Java Collections-List,Set,Map,Queue,Iterator,Generics

An overview of java-Collections-List,Set,Map,Queue Interface are explained in this post. For complete understanding please refer to Sun Certified Programmer for Java 6 Study Guide available in the scribd.com.
Please click on this Ebook Link->Kathy-sierra-PDFBooks

Refer to the below Links for Practise

Selenium Wiki
Complete List of Selenium Basics
50 Real Time Interview Questions
Real Time Scenario’s
Java Platform Standard Edition 8 Documentation
Complete List of Java Basics
Java Quiz
Java OOPS Quiz
Selenium Advanced Quiz
Selenium Quiz
Selenium WebDriver Resume

In Collection Framework they are mainly 4 interfaces which mentioned as follows:

List,Set,Map,Queue

List:

List Interface

List is a type of Collection useful to store duplicate objects. In List elements are stored in index based. List allows null value. ArrayList,Vector and LinkedList are the classes implementing the List interface.

1
2
3
4

List is a type of Collection useful to store duplicate objects.
In List elements are stored in index based.
List allows null value.
ArrayList,Vector and LinkedList are the classes implementing the List interface.

List Example:

List-ArrayList Example
Java

package org.totalqa.collections; import java.util.ArrayList; public class ListExample { public static void main(String[] args) { ArrayList aList = new ArrayList(); aList.add("test"); aList.add(123); aList.add(null); aList.add(55.22); System.out.println("Size of the list " + aList.size()); for(int i=0;i<aList.size();i++) { System.out.println("Element at index "+ i + " is " + aList.get(i)); } //Printing list of values using Enhanced For loop System.out.println("Printing the List of values using for-each"); for(Object o:aList) { System.out.println("Element is " + o); } } }

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26

package org.totalqa.collections;
import java.util.ArrayList;
public class ListExample
{
public static void main(String[] args)
{
ArrayList aList = new ArrayList();
aList.add("test");
aList.add(123);
aList.add(null);
aList.add(55.22);
System.out.println("Size of the list " + aList.size());

for(int i=0;i<aList.size();i++)
{
System.out.println("Element at index "+ i + " is " + aList.get(i));
}
//Printing list of values using Enhanced For loop
System.out.println("Printing the List of values using for-each");
for(Object o:aList)
{
System.out.println("Element is " + o);
}

}
}

List Console Output:

Console Output

Size of the list 4 Element at index 0 is test Element at index 1 is 123 Element at index 2 is null Element at index 3 is 55.22 Printing the List of values using for-each Element is test Element is 123 Element is null Element is 55.22

1
2
3
4
5
6
7
8
9
10

Size of the list 4
Element at index 0 is test
Element at index 1 is 123
Element at index 2 is null
Element at index 3 is 55.22
Printing the List of values using for-each
Element is test
Element is 123
Element is null
Element is 55.22

Set Interface:

Set Interface

Set is type of Collection useful to store unique objects Set Elements are stored without indexing. Set does not allow duplicate elements. Set allows null values.

1
2
3
4

Set is type of Collection useful to store unique objects
Set Elements are stored without indexing.
Set does not allow duplicate elements.
Set allows null values.

Set Example:

Set-HashSet Example
Java

package org.totalqa.collections; import java.util.HashSet; public class SetExample { public static void main(String[] args) { HashSet hSet = new HashSet(); hSet.add("test"); hSet.add(123); hSet.add(null); hSet.add(55.22); System.out.println("Size of the list " + hSet.size()); //Printing list of values using Enhanced For loop System.out.println("Printing the List of values using for-each"); for(Object o:hSet) { System.out.println("Element is " + o); } } }

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22

package org.totalqa.collections;
import java.util.HashSet;
public class SetExample
{
public static void main(String[] args)
{
HashSet hSet = new HashSet();
hSet.add("test");
hSet.add(123);
hSet.add(null);
hSet.add(55.22);
System.out.println("Size of the list " + hSet.size());

//Printing list of values using Enhanced For loop
System.out.println("Printing the List of values using for-each");
for(Object o:hSet)
{
System.out.println("Element is " + o);
}

