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02-datatypes.md

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Data Types, Variables, and Arrays

Data Types

  • Java is a Strongly typed language. it means that every variable and expression has a type and it is strictly defined and .
  • Java compiler checks for type compatibility during assignments ,parameters passing to methods.
  • Java has two types of data types.
    • Primitive Data Types(simple Data types)
    • Reference/Object Data Types
  • Java has eight primitive data types.They are byte,short,int,long,char, float,double and boolean.
Data Type Contents Size Default Value
Byte Signed two's complement integer from -128 to 127 8-bit 0
short signed two's complement integer from -2^15 to 2^15 -1 16-bit 0
int signed two's complement integer from -2^31 to 2^31-1 32-bit 0
long signed two's complement integer from -2^63 to 2^63-1 64-bit 0L
float single precision floating point number 32-bit 0.0f
double double precision floating point number 64-bit 0.0d
boolean stores boolean value true or false 1-bit false
char Stores 16-bit unicode character from '\u0000' to '\ffff' 16-bit -
  • Reference variables are created using the defined constructors of the class.These variables are declared to be of a specific type and that cannot be changed and that type is called reference data type.
  • class objects and various types of array variables come under reference data type.

Variables

  • A variable is a named storage that can store values that a program can manipulate.
  • The type of the variable determines the size ,layout and the range of values that can be stored in a variable.

Example for declaration and initialization the variable is as follows

int x, y;         // Declares two integers x,y.
int x = 10, y = 10;  // Example of initialization
byte B = 22;         // initializes a byte type variable B.
double pi = 3.14159; // declares and assigns a value of PI.
char a = 'a';        // the char variable a iis initialized with value 'a'
  • There are three kind of variable in Java . They are

    • Local Variables

      • Local variables are declared in methods, constructors, or blocks and are only visible inside them.
      • There is no default value for local variables, so local variables should be declared and an initial value should be assigned before the first use.

    • Instance Variables

      • Instance variables are declared in a class, but outside a method, constructor or any block and are visible for all methods, constructors and block in the class.
      • Instance variables have default values and also values can be assigned during the declaration or within the constructor.
      • There would only be one copy of each class variable per class, regardless of how many objects are created from it.

    • Class/Static Variables

      • Class variables also known as static variables are declared with the static keyword in a class, but outside a method, constructor or a block.

      • There would only be one copy of each class variable per class, regardless of how many objects are created from it.

      • Static variables can be accessed by calling with the class name ClassName.VariableName.

        public class Employee {

   // salary  variable is a     private static variable
   private static double salary;

   // DEPARTMENT is a constant
   public static final String   DEPARTMENT ="Development ";

   public static void main(String   args[]) {
       salary = 1000;
       System.out.println   (DEPARTMENT +"average  salary:" + salary);
   }
}

Arrays

  • Array is the data structure that stores fixed-size sequential collections of same type.
  • Declaration is done as follows 
    int   a[]; //dataType arrayRefVar[];
int[] b;  //dataType[] arrayRefVar; **prefered way**
  • Arrays can be created by using new keyword as follows. 
        //dataType[] arrayRefVar = new dataType[arraySize];
    double[] myList = new double[10];

    animal[] a = new animal[5];