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src/main/java/rocks/zipcode/io/quiz4/collections/SimpleStringGroup.java

 package rocks.zipcode.io.quiz4.collections;
 
 import java.util.ArrayList;
-import java.util.Collection;
 import java.util.Iterator;
 
 /**

src/main/java/rocks/zipcode/io/quiz4/collections/ZipCodeWilmington.java

 
 import rocks.zipcode.io.quiz4.objectorientation.Student;
 
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.List;
 import java.util.Map;
 
 /**
  * @author leon on 11/12/2018.
  */
 public class ZipCodeWilmington {
+    HashMap<Student, Double> students = new HashMap<>();
+
     public void enroll(Student student) {
+        students.put(student, 0.0);
     }
 
     public Boolean isEnrolled(Student student) {
-        return null;
+        return students.containsKey(student);
     }
 
     public void lecture(double numberOfHours) {
+        //students.forEach((student, aDouble) -> student.learn(numberOfHours));
+        students.keySet().forEach(student -> student.learn(numberOfHours));
     }
 
     public Map<Student, Double> getStudyMap() {
-        return null;
+        return students;
     }
 }

src/main/java/rocks/zipcode/io/quiz4/objectorientation/Student.java

  * @author leon on 11/12/2018.
  */
 public class Student {
+    private Integer id;
+    private Double totalStudyTime;
+
     public Student() {
-        this(null);
+        this.id = 0;
+        this.totalStudyTime = 0.0;
     }
 
     public Student(Integer id) {
+        this.id = id;
+        this.totalStudyTime = 0.0;
     }
 
     public void learn(Double amountOfHours) {
+        totalStudyTime += amountOfHours;
     }
 
     public Double getTotalStudyTime() {
-        return null;
+        return totalStudyTime;
     }
 }