関連ワード:  why do we use interface in java    why do we need interface in java    why should we use 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 do we need functional interface in java    why do we need functional interface in java 8    why do we use interface instead of abstract class in java