Java is a statically typed programming language. That is, it checks the data types of all values at compile time,

Data type

• Defined in terms of 3 components
  • A set of values,
  • A set of operations that can be applied,
  • A data representation, which determines how the values are stored,

Built-in data types

• Predefined data types,
• Ex: int, double, String etc,

Primitive data type

• Consists of an atomic, indivisible value,
• Defined without the help of any other data types,
• Programmers can't extend primitive data types,

Identifier

• Sequence of characters of unlimited length,
• Includes letters and digits,
• First letter must be a letter,
• Spaces aren't allowed in an identifier,
• Case sensitive,
• Ex: value, VALUE are different identifier,

Variables

• These memory locations,
• A variable has 3 properties:
  • A memory location for storing the value,
  • The type of the data to store at the memory location, and
  • A name (also called identifier) to refer to the memory location,
• Ex: int sum; where
  • memory for storing int is allocated,
  • data type = int,
  • name = sum,

Java supports 2 kinds of data types:

• Primitive

  • Holds a value,
  • Ex: int result;,
  • 2 categories:
    • Numeric: byte, short, int, long, char, float, double,
    • Boolean: boolean,

• Reference:

  • Holds a reference to an object,
  • Ex: String s;
    • Space is allocated for variable not for object,
  • Ex: String s = new String("Saeed");,
    • Space is allocated separately for variable and the object it is referring,
  • Be alert while modifying object of reference type inside method,

    private static void printHumanData(Human human){
      System.out.println(human.getName()+" -> "+human.getAge());
      human.setAge(26); // will modify main object
    }
    

    Using like this:

    Human human = new Human("Saeed",21);
    printHumanData(human); // Saeed -> 21
    System.out.println(human.getAge()); // 26
    

    • These are discussed briefly in Classes and Object section,