10 Arrays and ArrayLists

Most Important Concepts / Code

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.

Values stored in an array are called elements.

Elements are accessed by using the array name and then the subscript (also called index) in square brackets [ ].

You can use the array name and the subscript just like a variable.

Each array has a length attribute that tells you the number of elements.

Declare and create array:

All on one line, like:

dataType[] arrayRefVar = new dataType[arraySize];

Or on two lines, like:

dataType[] arrayRefVar; // this line names the array and declares the data type

arrayRefVar = new dataType[arraySize]; // this line actually creates the space in memory for the array

Example:

int[] anArray = new int[3];

Or

int[] anArray;

anArray = new int[3];

When you know the values you want to populate the array at the time of declaration you could use this syntax instead to declare, create, and initialize:

dataType[] arrayRefVar = {value0, value1, ..., valuek}; //

Example:

int[] anArray = { 100, 200, 300 };

Use a for loop and the length attribute to loop through an array.

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

System.out.println(anArray[i]);

}

A string is an array of characters. length is a method of the String class.

System.out.print(testWord.toCharArray()[letterPosition]);

You cannot assign an array to another. To achieve this you can use .clone()

destinationArray = sourceArry.clone();

A two-dimensional array is like a table.

Access elements with arrayName[row][col]

int[][] boxscore = new int[2][9]; // 2D integer array with 2 rows and 9 columns

// how to initialize two dimensional array in Java

// using for loop

int[][] board = new int[3][3];

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

for (int j = 0; j < board[i].length; j++) {

board[i][j] = i + j;

}

}

Array vs. List vs. ArrayList

With generics, the data type becomes like an argument.

ArrayList<String> list = new ArrayList<String>(); // creates ArrayList

list.add("Item1"); // adds item

int pos = list.indexOf("Item1"); // gets the position that matches argument

int size = list.size(); // returns the size

boolean element = list.contains("Item5"); // finds

String item = list.get(0); // returns element at index location

list.remove(0); // removes element at index location

list.remove("Item3"); // removes match

for (int i = 0; i < list.size(); i++) {

System.out.println("Index: " + i + " - Item: " + list.get(i));

}

for (String str : list) {

System.out.println("Item is: " + str);

}