Objective:

Learn about:

• The use of variables
• Working with different data types and their implications
• How to create different types of variables and to print their output to screen
• Garbage Collection
• Concept:

Primitive Data Types
Programs operate on data. Therefore we must be able to refer to various kinds of data. Each data value has a specific data type that determines the kinds of operations that we can perform on it. Several common data types are built into the java language and are called primitive data types in java: four variations of integers, two variations of floating point numbers, a Boolean data type, and a character. 

Integers and Floating Points
Java has two basic kinds of numeric values: integers (which have no fractional part) and floating points (which do). There are four integer data types (byte, short, int, and long) and two floating-point data types (float and double). All of the numeric types differ by the amount of memory space used to store a value of that type, which determine the range of the values that can be represented. The size of ech dta type is the same for all hardware platforms. All numeric types are signed, meaning that both negative and positive values can be stored in them. 

Characters
Characters are another fundamental type of data used and managed on a computer. A character set is alist of characters in a particular order. A character literal is expressed in a java program with single quotes, such as 'b', 'c'. Strings are the combination of characters. You can store a complete sentence in a string. String literals are used with the double quotes. The string type is not a primitive data type in java. 

Boolean
The Boolean data types has only two values: true and false. A Boolean variable is usually used to indicate if a particular condition is true or not. But it can also be used to represent any situation that has two states, such as a light bulb being or or off. Because a Boolean value represent one of two situations, only one byte is needed to store it. A Boolean value cannot be converted to any other data type and any other data type cannot be converted to a Boolean value. The words true and false are reserved in java as Boolean literals and cannot be used outside of this context.     

Identifiers
A programmer must make up words writing a program, such as a class name or a method name. Theses words are called identifiers. Most identifiers have no predefined meaning in language. 

Example: 

public static void main (String args[]){
          int a = 29;
          System.out.println("Hello " + a)
}

Here in this example, main, a , string, args and System.out.println are the identifiers. 

Keywords
Keywords are identifiers that have a special meaning in a programming language and can only be used in predefined ways. These are predefined so we can use them only according to the predefined rules. In above example key words are public, static, void.

Reserved Words
Reserved words have same behavior as keywords. These are one and the same things. But only in some java certification books keywords "false", "true", and "null" are marked as reserve words.

Literals
Literals are the values stored in a variable against any data type or we can say that the literals are the explicit data type that are used in a program. in above example 29 is a literal of int type.

Operators
Operators are the instructions: with the help of those we can do some operations on the data. For example in "a+b", a and b refers to the data and "+" sign is a operator that indicates the multiplication operation on the data. Operators are of different types such as: assignment operators, arithmetic operators, increment and decrement operators, and logical operators.

Operands
Operands are the values on which an operators performs a specific action. for example in "a+b" "a" and "b" that are referencing to the data are called the operands. The operands used in the operations might be literals, constants, variables, or other sources of data.

Expressions
Programming statements often involve expressions. These are combinations of operators and operands used to perform a calculation. In above examples "a+b" is called a whole expression.

Constants
A constant is some value that does not change e.g 1, "Sam", 'n' etc

Naming Convention
An identifier can be composed of any combination of letters, digits, the underscore character ( _ ) the dollar sign ($), but it cannot begin with a digit. Identifiers may be of any length. Therefore, total, leable7, next_stock_item, and $amount are all valid identifiers, but 4^th _word and coin# value are not valid.

Both uppercase and lowercase letters can be used in an identifier, and the difference is important. Java is case sensitive, which means hat two identifier names that only differ by the case of their letter are considered to be different identifiers. Therefore total, Total and ToTal are different identifiers.

Assignment Statements
Assignment statements are used to assign values to the variables. For example in expression "a=12" "a" is variable identifier and "=" is the assignment operator and "12" is the literal that is assigned to the variable a. In java or any other familiar programming languages, it is important to note that "=" sign does not means the equality; it is used to assign the values on the right side of this operator to the identifiers on the left side. So te above expressions does not mean "a is equal to 12", but it means place the value of 12 in the variable a.

Variables
A variable is a location in the computer's memory where a value can be stored for use by a program. All variables must be declared with some name and data type before they can be used in a program.

Declaration
        String first, second;
     int No1, No2;
This declaration specifies that variable first, second are of data type String.

Instance Variables
Java programmers usually create their own user defined data types called classes. Each class contains data and set of functions that manipulate the data. The data components of a class are called "Instance variables". An instance of built in data type Int or String is called a variable.    

Just as this an instance of a user defined data type is called Object. The difference between an object and a variable is that Object contains both data and methods in it while a variable contain only data in it. 

Static Variables
Each object of a class has its own copy of all the instance variables of the class. In certain cases, only a particular copy of a variable is shared by all the objects of the class. This particular variable is static in nature and called static variable. A static class variable represent class wide information all objects of the class share same piece of data among themselves. The declaration of static variable begins with keyword static. 

Declaration

public static int count;
private static String str;

A class public static class variable can be accessed through a class name using dot operator to an object of that class. A class's private static class variable can be accessed only through the methods of that class. Actually static class members exist even when no object of that class exists. 

Always remember that "A method declared static cannot access non static class members. Unlike simple methods a static method has no this reference because static class variables and methods exist independent of any objects of a class and before any object of the class instantiated. 

Final Variables
Java uses final keyword to declare constant variables. The constant variables must be initialized before they can be used in your program. They cannot be modified thereafter. Any attempt to make a change in the value of the final variables is an Error. Constant variables are also called Read only variables. They are often used to make a program more readable.

Declaration

final int count = 10;

Usage of variables
In this section we will discuss the usage of all above three types of variables. 

Examples:

Class variable {

String first, second;
int no1, no2;
static int st;
final int count = 10;

public static void main (String args[]) {

Variables v = new variables ();
v.first = "Pakistan";
v.second = "Zindabad";
System.out.println(v.first + "\n" +v.second);

//v.count = 11; //Error can't modify static variable 
System.out.println("final values: " + v.count);

v.no1 = 1999;
v.no2 = 2000;
System.out.println(v.no1 + "\n" + v.no2);
v.st = 36;
System.out.println("static values: " +v.st);
}

}

Java Keywords
Each of the word that we use in java program like if else, while, do while, final, static, etc are all java keywords. These words are reserved by the language to implement various feature. Keywords cannot be used by as identifiers (as variables names). List of java variables is shown as:

Garbage Collection
We have often seen in the Java language that new objects have been created for different classes and each object hold all the attributes and behaviors of its class, so it acquire various system resources such as memory (specially when new operator is used). Now we need a disciplined way to give resources back to the system when they are no longer in use. Java provides us with the facility of automatically reallocating the resources. The technique is called Garbage collection.

It works like this: when no references to an object exist, that object assumed to be no longer needed then memory occupied by the object can be automatically reclaimed. 

Declaration

System.gc();                    //explicit call to the garbage collector

this call uses public static void method gc() from class system in the java.lang package

finalize() method
Sometimes there could be a situation that an object need to perform some action when it is destroyed. For this purpose we use finalize() method. It is originally defined in the class object. A class's finalize() method always have name finalize receives no parameter and return no value. Method finalize() is normally called protected in a class.

Declaration

protected void finalize() {
        System.out.println("All the garbage has been collected");
}