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I understand the differences in theory, but what is the difference in the code implementation? Can somebody provide some examples?

amax
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    Thanks for the answers. http://stackoverflow.com/questions/4298177/association-vs-aggregation (third answer) also helped. I have a final uncertainty I hope someone can address. Am I correct in believing instances of the associated class in this diagram http://img152.imageshack.us/img152/4981/21083939.png would have to be owned by a class not shown in the diagram (otherwise an aggregate relationship would have to exist between whole and associated class?)? – amax Dec 06 '10 at 14:50

2 Answers2

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Purpose we have students and universities

class University {
    private final Set<Student> students = new HashSet<Student>();
    void addStudent(Student s){students.add(s);}
}

class Student {
    private final String name;
    public Student(String name) {
        this.name = name;
    }
}

We create this stuff in some way

University university = new University();
Student bob = new Student("Bob");
university.addStudent(bob);

And know we need to know does Bob studies in university. So we create some new method for university

boolean contains(Student student){
        for(Student s : students){
            if(s.equals(student)) return true;
        }
        return false;
    }

and, than do smt like university.contains(bob).

But what will be if we havent link to uniwersity. We need to ask it Bob. But Bob doesn't know. So we go from composition to bi-derection and create smt like 

class University {
    private final Set<Student> students = new HashSet<Student>();
    void addStudent(Student s){
        students.add(s);
        s.setUniversity(this);
    }
    boolean contains(Student student){
        for(Student s : students){
            if(s.equals(student)) return true;
        }
        return false;
    }
}

class Student {
    private final String name;
    private University university;
    public Student(String name) {
        this.name = name;
    }
    void setUniversity(University u){
        university = u;
    }
    boolean doYouStudyInUniversity(){
        return university != null;
    }
}
//ask
bob.doYouStudyInUniversity();
Stan Kurilin
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  • Should we enforce the setUniversity and addStudent in the constructor to make sure that they will always be called when a new Student is created? – Kenston Choi Jun 24 '12 at 03:49
  • @Kenston Choi, it depends from your coding style/paradigm you are using. Anyway it quite challenging to construct immutable bidirectional structures in Java (you can imitate it using proxies). – Stan Kurilin Jul 11 '12 at 16:10
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Composition is, in effect, uni-directional association - except that semantically, we interpret it as meaning "that thing is part of this thing" rather than simply "this thing holds a reference to that thing".

Karl Knechtel
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