2. Content
1. Variable declaration
2. Java Naming Rule
3. Comments
4. Primitive Data
5. Type/Wrapper Class
6. Auto Boxing
7. Promotion and Casting
8. Operators
3. Variable Declaration
● Variable is a name of memory location.
● Variable Declaration:
[access_modifier] Datatype varName [ = value];
● There are three types of variables in Java:
○ Instance variable
○ Class Variable or Static variable
○ Local variable
4. Variable Declaration
● Instance variable (non-static fields):
○ Declared outside a method, constructor, block
○ Are visible to all methods, constructors and block in the class
○ No “static” keyword
○ Values are unique to instance of class
○ Has a default value
5. Variable Declaration
● Static or Class variable (static fields):
○ Must be declared outside of methods
○ Has “static” keyword
○ Can be accessed by ClassName.VariableName
○ Has a default value
6. Variable Declaration
● Local Variables
○ Declared in a method, constructor or block
○ No access modifier
○ Lifetime is only within declared block
○ Automatically destroyed once it exits the block
○ No default value => Must be initial a value before used
○ Parameters are also local variables:
■ No access modifier
■ Used for passing a value for method
8. Java Naming Rules
● Variable is case-sensitive
● Can begin with letter (recommended), dollar sign ($), underscore( _ )
● Can contain letters, numbers, underscores and dollar sign (not recommended)
● Can not contain other special characters, whitespace, keywords
● Should start lowercase and with UPPERCASE for the character of the next word (eg.
teacherName)
9. Comments
● Describe the definition of code
● Remark the code
● Documenting the code
● Get ignored by the compiler
● There are 3 types of comments:
○ Single line
○ Multi line
○ Documents
13. Primitive Data Types / Wrapper Class
Primitive Data Types
● Java defines primitive types of data into 8 types: byte, short, int, long, char, float,
double, and boolean
● Primitive Data type is divided into 4 groups:
○ Integer: byte, short, int, long - that are used for whole-valued signed numbers
○ Floating-point numbers: float and double - that is used for decimal numbers
○ Character: char - is represented for symbols in character set such as letters and
numbers
○ Boolean: boolean – is a special data type that is represented true/false values.
14. Primitive Data Types / Wrapper Class
● Integers and Floating-point types
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● Character
15. Primitive Data Types / Wrapper Class
● Character
○ In Java, char data type has a size of 16-bit that is range from ‘u0000’ to ‘uffff’ or
0-65535
● Boolean
○ boolean data type has 1-Bit size that stores values of 0, 1 represented of false and
true
16. Primitive Data Types / Wrapper Class
Wrapper Class
● provides the mechanism for converting primitive data types to Object Wrapper Class.
● For convert data type from String to primitive data type
● provide some methods for usage
18. Auto Boxing
● Autoboxing is the automatic conversion that Java compiler makes between the
primitive data types and their corresponding object wrapper class
E.g Integer i = 9; // This is autoboxing
// Compiler will replace 9 with Integer.valueOf(9)
19. Unboxing
● Unboxing is the automatic conversion from object wrapper class to primitive data
type
E.g
Integer i = new Integer(9);
int j = i; // This is unboxing
// int j = i; is equal to int j = i.intValue();
21. Promotion & Casting
Casting
● Casting is type conversion to a specific data type.
● But there are some points that we can not cast such as:
○ Primitive type to Reference type
○ Null value to primitive type
○ Primitive type to Boolean
○ Boolean to other primitive types
22. Operators
● Operator in java is a symbol that is used to perform operations
● Here are some basic operators that are commonly used:
○ Assignment Operator
○ Arithmetic Operator
○ Relational / Comparison Operator - Bitwise Operator
○ Logical Operator
