java - Type of field 'myField' is concrete class 'MyClass' -

i preparing android application production have enabled lint warnings. 1 particular warning wide spread, don't understand it.

here warning:

type of field 'loginf' concrete class 'loginfragment'

and here description:

reports instance fields type declared concrete class, rather interface. such declarations may represent failure of abstraction, , may make testing more difficult. declarations classes come system or third-party libraries not reported inspection.

any elaboration appreciated.
note: ide using 'android studio'.

it's complaining because you're coupling class (which contains loginf field) loginfragment; if change loginfragment, may have change class.

alternatively, @ methods you're using on loginfragment in class. collectively form behaviour can specify in interface?

e.g. if method call on loginf initiatelogin(), perhaps can have:

interface login {     void initiatelogin(); }  class loginfragment extends fragment implements login {     @override     public void initiatelogin() {         // login     } } 

such instead of declaring loginfragment loginf = new loginfragment();, use login login = new loginfragment().

then can change loginfragment long keeps initiatelogin() method, or swap out new loginfragment() other implementation of login, might useful mocking in tests.

a contrived example. suggest using default lint config, not super strict one.