My Java Notebook I

Module 3: Arrays

I. Arrays

Arrays

An array is a collection of variables of the same type.
When you need to store a list of values, such as numbers, you can store them in an array, instead of declaring separate variables for each number.

To declare an array, you need to define the type of the elements with square brackets.
For example, to declare an array of integers:

The name of the array is arr. The type of elements it will hold is int.

Now, you need to define the array's capacity, or the number of elements it will hold. To accomplish this, use the keyword new.

The code above declares an array of 5 integers.
In an array, the elements are ordered and each has a specific and constant position, which is called an index.

To reference elements in an array, type the name of the array followed by the index position within a pair of square brackets.
Example:

This assigns a value of 42 to the element with 2 as its index.

Initializing Arrays

Java provides a shortcut for instantiating arrays of primitive types and strings.
If you already know what values to insert into the array, you can use an array literal.
Example of an array literal:

Place the values in a comma-separated list, enclosed in curly braces.
The code above automatically initializes an array containing 4 elements, and stores the provided values.

II. Summing Elements in Arrays

Array Length

You can access the length of an array (the number of elements it stores) via its length property.
Example:

Arrays

Now that we know how to set and get array elements, we can calculate the sum of all elements in an array by using loops.
The for loop is the most used loop when working with arrays, as we can use the length of the array to determine how many times to run the loop.

In the code above, we declared a variable sum to store the result and assigned it 0.
Then we used a for loop to iterate through the array, and added each element's value to the variable.

Result: (Program.java)

III. Enhanced for Loop

Enhanced for Loop

The enhanced for loop (sometimes called a "for each" loop) is used to traverse elements in arrays.
The advantages are that it eliminates the possibility of bugs and makes the code easier to read.
Example:

The enhanced for loop declares a variable of a type compatible with the elements of the array being accessed. The variable will be available within the for block, and its value will be the same as the current array element.
So, on each iteration of the loop, the variable t will be equal to the corresponding element in the array.

Result: (Program1.java)

IV. Multidimensional Arrays

Multidimensional Arrays

Multidimensional arrays are array that contains other arrays. The two-dimensional array is the most basic multidimensional array.
To create multidimensional arrays, place each array within its own set of square brackets.
Example of a two-dimensional array:

This declares an array with two arrays as its elements.
To access an element in the two-dimensional array, provide two indexes, one for the array, and another for the element inside that array.
The following example accesses the first element in the second array of sample.

You can get and set a multidimensional array's elements using the same pair of square brackets.
Example: int[][] myArr = { {1, 2, 3}, {4}, {5, 6, 7} };
myArr[0][2] = 42;
int x = myArr[1][0]; // 4

The above two-dimensional array contains three arrays. The first array has three elements, the second has a single element and the last of these has three elements. In Java, you're not limited to just two-dimensional arrays. Arrays can be nested within arrays to as many levels as your program needs. All you need to declare array with more than two dimensions, is to add as many sets of empty brackets as you need.
However, these are harder to maintain.
Remember, that all array members must be of the same type.

Result: (Program2.java)