Java - Array

About

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.

Management

Initialization

• 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

fromList

• for arrays of reference types

Foo[] array = new Foo[list.size()];
list.toArray(array); // fill the array

• for arrays of primitive types

int[] array = new int[list.size()];
for(int i = 0; i < list.size(); i++) {
         array[i] = list.get(i);
}

IsArray

obj.getClass().isArray()

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[]) personss.stream().map(Person::getName).toArray();

• Cons: This is a primitive data type and as therefore limited functionalities

Conversion

ToList

Arrays.asList

ToStream

Stream

Arrays.asList(array).stream()
// or shortcut
Arrays.stream(array)

Loop

For

for

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

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

Size

int size = array.length;

Resize

• 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;
}

@FunctionalInterface
public interface ArrayFactory<T> {
  T[] create(int count);
}

Copy

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);

Concat

• One element and an array, see add_an_element_in_the_first_position
• Several arrays (More)

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);
    result[0]=first;
    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];
System.arraycopy(arr,0,arrMinOne,0,arr.length-1);

Equality

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

Class

Class literal

type[].class

Example for a byte:

byte[].class

Functional Programming

Transformation

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

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])
      .collect(Collectors.toList());

Slice

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

Join

String.join(" ",args)

Documentation / Reference

• Arrays