Generics

• We use generics for type safety, which means only one type of element will get added

• It does not means Sun Microsystem has changed the add method of class ArrayList

• It is same as before and only changes done are in the compiler. In this case, the compiler will do more work on your behalf

• One of compiler tasks, besides compilation, is addition of extra code

Declaring ArrayList in 1.5

• ArrayList <Integer> al = new ArrayList <Integer>();

• This means that the compiler will start giving errors if we try to put any elements other than an integer.

• Point to be remembered: If we look at the byte code, these angular brackets are not considered. This is just for the compiler to give errors.

• In these <>, classes under Object hierarchy are allowed not primitives

• In both sides, the class type must be same, No superclass or subclass combination is allowed.
  

  ArrayList <Object> al = new ArrayList <Integer>();  // not allowedly
  ArrayList <int> al = new ArrayList <int>();  // not allowed – primitives       
  

  In jdk 1.7 right side <Integer> or <int> is not needed

  Like below

  ArrayList <Integer> al = new ArrayList<>();

Adding elements in jdk 1.5

1. al.add (33) - after the above declaration, we are able to directly add 33 primitive and there is no need of typecasting. This is one of the advantages of generics

2. Compiler will convert this line on the fly to al.add(new Integer(33))

Retrieving elements in jdk 1.5

1. al.get(index) in this case, in the earlier versions we must follow these three :

   Object obj=al.get (5) 
   Integer i = (Integer) obj; 
   int val=i.intValue ();
   

2. int val = al.get (index)- This is also one of the extra features of compiler. Object gets converted into the Integer. If we use the decompiler then byte code we see this in the byte code.

3. Another way is by using iterator:

   Iterator <Integer> itr=al.iterator (); 
     while (itr.hasNext()) {
       int element =itr.next(); [ Due to generics we return int directly]
     }
   

foreach loop

The foreach loop in java is used for traversing arrays or collection elements. It is sometimes referred to as the For In loops

• In this case we need to remember these points below:
  • No increment decrement
  • No size calculation
  • Must use generics for iterating

  • ArrayList<String> al=new ArrayList<String>(); 
      for (String element : al){
        System.out.println (element)
    }
    

  • This for loop is for compiler. It always gets converted into a simple ordinary for loop

  • ArrayList<Student> al=new ArrayList<Student>(); 
      for (Student stu: al) {
        System.out.println (stu.age); 
        System.out.println (stu.loc);
      }
    

  • This way of iterating element is most commonly used in industrial projects.

Business Use

Consider the following requirement:
Question: One student has age and location which needs to be added in the Arraylist, and give to ten students to other department.

package com.javabykiran.Collection.revisited;
/*
* @author Java By Kiran
*/
public class Student { 
    int age;
    String loc;
}

Answer: First we design student as below:

Example 1: Write class which prepares Students and returns from one of method.

package com.javabykiran.Collection.revisited; 
import java.util.ArrayList;
/*
* @author Java By Kiran
*/
public class PrepareStudent { 
ArrayList<Student> buildStudents() {
      Student student1 = new Student(); 
      student1.age = 28;
      student1.loc = "Pune";
      Student student2 = new Student(); 
      student2.age = 30;
      student2.loc = "Nagpur";
      // Students added in arraylist
      // remember a1 knows students’ address but not age and loc
      // Here we are placing addresses not actual data in ArrayList
    ArrayList<Student> array ListStu = new ArrayList<Student>();
      arrayListStu.add(student1); 
      arrayListStu.add (student2); 
      return arrayListStu;
   }
}

Now, write the class which gets students from above class's buildStudents () method and iterate over them to get students age and location.
Example 2:

package com.javabykiran.Collection.revisited; 
import java.util.ArrayList;
/*
* @author Java By Kiran
*/
public class RetriveStudents { 
 void fetchStudent() {
   PrepareStudent prepare = new PrepareStudent ();
   ArrayList<Student> listStu = prepare.buildStudents();
   // enhance for loop -- for each loop 
      for (Student student : listStu) {
       System.out.println("students age: " + student.age);
   System.out.println("students location: "+ student.loc);
  }	
 }
}

