Java: Primitive Data Types

• Java is a statically-typed language; the type of a variable is determined at compile time.
• Data types define the sizes and values storable in a variable.
• Java distinguishes between primitive types and reference types.
• Primitive types are predefined using keywords.
• Reference types are programmer-created.

Primitive Types

• Eight primitive data types exist in Java: byte, short, int, long, float, double, boolean, and char.
• byte: An 8-bit signed two's complement integer, with a minimum value of -128 and a maximum of 127. It's useful for conserving memory in large arrays when memory savings matter.
• short: A 16-bit signed two's complement integer, ranging from -32,768 to 32,767. It can also save memory in large arrays if the values are within its range.
• int: A 32-bit signed two's complement integer, spanning from -2,147,483,648 to 2,147,483,647. It's generally the default integer data type unless another type is specifically needed.
• long: A 64-bit signed two's complement integer, with a minimum value of -9,223,372,036,854,775,808 and a maximum of 9,223,372,036,854,775,807. This is used when a wider range than int is required.
• float: A single-precision 32-bit IEEE 754 floating point type, suitable when memory usage is a priority.
• double: A double-precision 64-bit IEEE 754 floating point type, commonly used as the default for decimal values.
• boolean: Represents a single bit of information, with values true or false, used in conditional tests.
• char: A single 16-bit Unicode character, with a minimum value of \u0000 (0) and a maximum of \uffff (65,535), for storing characters.

Type Casting

• Type casting converts one data type into another.
• Java supports implicit (widening) and explicit (narrowing) type casting.
• Widening Casting (Implicit): Automatic conversion from a smaller to a larger type: byte -> short -> char -> int -> long -> float -> double.
• Narrowing Casting (Explicit): Manual conversion using parentheses: double -> float -> long -> int -> char -> short -> byte. May result in data loss.
• Casting from float or double to an integral type (byte, short, char, int, or long) truncates the fractional part.
• Casting a double or float to an int when the value exceeds the int range results in the maximum or minimum int value.
• Widening Example:
  • int myInt = 9;
  • double myDouble = myInt; // Automatic int to double conversion
• Narrowing Example:
  • double myDouble = 9.78;
  • int myInt = (int) myDouble; // Manual double to int conversion

Variable Declaration

• Variables must be declared before use in Java.
• Declaration involves specifying the data type and name.
• Syntax: dataType variableName;
• Variables can be initialized during declaration.
• Syntax for declaration and initialization: dataType variableName = value;
• Examples:
  • int age; declares an integer variable named age.
  • double salary = 50000.0; declares a double variable named salary and initializes it to 50000.0.
• Declaration and initialization can occur on the same line or separately.
• Multiple variables of the same type can be declared on one line, separated by commas: int x, y, z;
• Variable names are case-sensitive.
• Variable names can start with a letter, underscore (_), or dollar sign ($), but not a digit.
• After the first character, variable names can also include letters, digits, underscores, or dollar signs.
• Java keywords (like int, double, class) cannot be used as variable names.
• Use descriptive camelCase names for better readability.
• Initialize variables to prevent potential errors.
• Local variables (inside a method) must be initialized before use.
• Instance variables (in a class, outside a method) are automatically initialized to default values (0 for numeric types, false for boolean, and null for reference types).
• A variable's scope defines where it can be accessed in the code.
• Local variables are only accessible within their method.
• Instance variables are accessible throughout the class.
• The final keyword makes a variable's value unchangeable after initialization.
  • final int CONSTANT_VALUE = 100;