Collection Operations on Lists–Simple and Bulk

Create the lists

List list1 = new ArrayList(); 
List list2 = new ArrayList(); 

Add the elements

// Add elements to the lists ...
list1.add("Hanuman");

Bulk Add
Copy all the elements from list2 to list1 (list1 += list2). list1 becomes the union of list1 and list2

list1.addAll(list2); 

Bulk Remove
Remove all the elements in list1 from list2 (list1 -= list2). list1 becomes the asymmetric difference of list1 and list2

list1.removeAll(list2); 

Intersection of Lists
Get the intersection of list1 and list2. list1 becomes the intersection of list1 and list2

list1.retainAll(list2); 

Remove all elements from a list

list1.clear(); 

Truncate the list

int newSize = 2;
list1.subList(newSize, list1.size()).clear(); 

Common methods of LinkedList in java

Often LinkedLists are selected for use because of the methods in this class. LinkedList has most of the common methods ArrayList has (add(), get(), set(), remove(), size(), etc) plus a number of new methods that can be very convenient:

addFirst(object) & addLast(object): adds the object to the beginning or end of the LinkedList.

peek(): This just returns the first element of the LinkedList. Appreciate this method name: The language architect here seemed to be feeling cutesy, which you don't see often.

poll():This returns and removes the first element of the LinkedList. Also this will return null if the LinkedListis empty.

offer(): Attempts to add object to the end of the LinkedLists, and returns a Boolean based on weather it was added or not.

removeFirst() & removeLast(): Returns and removes the last element.

Operations on Lists in java

Creating a list:

List listA = new ArrayList();

Adding Elements

To add elements to a List you call its add() method. This method is inherited from the Collection interface. Here are a few examples:

listA.add("element 1");
listA.add("element 2");
listA.add("element 3");

Adding element at particular index, like index 0 here:

listA.add(0, "element 0");

Accessing elements:

This is done by get method, which gets the element from particular index:

String element0 = listA.get(0);

Removing elements:

You can remove elements in two ways:

1. remove(Object element)
   
2. remove(int index)

remove(Object element) removes that element in the list, if it is present. All subsequent elements in the list are then moved up in the list. Their index thus decreases by 1.

remove(int index) removes the element at the given index. All subsequent elements in the list are then moved up in the list. Their index thus decreases by 1.

 

Iterating over elements:

This can be done in many ways, like using iterator, foreach style for loop, simple for loop with get method :

//access via Iterator
Iterator iterator = listA.iterator();
while(iterator.hasNext(){
  String element = (String) iterator.next();
}


//access via new for-loop i.e. foreach style
for(Object object : listA) {
    String element = (String) object;
}

//old style for loop
for(int i = 0;i<listA.size();i++) {
    String element = listA.get(i);
}

Performance of List interface implementations

LinkedList

- Performance of get and remove methods is linear time [ Big O Notation is O(n) ] - Performance of add and Iterator.remove methods is constant-time [ Big O Notation is O(1) ]

ArrayList

- The size, isEmpty, get, set, iterator, and listIterator operations run in constant time. [ Big O Notation is O(1) ]
- The add operation runs in amortized constant time [ Big O Notation is O(1) ] , but in worst case (since the array must be resized and copied) adding n elements requires linear time [ Big O Notation is O(n) ]
- Performance of remove method is linear time [ Big O Notation is O(n) ]
- All of the other operations run in linear time [ Big O Notation is O(n) ]. The constant factor is low compared to that for the LinkedList implementation.