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Arrays

Before discussing other collections in Java, recall these facts about arrays.

  • We must know the size of the array when we instantiate it. 
    String[] arr = new String[5];               // array with 5 null slots
String[] names = {"Alice", "Mad Hatter"};   // array with two Strings
  • To retrieve an item from an array, find it by zero-based index. 
    System.out.println(names[0]); // Alice
  • An empty array is filled with default values (zero for numeric primitives, \u0000 for char, false for boolean, and null for object references). 
    System.out.println(arr[4]);   // {null, null, null, null}
int[] ints = new int[3];      // {0, 0, 0}
  • An array's size cannot change. Its length field is immutable. 
    System.out.println(names.length); // 2
names.length = 1;                 // COMPILER ERROR - the final field
                                  // array.length cannot be assigned.

names[2] = "Queen of Hearts";     // ArrayIndexOutOfBoundsException

Practice Exercise

The fact that an array cannot grow or shrink is a major limitation of arrays.

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