Object array to generic array

Question

See the code

Integer[] array = (Integer[]) new Object[size];

this obviously do not works, I understand perfectly.

but why with generics works?

T[] array = (T[]) new Object[size];

if T is Integer class, after that line the array will be Object[] type, but why cast is possible? does not throw ClassCastException?

Avoid generics and arrays. Avoid casting arrays. This is a an area of the language that is somewhat unsafe - it's very easy to end up with an ArrayStoreException at runtime. — Boris the Spider
– Boris the Spider, Commented Sep 5, 2014 at 20:13

Bhesh Gurung · Accepted Answer · 2014-09-05 20:22:20Z

3

but why with generics works?

It's because the generic version is type-erased and compiled to following -

Object[] array = new Object[size];

answered Sep 5, 2014 at 20:22

Bhesh Gurung

51.1k23 gold badges96 silver badges147 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

Johnny Willer Over a year ago

SomeClass<T extends Comparable<T>> { T[] array; } will be translated to Comparable<Object>[] array; right?

Bhesh Gurung Over a year ago

Just compiled it on my eclipse and it's java.lang.Comparable[] array;.

Bhesh Gurung Over a year ago

You can get the idea behind it here - What is a reifiable type?.

tiorthan · Accepted Answer · 2014-09-05 20:27:21Z

2

Type casts are done at runtime that's why you get a cast exception. Generics on the other hand are compile time features of Java. When you declare

class Foo<T> {
    T bar;
}

the field bar is actually of type Object (or if you use bounds like ? extends ... of whatever base class you chose). When you use the class for example like this

Foo<Foobar> foo = new Foo<>();
foo.bar = new Foobar();
Foobar foobar = foo.bar;

the compiler will translate the last assignment to something equivalent to

Foobar foobar = (Foobar) foo.bar;

because it knows that the return value, even though internally of type Object will always a Foobar.

answered Sep 5, 2014 at 20:27

tiorthan

362 bronze badges

Collectives™ on Stack Overflow

Object array to generic array

2 Answers 2

3 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

3 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related