Added propper adding and removing of missions in country

src/main/java/cz/fi/muni/pa165/seminar4/group7/secretservice/entity/Country.java

 package cz.fi.muni.pa165.seminar4.group7.secretservice.entity;
 
 import com.sun.istack.NotNull;
+import lombok.AccessLevel;
 import lombok.Getter;
 import lombok.Setter;
 import lombok.experimental.Accessors;
 
 import javax.persistence.*;
-import java.util.List;
+import java.util.HashSet;
 import java.util.Objects;
+import java.util.Set;
 
 /**
  * @author Milan Mozolak
@@ -54,8 +56,19 @@ public class Country {
     @Column(nullable = false)
     private String military;
 
+    @Setter(AccessLevel.NONE)
     @OneToMany(mappedBy = "country")
-    private List<Mission> missions;
+    private Set<Mission> missions = new HashSet<>();
+
+    public void addMission(Mission mission) {
+        missions.add(mission);
+        mission.setCountry(this);
+    }
+
+    public void removeMission(Mission mission) {
+        missions.remove(mission);
+        mission.setCountry(null);
+    }
 
     @Override
     public boolean equals(Object o) {