Java Lesson 5: Arrays

• Creating Arrays
  • When creating an array, you must declare the array variable then define a new array assigned to the variable.
  • Define the array variable using the following syntax:
    int[] myArray;
  • Define the new array, including its size, and assign it to the existing variable using the following syntax:
    myArray = new int[10];
  • You may define both the variable and the array in a single line
    int[] myArray = new int[10];
  • You may also define the variable and populate the array in one line
    int[] myArray = {2, 5, 7, 31, 32}
  • Note that the array variable is distinct from the array itself.
  • Every item in the array must be of the same data type. You cannot have an array that has a mix of int and char or float values.
  • If an array is defined with a fixed size (instead of an implicit size garnered from a set of values), all values in the array are preset to a default value (such as 0 if it is an array of type int, or 0.0 for an array of floats)
  • Arrays are fixed sizes. Once you declare the size of an array, you cannot change it.
• Multi-dimensional Arrays
  • Arrays can be nested to create multidimensional arrays.
    int[][] myArray = new int [20][10]; A 2-dimensional array
    int[][][] myArray = new int [20][10][]; A 3-D array
  • Note that both the variable declaration and the array definition must have the same number of [] pairs.
  • The sizes of the deeper arrays may be left undefined, allowing each nested array to have a different size.
• Working with Arrays
  • Access array items by their index number. Note that arrays start with the index number 0 and use integers as index numbers.
    

            int[] myArray = {2, 5, 7, 31, 32};
            myAge = myArray[0];
            

  • You may also reassign array values by using their index number
    

            int[] myArray = {2, 5, 7, 31, 32};
            myArray[3] = 33;
            

  • Use the length attribute to find the size of an array
    

            int[] myArray = {2, 5, 7, 31, 32};
            myArraySize = myArray.length;
            

  • To access values in deeper arrays, use the following syntax
    myData = myArray[3][5];
• Example
  

  public class MyArrays
  {
      public static void main(String[] args)
      {
          // Create then populate an array
          int[] numbers = new int[5];
          numbers[0] = 2;
          numbers[1] = 4;
          numbers[2] = 6;
          numbers[3] = 8;
          numbers[4] = 10;
  
          // Create and populate in one line a nested array (2-D)
         char[][] letters = {{'A','a'}, {'B','b'}, {'C','c'}, {'D','d'}, {'E','e'}};
  
          // Ouput a value from each array
          for (int i = 0; i <=4; i++)
          {
              int capOrLower = (int)(2*Math.random()); // random 0 or 1
              System.out.println(numbers[i] +"-"+ letters[i][capOrLower]);
          }
      }
  }