Static can be used along with Variables, Methods, Class name
and block.
Java Static Variables
- Java instance variables are
given separate memory for storage. If there is a need for a variable to be
common to all the objects of a single java class, then the static modifier
should be used in the variable declaration.
- Any java object that belongs to
that class can modify its static variables.
- Also, an instance is not a must
to modify the static variable and it can be accessed using the java class
directly.
- Static variables can be
accessed by java instance methods also.
- When the value of a constant is
known at compile time it is declared ‘final’ using the ‘static’ keyword
- Unlike non-static variables, such variables can be
accessed directly in static and non-static methods.
Example:
//Program of static variable
Class
Department {
int deptNo;
String empName;
static String orgName="ABC";
Department (int deptNumber,String eName){
deptNo= deptNumber;
empName=eName;
}
void display (){
System.out.println("DeptNo:"+deptNo+"\t
EmpName:"+empName+"\t OrgName:"+orgName);
}
public static void main(String args[]){
Department s1 = new Department (111,"Karan");
Department s2 = new Department (222,"Aryan");
s1.display();
s2.display();
}
}
OutPut:
DeptNo:111 EmpName:Karan OrgName:ABC
DeptNo:222 EmpName:Aryan OrgName:ABC
Advantage of static variable:
It makes your program memory
efficient (i.e it saves memory).
Java Static Methods:
- Similar to static variables, java static methods are also common to classes and not tied to a java instance.
- Good practice in java is that, static methods should be invoked with using the class name though it can be invoked using an object. ClassName.methodName(arguments) or objectName.methodName(arguments)
- General use for java static methods is to access static fields.
- Static methods can be accessed by java instance methods.
- Java static methods cannot access instance variables or instance methods directly.
- Java static methods cannot use the ‘this’ & 'Super' keyword.
- Static methods can be accessed directly in static and non-static methods.
- The static method cannot use non static data member or call non-static method directly.
Example:
class StaticMethod{
static int i;
static String s;
//Static method
static void display()
{
//Its a Static method
StaticMethod obj1=new StaticMethod();
System.out.println("i:"+obj1.i);
System.out.println("i:"+obj1.i);
}
void funcn()
{
//Static method called in non-static
method
display();
}
public static void main(String args[]) //Its
a Static Method
{
//Static method called in another static
method
display();
}
}
OutPut:
i:0
i:0
Q. why java main method
is static?
Ans. because object is
not required to call static method if it were non-static method, jvm create
object first then call main() method that will lead the problem of extra memory
allocation.
Java Static Classes
- A Class can be made static only if it is a nested Class. The nested static class can be accessed without having an object of outer class.
- For java a static inner class it does not mean that, all their members are static. These are called nested static classes in java.
Example:
class StaticClass{
//Static class
static class X{
static String str="Inside Class
X";
}
public static void main(String args[])
{
X.str="Inside Class StaticClass ";
System.out.println("String stored in
str is- "+ X.str);
}
}
OutPut:
String
stored in str is- Inside Class StaticClass
Static Block
- Static block is mostly used for initialize the static variables. This block gets executed when the class is loaded in the memory.
- A class can have multiple Static blocks, which will execute in the same sequence in which they have been written into the program.
- It is executed before main method at the time of classloading.
Example:
class
StaticBlock{
static{
System.out.println("static block is
invoked");
}
public static void main(String args[]){
System.out.println("Hello
main");
}
}
OutPut:
static
block is invoked
Hello main
Q. Can we execute a program without main()
method?
Ans.
Yes, one of the way is static block but in previous version of JDK not in JDK
1.7.
Eg:
class
StaticBlock{
static{
System.out.println("static block is
invoked");
System.exit(0);
}
}
Output:
static block is invoked (if not JDK7)
In
JDK7 and above, output will be:
Output:
Error:
Main method not found in class StaticBlock, please define the main method as:
public
static void main(String[] args)
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