fix warnings

src/main/java/ru/ulstu/core/model/BaseEntity.java

@@ -5,7 +5,9 @@ import javax.persistence.GenerationType;
 import javax.persistence.Id;
 import javax.persistence.MappedSuperclass;
 import javax.persistence.Version;
+import javax.validation.constraints.NotNull;
 import java.io.Serializable;
+import java.util.Objects;
 
 @MappedSuperclass
 public abstract class BaseEntity implements Serializable, Comparable {
@@ -16,14 +18,6 @@ public abstract class BaseEntity implements Serializable, Comparable {
     @Version
     private Integer version;
 
-    protected BaseEntity() {
-    }
-
-    public BaseEntity(Integer id, Integer version) {
-        this.id = id;
-        this.version = version;
-    }
-
     public Integer getId() {
         return id;
     }
@@ -48,9 +42,7 @@ public abstract class BaseEntity implements Serializable, Comparable {
             return false;
         }
         BaseEntity other = (BaseEntity) obj;
-        if (id == null) {
-            return other.id == null;
-        } else return id.equals(other.id);
+        return Objects.equals(id, other.id);
     }
 
     @Override
@@ -70,7 +62,7 @@ public abstract class BaseEntity implements Serializable, Comparable {
     }
 
     @Override
-    public int compareTo(Object o) {
+    public int compareTo(@NotNull Object o) {
         return id != null ? id.compareTo(((BaseEntity) o).getId()) : -1;
     }