Learn Java Programming - Interface Variables (Fields) Tutorial Java 8+

An interface may contain a very specific type of a variable. When a variable (field) is declared in an interface it essentially becomes an implicitly declared constant. Here are the rules that apply to a variable in an interface:
Interface variables must be initialized when they are declared.
Interface variable values cannot be changed after they are declared.
The only modifiers that can be explicitly applied to an interface variable are: public, static, or final.
All interface variables are implicity marked public, static, and final.
Don't be fooled by what appears to be Default (Package-Private) access, public is the only access modifier allowed and implicitly required.