Write client class to test our logic.

package com.javabykiran.Collection.revisited;
/*
* @author Java By Kiran
*/
public class TestStudentsOperation { 
    public static void main(String[] args) {
    // prepare students
    PrepareStudent sp=new PrepareStudent();        
    sp.buildStudents();
    //retrieve students
    RetriveStudents retrivestudents = new RetriveStudents ();    
        retrivestudents.fetchStudent ();
  }
}

    Output:
     ArrayList >>[com.javabykiran.Collection.revisited.Student@182f0db, 
     com.javabykiran.Collection.revisited.Student@192d342]
     students age : 28 
     students location : Pune 
     students age : 30
     students location : Nagpur
  

Question: One student, who has age, location and many phone numbers, needs to be added in ArrayList and given to other department which has ten students.

package com.javabykiran.Collection.revisited; 
import java.util.ArrayList;
/*
* @author Java By Kiran
*/
public class Student { 
   int age;
   String loc;    
   ArrayList<String> alMobNo;
}

package com.javabykiran.Collection.revisited; 
import java.util.ArrayList;
/*
* @author Java By Kiran
*/
public class StudentPrepare {        
    ArrayList<Student> buildStudents(){
      Student student1 = new Student();   
      student1.age = 28;
      student1.loc = "Pune";
      
      //creates list of mobile numbers of student1 
    ArrayList<String> alMobNoStu1 = new ArrayList<>();
      alMobNoStu1.add("8888809416");   
      alMobNoStu1.add("9552343698"); 
      student1.alMobNo = alMobNoStu1; 
      Student student2 = new Student(); 
      student2.age = 28;
      student2.loc = "Pune";
      
      //creates list of mobilenumbers of student2 
    ArrayList<String> alMobNoStu2 = new ArrayList<>();
      alMobNoStu2.add("8234876642"); 
      alMobNoStu2.add("3455455441"); 
      student2.alMobNo = alMobNoStu2;
      // Students added in arraylist
      //remember al knows students’ address not their age and loc
      
      //Here we are placing address, not actual data in ArrayList    
    ArrayList<Student> arrayListStu = new ArrayList<>();
      arrayListStu.add(student1);  
      arrayListStu.add (student2);
      System.out.println("ArrayList >> "+ arrayListStu); 
      return arrayListStu;
   }	
}

For Fetching Students use the method given below:
Example 4:

void fetchStudent() {
  StudentPrepare prepare = new StudentPrepare();
  ArrayList <Student> listStu = prepare.buildStudents();
    //Enhance for loop -- foreach loop 
    for(Student student : listStu) {
     System.out.println("students age: " + student.age);  
     System.out.println("students location: " + student.loc);
       ArrayList<String> alMobNo = student.alMobNo;    
         for (String mobNo : alMobNo) {       
         System.out.println("al mob no" + mobNo);
       } // end 1st For loop
    }// end 2nd for loop
}

    Output:
     ArrayList >> [com.javabykiran.Collection.revisited.Student@182f0db, 
     com.javabykiran.Collection.revisited.Student@192d342]
     students age : 28 
     students location : Pune 
     al mob no 8888809416 
     al mob no 9552343698
     students age : 30
     students location : Nagpur 
     al mob no 8234876642
     al mob no 3455455441
  

Example 5:
Question: One student has age, location and a phone having two types, one landline number and many mobile numbers. It needs to be added in ArrayList and given to other department which has ten students.
First we design Phone as below:

package com.javabykiran.Collection.revisited; 
import java.util.ArrayList;
/*
* @author Java By Kiran
*/
public class Phone { 
  int landLineNo;
  ArrayList<String> mobNos;

First we design Student as below having phone attribute: This also shows one to one relationship.

package com.javabykiran.Collection.revisited; 
import java.util.ArrayList;
/*
* @author Java By Kiran
*/
public class Student { 
  int age;
  String loc; 
  Phone p;
}