What kind of constructor is this, beginning with <T extends Drawable & DrawerToggle>? [duplicate]

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?

Answer 1

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.

Answer 2

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