Static final is used to modify member variables and member methods, which can be simply understood as "global constants"!
For variables, it means that once a value is given, it cannot be modified and can be accessed through the class name.
For methods, it means that they cannot be overridden and can be accessed directly through the class name.
Sometimes you want to define a class member so that its use is completely independent of any objects of that class. Normally, a class member must be accessed through an object of its class, but it is possible to create a member that can be used by itself without referencing a specific instance. Such a member can be created by adding the keyword static (static) before the member's declaration. If a member is declared static, it can be accessed before any object of its class is created without having to reference any object. You can declare both methods and variables as static. The most common example of a static member is main(). Because main() must be called when the program begins execution, it is declared static.
A variable declared as static is essentially a global variable. When an object is declared, a copy of the static variable is not generated, but all instance variables of the class share the same static variable. Methods declared as static have the following restrictions:
· They can only call other static methods.
· They can only access static data.
· They cannot refer to this or super in any way (the keyword super is related to inheritance and is described in the next chapter).
If you need to initialize your static variables through calculation, you can declare a static block. The Static block will only be executed once when the class is loaded. The following example shows a class with a static method, some static variables, and a static initialization block:
class UseStatic {
static int a = 3;
static int b;
static void meth(int x) {
System.out.println("x = " + x);
System.out.println("a = " + a);
System.out.println("b = " + b);
}
static {
System.out.println("Static block initialized.");
b = a * 4;
}
public static void main(String args[]) {
meth(42);
}
}Once the UseStatic class is loaded, all static statements are run. First, a is set to 3, then the static block is executed (printing a message), and finally, b is initialized to a*4 or 12. Then call main(), main() calls meth(), passing the value 42 to x. The three println () statements refer to two static variables a and b, and the local variable x.
Note: It is illegal to reference any instance variables in a static method.
Here is the output of the program:
Static block initialized. x = 42 a = 3 b = 12
Static methods and variables can be used independently of any object outside the class in which they are defined. This way, you just add the dot operator after the class name. For example, if you want to call a static method from outside the class, you can use the following general format:
classname.method( )
Here, classname is the class Name, define static method in this class. As you can see, this format is similar to the format of calling non-static methods through object reference variables. A static variable can be accessed in the same format - the class name followed by the dot operator. This is how Java implements a controlled version of global functions and global variables.
Below is an example. In main(), the static method callme() and the static variable b are accessed outside their classes.
class StaticDemo {
static int a = 42;
static int b = 99;
static void callme() {
System.out.println("a = " + a);
}
}
class StaticByName {
public static void main(String args[]) {
StaticDemo.callme();
System.out.println("b = " + StaticDemo.b);
}
}The following is the output of the program:
a = 42 b = 99
Static members cannot be accessed by instances created by the class in which they are located.
If the members without static modification are object members, they are owned by each object.
Members modified with static are class members, which can be directly called by a class and are common to all objects.
The above is the detailed content of What effect does static and final achieve when used together in Java?. For more information, please follow other related articles on the PHP Chinese website!