Java Variable and Constant

Java Variable

• A variable is a location in your computer's memory in which you can store a value and from which you can later retrieve that value.
• Your computer's memory is normally Random Access Memory (RAM).
• A variable must always be defined inside a Java class.
• When you define a variable in Java, you must tell the compiler what kind of variable it is. (this is usually referred to as variable type).

Java Primitive variable types

• Java works with Primitive variable types, which represent simple values that have built-in functionality in the language.
• They are fixed elements, such as characters, integers and floating-point.
  Primitive types in Java:

  Type	Definition
  boolean	true or false
  char	16-bit, Unicode character
  byte	8-bit, signed, two's complement integer
  short	16-bit, signed, two's complement integer
  int	32-bit, signed, two's complement integer
  long	64-bit, signed, two's complement integer
  double	64-bit, 1.7e-308 – 1.7e+308, floating point value
  float	32-bit, 3.4e-38 -- 3.4e+38, floating-point value

  Variable declaration example:

  ...
  int foo;
  double d1, d2;
  boolean isFun;
  ...

  Variable declaration with initialization example:

  ...
  int foo = 42;
  double d1 = 3.14, d2 = 2 * 3.14;
  boolean isFun = true;
  ...

  Integer literals example:

  ...
  int i1 = 1230;   // specified in decimal (base 10) 
  int i2 = 01230;  // specified in octal (base 8)          // i2 = 664 decimal 
  int i3 = 0xFFFF; // specified in hexadecimal (base 16)   // i3 = 65535 decimal 
  ...

  Automatic casting/converting example:

  ...
  long l = 13L;    // Int. literals are of type int 
                   // unless they are suffixed with an L
  long l = 13;     // Equivalent: 13 is converted from type int
  
  byte b = 42;     // Works fine as long as values < 255
  int i = 43;
  int result = b * i;  // b is promoted to int before multiplication 
  
  int x = 13;
  byte b = x;           // Compile-time error, explicit cast needed 
  byte b = (byte) x;    // OK - as x is casted to byte
  ...

  Floating-point literals example:

  ...
  double d = 8.31;      // Floating-point values can be specified in decimal 
  double e = 3.00e+8;   // or specified in scientific notation 
  float f = 8.31F;      // Default of type double unless they 
                        // are suffixed with an f or F 
  float g = 3.00e+8F;;
  ...

  Character literals example:

  ...
  char a = 'a';            // Specified either as a single-quoted character 
                           // or
  char newline = '\n';     // as an escaped ASCII 
                           // or
  char smiley = '\u263a';  // written with special Unicode escape sequences 
  ...