Generics -Intermediary

Using Generic code with different Collection types

Lists

The usage with List Collections is quite straightforward as most of the java developers are familier with its API & its methods. Example of a code involving List interface is as follows:




List myList=new ArrayList();

MyObject a=new MyObject();

myList.add(a);

...

...




//Use this code to iterate

Iterator iterator=myList.Iterator();

while(iterator.hasNext()){

MyObject temp=iterator.next();

System.out.print(""+temp.toString());

}

//Or use the foll code

for(MyObject ss:myList){

MyObject temp=iterator.next();

System.out.print(""+temp.toString());

}

..

...

Sets

Set s=new HashSet();

System.out.println("Added an element"+s.add(new MyObject()));

s.add(new MyObject());

....




for(MyObject st:s)

System.out.println(st.toString());




Using TreeSets

Set mytreeSet=new TreeSet();




Now, class Power must be comparable, i.e. You can compare its elements .Eg. You can compare 1 & 2, "A" & "B", but how will you compare boy("Sumit") & boy("Rahul") ?

In order to do this, use java.util.Comparator interface.




Class Power implements Comparable{

public int FactorValue;

public int OtherValue;

.... other code including constructors

.

..

public int compareTo(Power t){ //Overriding the method through programmer implementation

if(this.FactorValue==t.FactorValue)

return 0;

if(this.FactorValue
return -1;

if(this.FactorValue>t.FactorValue)

return 1;

else

return 0;//Maybe throw an Assertion Error or an exception

}

//OR, You Could Also Use The Following Override


public int compareTo(Power t){ //Overriding through delegation

return(this.FactorValue.compareTo(t.FactorValue()));

/*

Works because Comparison is delegated to the int variable FactorValue in our class;

*/

}

}




If you wish to sort your object in a different manner, then use a Comparator. You can use it to sort even those class that you cannot modify. In such a case, create a (3rd Party) class like:




class OtherSort implements Comparator{

public int compare(MyObjectme,MyObject you){

return(me.OtherValue.compareTo(you.OtherValue));

}

}

Maps

Any class you use in a Map must implement the hashCode() and equals() methods. If you don't do so, then no compile/run time errors will occur, the code will just would'nt function the way it was supposed to.

Usage:

Map myMap=new HashMap();

Type1=Type Of KEY

Type2=Type Of VALUE

eg. Map shopList=new HashMap();




class Human{

String name;

Human(String s){

name=s;

}

public boolean equals(Object o){

if((o instanceof Human)&& ((Human)o).name==name)

return true;

else

return false;

}

public int hashCode(){ //More unique the hashCode returns a value,

return 5; //More efficient your class will be in retrieveing tho object from Map

}

public static void main(String[] args){

Map map=new HashMap();

map.put("we",new Human("Sumit"));

...

...

Human us;

us=map.get("we");

System.out.print(""+us.name);

}

}

Also, this is a valid code segment

Map> allWineLists=new LinkedHashMap>();

PriorityQueue

Queue queue=new PriorityQueue();

int[] values={1,6,3,8,34,55};

for(int i:values)

queue.offer(i); //Populate

System.out.println(""+queue.peek());//Print highest priority int

System.out.println(""+queue.size());

for(int i:values)

System.out.print(""+queue.poll(i));//Print and remove




A case where Mixing of Generic & non Generic collection creates Havoc




class Old{ //Old, Pre-Generic code

void InsertInt(List lst){

lst.add(new Integer(42));

}

}

class New{ //Newer class, under study

void go(){

List l=new ArrayList();

Old oldobject=new Old();

oldobject.InsertInt(l); //Adds Integer in an ArrayList OK!

}

void nogo(){

List l=new ArrayList();

Old oldobject=new Old();

oldobject.InsertInt(l); //Adds Integer in an ArrayList OOPS!, RuntimeException

}

}

This is primary reason why the compiler 'unhappily' chunks out an -Xlint:unchecked warning on compilation.




Polymorphism in Generics- Not Allowed

Hence you can't use

List