Java - Array

Java Conceptuel Diagram


Collection - Array in Java.

Arrays implement most of the List operations, but not remove and add. They are “fixed-size” Lists.

An array is a container object that holds :

  • a fixed number of values
  • of a single type.

The length of an array is established when the array is created. After creation, its length is fixed.

In Java, arrays cannot be resized dynamically. Another approach is to use a list.



  • Declaration
// declares an array of 
//     - integers
int[] anArray;
//     - string
String[] anArray
// A generic
T[] anGenericArray = (T[]) Array.newInstance(clazz, values.size())
// ...
  • Array Initialization
int[] anArray= {1,2,3,4,5,6,7,8,9,10};
anArray= new Integer[]{1,2,3,4};
string[] stringArray = new String[]{"foo","bar"}
  • Memory Allocation
// allocates memory for 3 integers with the index (0,1,2)
anArray = new int[3];
// allocates memory for 3 strings
anArray = new String[3];
  • Unit Initialization
// initialize first element
anArray[0] = "Nic";
// initialize second element
anArray[1] = "o";

anArray[2] = "Nic";
anArray[3] = "Nicooooo";
// and so forth


Foo[] array = new Foo[list.size()];
list.toArray(array); // fill the array
int[] array = new int[list.size()];
for(int i = 0; i < list.size(); i++) {
         array[i] = list.get(i);



Array vs List - Pro / Cons

  • Pro: You can cast an array but not a list. Example: casting an array at the end of a stream
String[] names = (String[]);





Java - Stream Processing

// or shortcut




for (int i = 0; i < anArray.length; i++) {

	System.out.println(i+" : "+anArray[i]);


int size = array.length;


  • Integer array:
public static double [] resize(int[] array, int newSize){
    if (newSize == 0) {
      return [];
    return array.length == newSize ? array : Arrays.copyOf(array, newSize);
  • Generic where array factory is a functional interface that creates the array in the good type
public static <T> T [] resize(T[] array, int newSize, ArrayFactory<? extends T> factory) {
    int oldSize = array.length;
    if (oldSize == newSize) {
      return array;

    T[] result = factory.create(newSize);
    if (newSize == 0) {
      return result;

    System.arraycopy(array, 0, result, 0, Math.min(oldSize, newSize));
    return result;
public interface ArrayFactory<T> {
  T[] create(int count);


char[] copyTo = java.util.Arrays.copyOfRange(copyFrom, 2, 9);
char[] copyTo = java.util.Arrays.copyOf(copyFrom, copyFrom.length)
System.arraycopy(copyFrom, 2, copyTo, 0, 7);


public static <T> T[] concatAll(T[] first, T[]... rest) {
    int totalLength = first.length;
    for (T[] array : rest) {
      totalLength += array.length;
    T[] result = Arrays.copyOf(first, totalLength);
    int offset = first.length;
    for (T[] array : rest) {
      System.arraycopy(array, 0, result, offset, array.length);
      offset += array.length;
    return result;

Add an element in the first position

public static <T> T[] addElementFirst(T[] elements, T first) {
    int totalLength = elements.length + 1;
    T[] result = Arrays.copyOf(elements, totalLength);
    System.arraycopy(elements, 0, result, 1, totalLength-1);
    return result;

Remove the last one

String[] arr = {"1","2","3"};
String[] arrMinOne = new String[arr.length-1];


import java.util.Arrays;
Arrays.equals(firstArray, secondArray));


Class literal


Example for a byte:


Functional Programming


String[] arr = {"1","2",""};
arr = Arrays
	.map(s -> {
		if (s.equals("")) {
			return null;
		} else {
			return s;

Select by index

Example: select the even elements

List<String> evenElement = IntStream
      .range(0, elements.length)
      .filter(i -> i % 2 == 0)
      .mapToObj(i -> elements[i])


String[] args = Arrays.copyOfRange(args, 1, args.length);


String.join(" ",args)

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