#37 Static Variable in Java
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In this lecture we are discussing:
1)What is use static keyword in java?
2)At which place we can use static keyword. (discussing in upcoming next video)
3)when memory get allocated (extra information how static variable memory get allocated)
4)static variable vs instance method
Use of static keyword
= when we are using static keyword with variable, block, method it become
independent of object.
e.g class{
static int a=5;
}
= a is not dependent on object .if we have two object obj1 and obj2 then both object able to access this
variable.
= when we are using static it become independent to object.
Four place we can use static keyword
a) before variable declaration e.g class Calc{ static int s; }
b) before a block class Calc{ static {System.out.println("this is static block);}}
c) during method creation class Calc{public static void add(int num1,int num2){};}
d) with concept of inner class (It is not discussed in this lecture)
class OuterClass {
int x = 10;
static class InnerClass {
int y = 5;
}
}
When memory get allocated
= for that we need to know some term
a)stack area b)heap area c)class loader system
Step 1: when you compile the code you get .class file
Step 2: when you are executing (java MainClass) first class loading to class loader System.
Step 3: During class loading static variable initialize, static block get executed.
Step 4: Since, static variable got memory in heap before object creation. Now we can say that it is independent of object.
Static variable vs instance variable
Class Calc{
static int stA=100; //independent of object // we can use this by class name as well as object
int inB=100; //dependent of object //we can only use this by object
public static void main(String []args){
Calc obj1=new Calc();
Calc obj1=new Calc();
//for static variable
System.out.println(Calc.stA); //use with class name
System.out.println(obj1.stA); //use by object name
//for instance variable
//System.out.println(Calc.inB); //got an error --Cannot make a static reference to the non-static
System.out.println(obj1.inB); //used by object name reference
// = if we can change value static or instance what happen
obj1.inB=1000;
obj1.stA=2000;
//static
System.out.println(obj1.stA);//output: 2000
System.out.println(obj2.stA);//output: 2000 value changed for both obj1 and obj2
//it also show that static variable independent of objects
//instance
System.out.println(obj1.inB); // output: 1000
System.out.println(obj2.inB); // output: 100 no change in obj2
}
}
Remember one thing:
i)we can use static property, member inside non static block or method without object creation.
ii) But we cannot use non static property or method inside static block or method without object creation.
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