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Arrays and ArrayLists

Java ArrayList

In Java, an ArrayList is used to represent a dynamic list.

While Java arrays are fixed in size (the size cannot be modified), an ArrayList allows flexibility by being able to both add and remove elements. 

// import the ArrayList package
import java.util.ArrayList;


// create an ArrayList called students
ArrayList<String> students = new ArrayList<String>();

Index

An index refers to an element’s position within an array.

The index of an array starts from 0 and goes up to one less than the total length of the array.

int[] marks = {50, 55, 60, 70, 80};


System.out.println(marks[0]);
// Output: 50


System.out.println(marks[4]);
// Output: 80

Arrays

In Java, an array is used to store a list of elements of the same datatype.

Arrays are fixed in size and their elements are ordered.

// Create an array of 5 int elements
int[] marks = {10, 20, 30, 40, 50};

Array creation in Java

In Java, an array can be created in the following ways:

  • Using the {} notation, by adding each element all at once.
  • Using the new keyword, and assigning each position of the array individually.
int[] age = {20, 21, 30};


int[] marks = new int[3];
marks[0] = 50; 
marks[1] = 70;
marks[2] = 93;

Changing an Element Value

To change an element value, select the element via its index and use the assignment operator to set a new value.

int[] nums = {1, 2, 0, 4};
// Change value at index 2
nums[2] = 3;

Modifying ArrayLists in Java

An ArrayList can easily be modified using built in methods.

To add elements to an ArrayList, you use the add() method. The element that you want to add goes inside of the ().

To remove elements from an ArrayList, you use the remove() method. Inside the () you can specify the index of the element that you want to remove. Alternatively, you can specify directly the element that you want to remove.

import java.util.ArrayList;


public class Students {
  public static void main(String[] args) {
    
     // create an ArrayList called studentList, which initially holds []
		ArrayList<String> studentList = new ArrayList<String>();
    
    // add students to the ArrayList
    studentList.add("John");
    studentList.add("Lily");
    studentList.add("Samantha");
    studentList.add("Tony");
    
    // remove John from the ArrayList, then Lily
    studentList.remove(0);
    studentList.remove("Lily");
    
    // studentList now holds [Samantha, Tony]
    
    System.out.println(studentList);
  }
}
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