Introduced interfaces for entity, value object and domain event, extracted from the sameIdentityAs/sameValueAs/sameEventAs methods

dddsample/src/main/java/se/citerus/dddsample/domain/DomainEvent.java

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+package se.citerus.dddsample.domain;
+
+/**
+ * A domain event is something that is unique, but does not have a lifecycle.
+ * The identity may be explicit, for example the sequence number of a payment,
+ * or it could be derived from various aspects of the event such as where, when and what
+ * has happened.
+ */
+public interface DomainEvent<T> {
+
+  /**
+   * @param other The other domain event.
+   * @return <code>true</code> if the given domain event and this event are regarded as being the same event.
+   */
+  boolean sameEventAs(T other);
+
+}

dddsample/src/main/java/se/citerus/dddsample/domain/Entity.java

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+package se.citerus.dddsample.domain;
+
+/**
+ * An entity, as explained in the DDD book.
+ *  
+ */
+public interface Entity<T> {
+
+  /**
+   * Entities compare by identity, not by attributes.
+   *
+   * @param other The other entity.
+   * @return true if the identities are the same, regardles of other attributes.
+   */
+  boolean sameIdentityAs(T other);
+
+}

dddsample/src/main/java/se/citerus/dddsample/domain/ValueObject.java

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+package se.citerus.dddsample.domain;
+
+/**
+ * A value object, as described in the DDD book.
+ * 
+ */
+public interface ValueObject<T> {
+
+  /**
+   * Value objects compare by the values of their attributes, they don't have an identity.
+   *
+   * @param other The other value object.
+   * @return <code>true</code> if the given value object's and this value object's attributes are the same.
+   */
+  boolean sameValueAs(T other);
+
+}