Java Collection

It is a resizable array or a dynamic array. It uses this dynamic array mechanism to store its elements. ArrayList class is found in the java.util package in java.

Hierarchy of ArrayList class

            
public class ArrayList<E> extends AbstractList<E> implements List<E>, 
                                  RandomAccess, Cloneable, Serializable  
            

Features of ArrayList

• An ArrayList allows duplicate elements.
• An ArrayList maintains the insertion order.
• An ArrayList is not synchronized, hence not thread safe.
• An ArrayList allows RandomAccess of elements as it implements the same interface functionality.
• Insertion/deletion operation is a little slow if compared to a linkedList as arrayList does a lot of shifting of elements(being a resizable array).
• The iterators returned by iterator and listIterator methods of ArrayList are fail-fast. Which means, if the list is structurally modified at any time after the iterator is created, in any way except through the iterator’s own remove or add methods, the iterator will throw a ConcurrentModificationException.

ArrayList Constructors

1. ArrayList() : This is used to build an empty arrayList.
2. ArrayList(int capacity) : This is used to build an array list that has the specified initial capacity.
3. ArrayList(Collection <? extends E> c) : This is used to build an array list that is initialized with the elements of the collection c .

Coding Examples of HashMap

1. Declare & Add Items to ArrayList

import java.util.ArrayList; // import this class

public class TestArrayList { 
  public static void main(String[] args) { 
    ArrayList<String> vehicles = new ArrayList<String>();
    vehicles.add("Merc");
    vehicles.add("BMW");
    vehicles.add("Ford");
    vehicles.add("JLR");
    System.out.println(vehicles);
  } 
}

OUTPUT

[Merc, BMW, Ford, JLR]

2. Iterating Collection through the for-each loop

import java.util.*; 

class TraverseList{  
 public static void main(String args[]){  
  ArrayList<String> al=new ArrayList<String>();  
  al.add("Alpha");  
  al.add("Beta");  
  al.add("Gamma");  

  //Traversing list using for-each loop  
  for(String obj:al)  
    System.out.println(obj);  
  }  

 System.out.println("-------------");

 // Traversing list using java 8 forEach loop and lambda
  al.forEach(x -> {    // "x" can be replaced by any variable name
    System.out.println(x);
  });
}  

OUTPUT

Alpha
Beta
Gamma
-------------
Alpha
Beta
Gamma

3. set(), get(), isEmpty(), remove() methods of ArrayList

public static void main(String[] args) {
  
      List<String> groceryList = new ArrayList<String>();

       // Check if an ArrayList is empty
       System.out.println("Is the groceryList empty? : " + groceryList.isEmpty());

       groceryList.add("Deo");
       groceryList.add("Cookies");
       groceryList.add("Cereals");
       groceryList.add("Bread");
       groceryList.add("Cheese");

       // Find the size of an ArrayList
       System.out.println("Here is the grocery list of " + groceryList.size() +" items");
       System.out.println(groceryList);

       // Retrieve the element at a given index
       String perfume = groceryList.get(0);
       String lastItem = groceryList.get(groceryList.size() - 1);

       System.out.println("Best perfume: " + perfume);
       System.out.println("Last item in the list: " + lastItem);

       // Modify the element at a given index
       groceryList.set(4, "Amazon");
       System.out.println("Modified grocery list: " + groceryList);
       
       //remove the item from the list
       groceryList.remove(3);
       System.out.println("Grocery list after removing item 3: "+groceryList);
}

OUTPUT

Is the groceryList empty? : true
Here is the grocery list of 5 items
[Deo, Cookies, Cereals, Bread, Cheese]
Best perfume: Deo
Last item in the list: Cheese
Modified grocery list: [Deo, Cookies, Cereals, Bread, Amazon]
Grocery list after removing item 3: [Deo, Cookies, Cereals, Amazon]

4. ArrayList of User defined objects

import java.util.ArrayList;
import java.util.List;

class Employee {
    private String name;
    private int empId;

    public Employee(String name, int empId) {
        this.name = name;
        this.empId = empId;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getEmpId() {
        return empId;
    }

    public void setEmpId(int empId) {
        this.empId = empId;
    }
}

public class TestArraylist {
    public static void main(String[] args) {
        List<Employee> empList = new ArrayList<>();
        empList.add(new Employee("Jason", 30));
        empList.add(new Employee("Winston", 34));
        empList.add(new Employee("Bourne", 29));

        empList.forEach(Employee -> {
          System.out.println("Name : " + Employee.getName() + 
                             ", Employee Id : " + Employee.getEmpId());
        });
    }
}

OUTPUT

Name : Jason, Employee Id : 30
Name : Winston, Employee Id : 34
Name : Bourne, Employee Id : 29

Summary

So here we are, we hope now you are confident and know how to use ArrayList in java.

We tried our best to simplify the explanation, if you feel any difference or you need help in understanding any such thing, then feel free to contact us through email and subscribe to ProgrammerToday.