}
}

Set Console Output:

Set-HashSet Console Output
Java

Size of the Set 4 Printing the Set of values using for-each Element is null Element is test Element is 123 Element is 55.22

1
2
3
4
5
6

Size of the Set 4
Printing the Set of values using for-each
Element is null
Element is test
Element is 123
Element is 55.22

Queue Interface:

Queue Interface

Queue is type of collection allows duplicate Objects. Queue Elements are stored without index Queue implements FIFO(Fist In First Out) Queue does not allow null values PriorityQueue is a Class implementing the Queue Interface PriorityQueue is autosorted. poll() method in queue removes the head element and the queue size reduced by 1

1
2
3
4
5
6
7

Queue is type of collection allows duplicate Objects.
Queue Elements are stored without index
Queue implements FIFO(Fist In First Out)
Queue does not allow null values
PriorityQueue is a Class implementing the Queue Interface
PriorityQueue is autosorted.
poll() method in queue removes the head element and the queue size reduced by 1

Queue Example:

Queue-PriorityQueue Example
Java

package org.totalqa.collections; import java.util.PriorityQueue; public class QueueExample { public static void main(String[] args) { PriorityQueue pQueue = new PriorityQueue(); pQueue.add(123); pQueue.add(46); pQueue.add(78); pQueue.add(99); System.out.println("Size of the Queue " + pQueue.size()); System.out.println("Queue Elements" + pQueue); System.out.println("First Element in the Queue"+pQueue.poll()); System.out.println("Size of the Queue " + pQueue.size()); System.out.println("First Element in the Queue"+pQueue.poll()); System.out.println("Size of the Queue " + pQueue.size()); } }

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19

package org.totalqa.collections;
import java.util.PriorityQueue;
public class QueueExample
{
public static void main(String[] args)
{
PriorityQueue pQueue = new PriorityQueue();
pQueue.add(123);
pQueue.add(46);
pQueue.add(78);
pQueue.add(99);
System.out.println("Size of the Queue " + pQueue.size());
System.out.println("Queue Elements" + pQueue);
System.out.println("First Element in the Queue"+pQueue.poll());
System.out.println("Size of the Queue " + pQueue.size());
System.out.println("First Element in the Queue"+pQueue.poll());
System.out.println("Size of the Queue " + pQueue.size());
}
}

Queue Console Output:

Queue-PriorityQueue Example
Java

Size of the Queue 4 Queue Elements[46, 99, 78, 123] First Element in the Queue46 Size of the Queue 3 First Element in the Queue78 Size of the Queue 2

1
2
3
4
5
6

Size of the Queue 4
Queue Elements[46, 99, 78, 123]
First Element in the Queue46
Size of the Queue 3
First Element in the Queue78
Size of the Queue 2

Map Interface:

MapInterface

Map is type of collection allows key-value pair. In Map, Key is Object type and it is unique. In Map, Value is referenced using Key. Map is not related to Collection Interface.

1
2
3
4

Map is type of collection allows key-value pair.
In Map, Key is Object type and it is unique.
In Map, Value is referenced using Key.
Map is not related to Collection Interface.

Map Example:

Map Example
Java

package org.totalqa.collections; import java.util.HashMap; public class MapExample { public static void main(String[] args) { HashMap hMap = new HashMap(); hMap.put(123,"name1"); hMap.put("key",12.34); hMap.put(null, "123"); System.out.println("Map Elements ::" + hMap); System.out.println("Value is ::" + hMap.get(null)); System.out.println("Keys are ::"+ hMap.keySet()); System.out.println("Values are ::"+ hMap.values()); } }

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16

package org.totalqa.collections;
import java.util.HashMap;
public class MapExample {
public static void main(String[] args) {

HashMap hMap = new HashMap();
hMap.put(123,"name1");
hMap.put("key",12.34);
hMap.put(null, "123");
System.out.println("Map Elements ::" + hMap);
System.out.println("Value is     ::" + hMap.get(null));
System.out.println("Keys are     ::"+ hMap.keySet());
System.out.println("Values are   ::"+ hMap.values());

}
}

Iterator Interface:

Iterator Interface
Java

Collection API provides an iterator to Iterate each element of any collection type. Iterator has 3 methods: next():This method iterates to next Element and returns the element. If there is no next Element then NoSuchElementException thrown. hasNext():This method checks whether next element exists or not. If next element is present it returns true otherwise it returns false. remove():This method removes the Element from the Collection. When an Element is removed,the previous element point to the next Element.

