The data type specifies the size and type of values that can be stored in an identifier. Variables are the entities that include values and data that act or are acted upon. All variables must have a data type.
A data type is a classification of data, which can store a specific type of information. Data types are primarily used in computer programming, in which variables are created to store data. Each variable is assigned a data type that determines what type of data the variable may contain.
The term data type and primitive data type are often used interchangeably.
Primitive data type: Primitive data types are predefined types of data, which are supported by the programming language. For example,
integer,character, andstringare all primitive data types. Programmers can use these data types when creating variables in their programs. For example, a programmer can create a variable calledlastnameand define it as astringdata type. The variable will then store data as a string of characters.Non-primitive data type: These too are used in a programming language, but are instead created by the programmer. They are sometimes called reference variables, or object references, since they reference a memory location, which stores the data. In the Java programming language, non-primitive data types are simply called objects because they are created, rather than predefined. While an object may contain any type of data, the information referenced by the object may still be stored as a primitive data type.
There are three kinds of variables in Java. They are local variables, instance variables, and class variables, sometimes also known as static variables because the static variable can be used to refer the common property of all objects and the static variable gets memory only once in the class area at the time of class loading.
The following are the properties of local variables:
Local variables are declared in methods, constructors, or blocks
Local variables are created when the method, constructor, or block is entered and the variable will be destroyed once it exits the method, constructor, or block
Local variables are visible only within the declared method, constructor, or block
There is no default value for local variables, so local variables should be declared and an initial value should be assigned before the first use
Here, age is a local variable. This is defined inside the broAge() method and its scope is limited to this method only:
package MyFirstPackage;
public class localVariableExample {
public void broAge(){
int age = 0;
age = age + 7;
System.out.println("My younger brother's age is : " + age);
}
public static void main(String args[]){
localVariableExample localVariableExample = new localVariableExample();
localVariableExample.broAge();
}
}The output will be as follows:
My younger brother's age is : 7
Instance variables are declared in a class, but outside a method, constructor or any block.
When a space is allocated for an object in the heap, a slot for each instance variable value is created.
Instance variables are created when an object is created with the use of the
newkeyword and destroyed when the object is destroyed.Instance variables can be declared in class level before or after use.
The instance variables are visible for all methods, constructors, and blocks in the class. Normally, it is recommended that you make these variables
private(access level). However, visibility for subclasses can be given for these variables with the use of access modifiers.Instance variables have default values. For numbers, the default value is
0; for Boolean, it isfalse; and for object references, it isnull. Values can be assigned during the declaration or within the constructor.Instance variables can be accessed directly by calling the variable name inside the class. However, within static methods and different classes, (when instance variables are given accessibility) it should be called using the fully qualified name,
ObjectReference.VariableName.
The following is an example of instance variables in a Java program:
package MyFirstPackage;
public class instanceVariableExample {
public class Employee{
// this instance variable is visible for any child class.
public String name;
// salary variable is visible in Employee class only.
private double salary;
// The name variable is assigned in the constructor.
public instanceVariableExample (String empName){
name = empName;
}
// The salary variable is assigned a value.
public void setSalary(double empSal){
salary = empSal;
}
// This method prints the employee details.
public void printEmp(){
System.out.println("name : " + name );
System.out.println("salary :" + salary);
}
public static void main(String[] args){
instanceVariableExample empOne = new instanceVariableExample("Raghu");
empOne.setSalary(1000000);
empOne.printEmp();
}
}Output:
name : Raghu salary :1000000.0
Class variables, also known as static variables, are declared with the
statickeyword in a class, but outside a method, constructor or block.There will only be one copy of each class variable per class, regardless of how many objects are created from it.
Static variables are rarely used other than being declared as constants. Constants are variables that are declared as
public/private,final, andstatic. Constant variables never change from their initial value.Static variables are stored in static memory. It is rare to use static variables other than declared
finaland used as eitherpublicorprivateconstants.Static variables are created when the program starts and destroyed when the program stops.
Visibility is similar to instance variables. However, most static variables are declared
publicsince they must be available for users of the class.Default values are the same as instance variables. For numbers, the default value is
0; for Boolean, it isfalse; and for object references, it isnull. Values can be assigned during the declaration or within the constructor. Additionally, values can be assigned in special static initializer blocks.Static variables can be accessed by calling the class name,
ClassName.VariableName.When declaring class variables as public static final, the variables names (constants) are all in uppercase. If the static variables are not
publicandfinal, the naming syntax is the same as instance and local variables.
The following is an example of class/static variables in a Java program:
package MyFirstPackage;
public class classSstaticvariableExample {
// salary variable is a private static variable
private static double salary;
// DEPARTMENT is a constant
public static final String DEPT = "Engg ";
public static void main(String args[]){
salary = 100000;
System.out.println(DEPT+"average salary:"+salary);
}
}The output will be as follows:
Engg average salary:100000.0
The following are the different types of type casting:
The following is an example for widening and narrowing in a Java program:
package MyFirstPackage;
public class Narrowingandwdining {
public static void main(String[] args) {
float i = 10 ;
int j;
System.out.println(i);
//double j = 20.89;
i = (float) 20.89;
System.out.println(i);
j = (int) i; // can also written as (int) j = (int) i;
System.out.println(j);
}
}The output for the preceding code is as follows:
10.0 20.89 20