Arrays

An array is a container object that holds values of a same type.

Each item in the array is called an element.
Each element has a numeric index, starting with 0.
To access an element, use an index and square brackets, e.g., arr[4].
Java arrays are also objects.
So, you need to be able to create an array object.

Examples

int[] arr = null; //declares an array of integers.

arr = new int[4];
/*
4 is the size of the array. 
It is stored in the variable length of the array and never changes.
JVM initializes all elements of arr to 0. 
*/

arr[0] = 1;//assign 1 to 1st element
arr[1] = 2;//assign 2 to 2nd element
arr[2] = 3;//assign 3 to 3rd element
arr[3] = 4;//assign 4 to last element

Array literals reduce all the code so far to one line.

int[] arr2 = {1, 2, 3, 4};

Let's create an array that stores references to elements.

String[] arr3 = null;//declares an array of Strings.
arr3 = new String[2];//JVM creates the string array and initializes all elements to null.
//You cannot increase or decrease the size of the array.
arr3[0] = "Java";//1st element is assigned "Java" object’s reference.
arr3[1] = "School";//2nd element is assigned "School" object’s reference.

Array literals allow you to shorten all the code of the above string array to one line.

String[] arr4 = {"Java", "School"};

How to express strings

To represent strings in code, you need to create a string object from the String class.
If you need the string "Hello", code it like below:

char[] arrayOfHello = {'H','e','l','l','o'};
String greetings = new String(arrayOfHello);

However, most Java programmers use string literals instead of the above code.

String hello = "Hello";//"Hello" is a String Literal.

When JVM calls method main, it creates and passes an array of strings.

class ArrayTest {
  public static void main(String[] args) {
    System.out.println(args.length);
  }
}

C:\javaApp>javac ArrayTest.java

C:\javaApp>java ArrayTest
0

Let's make the JVM create an array with size greater than zero.

class ArrayTest {
  public static void main(String[] args) {
    System.out.println(args[0]);
    System.out.println(args[1]);
    System.out.println(args[2]);
 }
}

C:\javaApp>javac ArrayTest.java

C:\javaApp>java ArrayTest Be The One 
Be
The
One

class ArrayTest {
  public static void main(String[] args) {
    int[] arr = null; //declares an array of integers.
    arr = new int[4];

    arr[0] = 1;//assign 1 to 1st element
    arr[1] = 2;//assign 2 to 2nd element
    arr[2] = 3;//assign 3 to 3rd element
    arr[3] = 4;//assign 4 to last element

    int[] arr2 = {1, 2, 3, 4};

    String[] arr3 = null;//declares an array of Strings.
    arr3 = new String[2];

    arr3[0] = "Java";
    arr3[1] = "School";

    String[] arr4 = {"Java", "School"};

    char[] arrayOfHello = {'H','e','l','l','o'};
    String greetings = new String(arrayOfHello);

    String hello = "Hello"; //"Hello" is a String Literal.

    System.out.print("arr's length:");
    System.out.println(arr.length);

    System.out.println("output all elements of arr");
    int arrLength = arr.length;
    for (int i = 0; i < arrLength; i++) {
      System.out.println(arr[i]);
    }

    System.out.print("arr2's length:");
    System.out.println(arr2.length);

    System.out.println("output all elements of arr2");
    int arr2Length = arr2.length;
    for (int i = 0; i < arr2Length; i++) {
      System.out.println(arr2[i]);
    }

    System.out.print("arr3's length:");
    System.out.println(arr3.length);
    System.out.println("output all elements of arr3");
    int arr3Length = arr3.length;
    for (int i = 0; i < arr3Length; i++) {
      System.out.println(arr3[i]);
    }

    System.out.print("arr4's length:");
    System.out.println(arr3.length);

    System.out.println("output all elements of arr4");
    int arr4Length = arr4.length;
    for (int i = 0; i < arr4Length; i++) {
      System.out.println(arr4[i]);
    }

    System.out.print("output variable greetings:");
    System.out.println(greetings);

    System.out.print("output variable hello:");
    System.out.println(hello);

    System.out.print("args's length:");
    int argsLength = args.length;
    System.out.println(argsLength);

    System.out.println("ouput all elements of args");
    if (argsLength > 0) {
      for (int i = 0; i < argsLength; i++) {
        System.out.println(args[i]);
      }
    }

  }

}

C:\javaApp>javac ArrayTest.java

C:\javaApp>java ArrayTest Self-pity "D.H. Lawrence"
arr's length:4
output all elements of arr
1
2
3
4
arr2's length:4
output all elements of arr2
1
2
3
4
arr3's length:2
output all elements of arr3
Java
School
arr4's length:2
output all elements of arr4
Java
School
output variable greetings:Hello
output variable hello:Hello
args's length:2
output all elements of args
Self-pity
D.H. Lawrence