What kind of constructor is this, beginning with ?

Question

<T extends Drawable & DrawerToggle> ActionBarDrawerToggle(
        Activity activity, 
        Toolbar toolbar,
        DrawerLayout drawerLayout, 
        T slider,
        @StringRes int openDrawerContentDescRes,
        @StringRes int closeDrawerContentDescRes) {

During browsing source code of the class ActionBarDrawerToggle.java, I've found this constructor is declared without access specifier. Rather, its declaration starts with

<T extends Drawable & DrawerToggle>

Please explain, what does it really mean?

"without access specifier" specifies that it has package-private visibility. In other words, unless you're developing code in the same package, you don't really need to know this exists. — Andy Turner, Feb 05 '16 at 11:19
It is a method type parameter. This might help you. https://docs.oracle.com/javase/tutorial/java/generics/boundedTypeParams.html It is not related to whether the constructor is declared `public` or not (but instead, as @AndyTurner mentioned, package-private, which is a kind of "default" access modifier as well). — SOFe, Feb 05 '16 at 11:20
@Stultuske You can just `Ctrl+Click` and browse in Android Studio once you downloaded the source code, but I have edited and linked source url as well. — Vikas Patidar, Feb 05 '16 at 11:22
@VikasPatidar : Please avoid pasting images for code, rather simply paste the code itself, it's easier to copy/edit/... — Zbynek Vyskovsky - kvr000, Feb 05 '16 at 11:38
the summary of what i understood from below answers is **** acts like a reference for all parameters that use **T** . so you can have 10 parameters of T without specifying ** 10 times — Srinath Ganesh, Feb 05 '16 at 12:36
Look [here](http://www.java2s.com/Tutorials/Java/Java_Object_Oriented_Design/0370__Java_Generic_Methods_Constructors.htm) and search for "Generic Constructor" — Matteo Umili, Feb 05 '16 at 14:36

Zbynek Vyskovsky - kvr000 · Accepted Answer · 2016-02-05T11:22:20.047

23

T is generic type declaration which is then used as a type for one of the arguments - slider.

<T extends Drawable & DrawerToggle > specifically means that T must extend/implement both Drawable and DrawerToggle classes/interfaces.

The access specifier is not mandatory. In case it's missing it means the class/method is accessible only from classes within the same package.

edited Feb 05 '16 at 11:22

answered Feb 05 '16 at 11:18

Zbynek Vyskovsky - kvr000

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which doesn't really answer the question. – Stultuske Feb 05 '16 at 11:21
1

he's not really asking what a Generic is, he 's aware of that, but being that it's a constructor, he doesn't understand the syntax. – Stultuske Feb 05 '16 at 11:36
3

for a method, yes. for a constructor, no. – Stultuske Feb 05 '16 at 11:59
5

@Stultuske : I don't understand this. There is no difference for generics definitions in methods and constructors. – Zbynek Vyskovsky - kvr000 Feb 05 '16 at 12:04
@ZbynekVyskovsky-kvr000 from what i understand method generics determines either the parameters or return type. what does generics in a constructor do ? – Srinath Ganesh Feb 05 '16 at 12:21
3

@SrinathGanesh : In constructor they may define only constructor parameters types. This is obvious as constructor doesn't have any return type. – Zbynek Vyskovsky - kvr000 Feb 05 '16 at 12:23
@ZbynekVyskovsky-kvr000 ok so that way * A(Object obj)* .. wont work for *new A(new Object())* ? – Srinath Ganesh Feb 05 '16 at 12:27
1

@ZbynekVyskovsky-kvr000 oh ya i understood now . its nice , i should have tried out *A(T t)* – Srinath Ganesh Feb 05 '16 at 12:29

M A · Answer 2 · 2016-02-05T12:10:39.573

Like normal methods, constructors can take type parameters. This is mentioned in this section of the Java Language Specification:

https://docs.oracle.com/javase/specs/jls/se8/html/jls-8.html#jls-8.8

From the syntax mentioned there, you can have type parameters in the signature:

TypeParameters:

< TypeParameterList >

TypeParameterList:

TypeParameter {, TypeParameter}

and where TypeParameter is specified as

TypeParameter:

{TypeParameterModifier} Identifier [TypeBound]

TypeParameterModifier:

Annotation

TypeBound:

extends TypeVariable

extends ClassOrInterfaceType {AdditionalBound}

AdditionalBound:

& InterfaceType

score 3 · Answer 3 · answered Feb 05 '16 at 11:19

3

This block specifies slider parameters type (value of slider parameter must extends drawable and drawabletoggle classes (or interfaces)). Search generics for more info.

answered Feb 05 '16 at 11:19

Hyperion

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What kind of constructor is this, beginning with ?

3 Answers3