Declaration and Access Control

Array Fundamentals

Arrays are used to represent fixed number of elements of the same type. The following are legal syntax for declaring one-dimensional arrays.

int anArray[];
int[] anArray;
int []anArray;


It is important to note that the size of the array is not included in the declaration. Memory is allocated for an array using the new operator as shown below.
anArray = new int[10];
The declaration and memory allocation may be combined together as shown below.
int anArray[] = new int[10];
The elements of the array are implicitly initialized to default values based on array types (0 for integral types, null for objects etc.). This is true for both local arrays as well as arrays which are data members. In this respect arrays are different from normal variables. Variable defined inside a method are not implicitly initialized, where as array elements are implicitly initialized.

Array Initializations

Arrays are initialized using the syntax below
int intArray[] = {1,2,3,4};
The length operator can be used to access the number of elements in an array (for example - intArray.length).

Multidimensional Arrays

The following are legal examples of declaration of a two dimensional array.
int[] arr[];
int[][] arr;
int arr[][];
int []arr[];


When creating multi-dimensional arrays the initial index must be created before a later index. The following examples are legal.

int arr[][] = new int[5][5];
int arr[][] = new int[5][];


The following example will not compile;
int arr[][] = new int[][5];


Class Fundamentals

A class defines a new type and contains methods and variables. The example below illustrates a simple class.

class City {
   String name;	     // member variable		
   String getName()    // member method
   {
      return name;
   }
   public static void main(String arg[]) {
   }
}

Method overloading

Java^TM technology allows two methods to have the same name as long as they have different signatures. The signature of a method consists of name of the method, and count and type of arguments of the method. Thus as long as the argument types of two methods are different, they may be over-loaded (have the same name).

Class constructors

Constructors are member methods that have same name as the class name. The constructor is invoked using the new operator when a class is created. If a class does not have any constructors then Java language compiler provides an implicit default constructor. The implicit default constructor does not have any arguments and is of the type -
class_name() { }

If a class defines one or more constructors, an implicit constructor is not provided. The example below gives a compilation error.

class Test {
   int temp;
   Test(int x) {
      temp = x;
   }
   public static void main() {
      Test t = new Test(); /* This would generate a 
      compilation error, as there is no constructor 
      without any arguments. */
   }
}