関連ワード:  why do we use an interface in java    why do we need an interface in java    why do we use functional interface in java    why do we use marker interface in java    why do we use runnable interface in java    why do we use serializable interface in java    why should we use interface in java    why do we need functional interface in java    why do we need functional interface in java 8    why do we need static methods in interface java 8