All about "static"
The static keyword can be used in 3 scenarios
1) static variables2) static methods3) static blocks of code.
Lets look at static variables and static methods first.
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static variable
- It is a variable which belongs to the class and not to object(instance)
- Static variables are initialized only once , at the start of the execution . These variables will be initialized first, before the initialization of any instance variables
- A single copy to be shared by all instances of the class
- A static variable can be accessed directly by the class name and doesn’t need any object
- Syntax : <class-name>.<variable-name>
static method
It is a method which belongs to the class and not to the object(instance)
- A static method can access only static data. It can not access non-static data (instance variables)
- A static method can call only other static methods and can not call a non-static method from it.
- A static method can be accessed directly by the class name and doesn’t need any object
- Syntax : <class-name>.<method-name>
- A static method cannot refer to "this" or "super" keywords in anyway
Important Note
main method is static , since it must be be accessible for an application to run , before any instantiation takes place.
Lets learn the nuances of the static keywords by doing some excercises!
Assignment
The goal of this assignment is to learn working of static variables & method
Step 1) Copy the following code into a editor
-
class Student { -
int a; //initialized to zero -
static int b; //initialized to zero only when class is loaded not for each object created. -
Student(){ -
//Constructor incrementing static variable b -
b++; -
} -
public void showData(){ -
System.out.println("Value of a = "+a); -
System.out.println("Value of b = "+b); -
} -
//public static void increment(){ -
//a++; -
//} -
} -
class Demo{ -
public static void main(String args[]){ -
Student s1 = new Student(); -
s1.showData(); -
Student s2 = new Student(); -
s2.showData(); -
//Student.b++; -
//s1.showData(); -
} -
}
Step 2) Save & Compile the code. Run the code as, java Demo.
Step 3) Expected output show below
Following diagram shows , how reference variables & objects are created and static variables are accessed by the different instances.
Step 4) It is possible to access a static variable from outside the class using the syntax ClassName.Variable_Name. Uncomment line # 27 & 28 . Save , Compile & Run . Observe the output.
Step 5) Uncomment line 15,16 & 17 . Save , Compile & Run.
Step 5) Error = ? This is because it is not possible to access instance variable "a" from static method "increment".
static block
The static block, is a block of statement inside a Java class that will be executed when a class is first loaded in to the JVM
-
class Test{ -
static { -
//Code goes here -
} -
}
A static block helps to initialize the static data members, just like constructors help to initialize instance members
转载http://www.javatutorialhub.com/wiki/All_about_%22static%22
Question：
Quick question -
When are static fields initialized? If I never instantiate a class, but I access a static field, are ALL the static blocks and private static methods used to instantiate private static fields called (in order) at that instant?
What if I call a static method? Does it also run all the static blocks? Before the method?
Answer：
A classes static initialization normally happens immediately before the first time one of the following events occurs:
- an instance of the class is created,
- a static method of the class is invoked,
- a static field of the class is assigned,
- a non-constant static field is used, or
- for a top-level class, an assert statement lexically nested within the class is executed.
See JLS 12.4.1.
It is also possible to force a class to initialize (if it hasn't already initialized) by using
Class.forName(String, boolean, ClassLoader)
转载http://stackoverflow.com/questions/3499214/java-static-class-initialization