1. Regular interface, including abstract methods.
2. They are all constants.
Do not add any modifiers to the interface category (public is not required either).
Since any field (member variable) in the interface is static and final automatically, the interface becomes a very convenient tool for creating constant groups.
public interface Months { int JANUARY = 1, FEBURARY = 2, ..., DECEMBER = 12; }
3. The empty interface is also called the mark interface, and its functions are as follows:
is used as a mark.
Logically, a class can become a subclass of an interface.
// null interface public interface EnvironmentObject{} // public interface Agent extends EnvironmentObject { .... }
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