Fundamental Programming Structures in Java: Arrays

What we’ll cover

• Arrays
• ArrayLists
• Wrapper Classes

Arrays

• An array is a data structure that stores a collection of values of the same type.
• each individual value is accessible by using specifting its index.
• Declare an array variable by specifying the array type, which is the element type, followed by [], and the array variable name
  • In the example below, the array-type is int
  • In the example below, the array variable name is array

int[] array;

• Use the new operator to create the array.
• Once an array is created its length cannot change
  • In the example below, the array is of length 100.

int[] array = new int[100];

• To find the number of elements of an array, use array.length

int[] array = new int[100];
int numberOfElements = array.length;

Array Initializers and Anonymous Arrays

• Below is are two shortcuts for creating an array object and supplying initial values at the same time.

int[] smallPrimes = new int[] { 17, 19, 23, 29, 31, 37 };
int[] smallerPrimes = { 2, 3, 5, 7, 11, 13 };

Array Copying

Arrays can be set from one variable to another. However, then both variables refer to the same array.

int[] smallPrimes = new int[] { 17, 19, 23, 29, 31, 37 };
int[] luckyNumbers = smallPrimes;
luckyNumbers[5] = 12; // now smallPrimes[5] is also 12

• To copy all values of one array to a new array, use Arrays.copyOf method.

int[] copiedLuckyNumbers = Arrays.copyOf(luckyNumbers, luckyNumbers.length);

The “for each” Loop

• Used to iterate through each element in an iterable object (Array, List, Collection, etc.)

• General Syntax

for (VariableType variableName : iterableType) {
  // body of for loop
  // ...
}

• Example Syntax

int[] smallPrimes = new int[] { 17, 19, 23, 29, 31, 37 };
for (int currentPrime : smallPrimes) {
  System.out.println(currentPrime);
} 

• Standard-For Equivalent

int[] smallPrimes = new int[] { 17, 19, 23, 29, 31, 37 };
int numberOfElements = smallPrimes.length;
for (int currentLoopNumber = 0; currentLoopNumber < numberOfElements; currentLoopNumber++) {
  int currentPrime = smallPrimes[currentLoopNumber];
  System.out.println(currentPrime);
}

• While Equivalent

int[] smallPrimes = new int[] { 17, 19, 23, 29, 31, 37 };
int numberOfElements = smallPrimes.length;
int currentLoopNumber = 0;
while(currentLoopNumber < numberOfElements) {
  int currentPrime = smallPrimes[currentLoopNumber];
  System.out.println(currentPrime);
  currentLoopNumber++;
}

Array Sorting

• To sort an array of numbers, you can use one of the sort methods in the Arrays class:
• This method uses a tuned version of the QuickSort algorithm that is claimed to be very efficient on most data sets.
  • Arrays API

int[] a = new int[10000];
Arrays.sort(a)

Multidimensional Arrays

• Multidimensional arrays use more than one index to access array elements
• Declaring a two-dimensional array in Java

double[][] balances;

You cannot use the array until you initialize it.

balances = new double[NYEARS][NRATES];

int[][] magicSquare =
{
    {16, 3, 2, 13},
    {5, 10, 11, 8},
    {9, 6, 7, 12},
    {4, 15, 14, 1}
};

balances[i][j]