructor().newInstance());
    return list;
}

Usage example:

List<String> strings = new ArrayList<String>();
pushBack(strings, String.class);

Solution 2

Old question but I would imagine this is the preferred way of doing it in java8+

public <T> ArrayList<T> dynamicAdd(ArrayList<T> list, Supplier<T> supplier) {
  list.add(supplier.get());
  return list;
}

and it could be used like this:

AtomicInteger counter = ...;
ArrayList<Integer> list = ...;

dynamicAdd(list, counter::incrementAndGet);

this will add a number to the list, getting the value from AtomicInteger's incrementAndGet method

Also possible to use constructors as method references like this: MyType::new

Solution 3

simple solution!

private <GenericType> ArrayList increaseSizeArray(ArrayList array_test, GenericType genericObject)
{
    array_test.add(new genericObject());
    return ArrayList;
}

Comments

[Pierre Guilbert]

How do you call that method? I don't know what Class<T> typekey should be. I can't find anything about it. If you have any reading advice, it will be welcome. Thank you for your response!

[C. K. Young]

@Pierre: Just added a usage example. Basically, if you take a class name (String in this case) and add .class after it, you have a type key (class object) for that class. String.class has type Class<String>.

[Pierre Guilbert]

I just used it like this: static <T> List<T> pushBack(List<T> list, Class typeKey, int size) and it worked. Thank you, you helped me a lot. Avoiding copy/paste code is always a joy.

[Deepak Gangore]

Why <T> is used just before return type ? I am also doing the same thing but i want to know the actual usage of <T>. Thanks in advance.

[Douglas Held]

docs.oracle.com/javase/tutorial/extra/generics/methods.html introduces the construct of static <T> void method(...)

[Douglas Held]

Correction... docs.oracle.com/javase/tutorial/java/generics/methods.html actually introduces the syntax (the above link I provided does not).