1
2
3
4
5
6
7
8

Collection API provides an iterator to Iterate each element of any collection type.
Iterator has 3 methods:
next():This method iterates to next Element and returns the element.
If there is no next Element then NoSuchElementException thrown.
hasNext():This method checks whether next element exists or not.
If next element is present it returns true otherwise it returns false.
remove():This method removes the Element from the Collection.
When an Element is removed,the previous element point to the next Element.

Iterator Example:

Iterator Example
Java

package org.totalqa.collections; import java.util.HashSet; import java.util.Iterator; public class IteratorExample { public static void main(String[] args) { HashSet hSet = new HashSet(); hSet.add("test"); hSet.add(123); hSet.add(null); hSet.add(55.22); System.out.println("Elements of the Set are " + hSet); Iterator itr = hSet.iterator(); while(itr.hasNext()) { System.out.println("Element is " + itr.next()); } } }

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20

package org.totalqa.collections;
import java.util.HashSet;
import java.util.Iterator;
public class IteratorExample
{
public static void main(String[] args)
{
HashSet hSet = new HashSet();
hSet.add("test");
hSet.add(123);
hSet.add(null);
hSet.add(55.22);
System.out.println("Elements of the Set are  " + hSet);
Iterator itr = hSet.iterator();
    while(itr.hasNext())
    {
     System.out.println("Element is " + itr.next());
    }
}
}

Generics

Generics
Java

If all the Elements in the Collection of are of Same Type, then we should use Generics. If a Collection is declared with Generics,we cannot store elements apart from Generic Type.

1
2
3
4

If all the Elements in the Collection of are of Same Type,
then we should use Generics.
If a Collection is declared with Generics,we cannot store elements
apart from Generic Type.

Generics Example:

Generics Example
Java

package org.totalqa.collections; import java.util.ArrayList; public class GenericsExample { public static void main(String[] args) { ArrayList<String> aList = new ArrayList<String>(); aList.add("test"); aList.add("test1"); aList.add(null); aList.add("test2"); System.out.println("Size of the list " + aList.size()); for(int i=0;i<aList.size();i++) { System.out.println("Element at index "+ i + " is " + aList.get(i)); } //Printing list of values using Enhanced For loop System.out.println("Printing the List of values using for-each"); for(Object o:aList) { System.out.println("Element is " + o); } } }

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25

package org.totalqa.collections;
import java.util.ArrayList;
public class GenericsExample
{
public static void main(String[] args)
{
ArrayList<String> aList = new ArrayList<String>();
aList.add("test");
aList.add("test1");
aList.add(null);
aList.add("test2");
System.out.println("Size of the list " + aList.size());
for(int i=0;i<aList.size();i++)
{
System.out.println("Element at index "+ i + " is " + aList.get(i));
}
//Printing list of values using Enhanced For loop
System.out.println("Printing the List of values using for-each");
for(Object o:aList)
{
System.out.println("Element is " + o);
}

}
}

5 comments on “Java Collections-List,Set,Map,Queue,Iterator,Generics”

Cerosh Jacob June 26, 2018 at 2:23 am

Really Good information. nicely collated

Reply ↓
1. total-qa Post authorJune 26, 2018 at 8:09 am
  
  Thanks for the comments.
  
  Reply ↓
Pingback: Sorting the Dynamic HTML Table List in Selenium WebDriver & Java - TotalQA
Pingback: [Solved]Error in Launching Eclipse on a MAC-Failed to find a Main Class - Total-QA
San February 6, 2021 at 4:47 pm

Very useful information. Thank you

Reply ↓

Total-QA

Automation Tools

5 comments on “Java Collections-List,Set,Map,Queue,Iterator,Generics”

Leave a Reply Cancel reply