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Aggregation in java with example codes

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What is aggregation? 
  •  "Have a" or "May have" relationship
  •  Aggregation is special type of association
  •  Life time of Component object is independent from its  container object
E.g A school has one student


class Student {
String name;
Student () {name=null; }
Student (String n) {name=n; }
void setName(String n) {name=n;}
String getName() { return name;}
}
class School{
Student  st;
School (  ) { st=null;}
School (Student s){st=s;}
void studentName(){
  System.out.println(st.getName());
}
}
class Post{
public static void main (String a[]){
Student st=new Student("Imran");
School s= new School(st);
s.studentName();
}
}



Output
Hello



Problem : A Car must has Engine and body.
          A Car may or may not has Ac

Adding Ac as an Aggregate object in car

class Engine{
void start() {
System.out.println("Engine Start");
}
}
class Body{
void type() {
System.out.println("Saloon Type");
}
}
class Ac{
void cool() {
System.out.println("Super Cool");
}
}

class Car{
Engine e;
Body b;
Ac ac;
Car () { e=new Engine();
         b= new Body();
         ac=null;        
}
Car (Ac p) { e=new Engine();
             b= new Body();
             ac=p;        
}
void specification(){
e.start();
b.type();
if (ac!=null)
    ac.cool();
}
}

class Post{
  public static void main(String a[]){
  Ac ac= new Ac();
  Car c1= new Car(ac); // car with AC
  c1.specification();
  Car c2= new Car(); // car without AC
  c2.specification();
  }
  }


Output:

 Engine Start
Saloon Type
Super Cool
Engine Start

Saloon Type


 Same with Data Members

class Engine{
int hrp;
Engine () { }
Engine(int hp) { hrp=hp;}
void start() {
System.out.println("Engine Power "+hrp);
}
}
class Body{
int size;
Body()  {}
Body(int s){size=s;}
void type() {
System.out.println("Saloon Size "+size);
}
}
class Ac{
int price;
Ac(int p){price=p;}
void cool() {
System.out.println("Super Cool with price =" + price);
}
}
class Car{
Engine e;
Body b;
Ac  ac;
int price;
Car () { e=new Engine();
         b= new Body();
         ac=null;
}
Car (int p,int hp,int s,Ac a) { 
e=new Engine(hp);
b= new Body(s); 
price=p; 
ac=a;
}
void specification(){
e.start();
b.type();
System.out.println("Car Price "+price);
if (ac!=null)
    ac.cool();
}
}

class Post{
public static void main(String a[]){
Ac ac=new Ac(1000);
Car c= new Car(1000,800,5,ac);
c.specification();

}
}

 Output:

 Engine Power 800
Saloon Size 5
Car Price 1000

Super Cool with price =1000

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