java - Difference between Static and final?

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Top 5 Answer for java - Difference between Static and final?

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98

The static keyword can be used in 4 scenarios

  • static variables
  • static methods
  • static blocks of code
  • static nested class

Let's look at static variables and static methods first.

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.variable

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) unless it has/creates an instance of the class.
  • A static method can call only other static methods and can not call a non-static method from it unless it has/creates an instance of the class.
  • A static method can be accessed directly by the class name and doesn’t need any object.
  • Syntax: Class.methodName()
  • A static method cannot refer to this or super keywords in anyway.

Static class

Java also has "static nested classes". A static nested class is just one which doesn't implicitly have a reference to an instance of the outer class.

Static nested classes can have instance methods and static methods.

There's no such thing as a top-level static class in Java.

Side note:

main method is static since it must be be accessible for an application to run before any instantiation takes place.

final keyword is used in several different contexts to define an entity which cannot later be changed.

  • A final class cannot be subclassed. This is done for reasons of security and efficiency. Accordingly, many of the Java standard library classes are final, for example java.lang.System and java.lang.String. All methods in a final class are implicitly final.

  • A final method can't be overridden by subclasses. This is used to prevent unexpected behavior from a subclass altering a method that may be crucial to the function or consistency of the class.

  • A final variable can only be initialized once, either via an initializer or an assignment statement. It does not need to be initialized at the point of declaration: this is called a blank final variable. A blank final instance variable of a class must be definitely assigned at the end of every constructor of the class in which it is declared; similarly, a blank final static variable must be definitely assigned in a static initializer of the class in which it is declared; otherwise, a compile-time error occurs in both cases.

Note: If the variable is a reference, this means that the variable cannot be re-bound to reference another object. But the object that it references is still mutable, if it was originally mutable.

When an anonymous inner class is defined within the body of a method, all variables declared final in the scope of that method are accessible from within the inner class. Once it has been assigned, the value of the final variable cannot change.

vote vote

88

static means it belongs to the class not an instance, this means that there is only one copy of that variable/method shared between all instances of a particular Class.

public class MyClass {     public static int myVariable = 0;  }  //Now in some other code creating two instances of MyClass //and altering the variable will affect all instances  MyClass instance1 = new MyClass(); MyClass instance2 = new MyClass();  MyClass.myVariable = 5;  //This change is reflected in both instances 

final is entirely unrelated, it is a way of defining a once only initialization. You can either initialize when defining the variable or within the constructor, nowhere else.

note A note on final methods and final classes, this is a way of explicitly stating that the method or class can not be overridden / extended respectively.

Extra Reading So on the topic of static, we were talking about the other uses it may have, it is sometimes used in static blocks. When using static variables it is sometimes necessary to set these variables up before using the class, but unfortunately you do not get a constructor. This is where the static keyword comes in.

public class MyClass {      public static List<String> cars = new ArrayList<String>();      static {         cars.add("Ferrari");         cars.add("Scoda");     }  }  public class TestClass {      public static void main(String args[]) {         System.out.println(MyClass.cars.get(0));  //This will print Ferrari     } } 

You must not get this confused with instance initializer blocks which are called before the constructor per instance.

vote vote

76

The two really aren't similar. static fields are fields that do not belong to any particular instance of a class.

class C {     public static int n = 42; } 

Here, the static field n isn't associated with any particular instance of C but with the entire class in general (which is why C.n can be used to access it). Can you still use an instance of C to access n? Yes - but it isn't considered particularly good practice.

final on the other hand indicates that a particular variable cannot change after it is initialized.

class C {     public final int n = 42; } 

Here, n cannot be re-assigned because it is final. One other difference is that any variable can be declared final, while not every variable can be declared static.

Also, classes can be declared final which indicates that they cannot be extended:

final class C {}  class B extends C {}  // error! 

Similarly, methods can be declared final to indicate that they cannot be overriden by an extending class:

class C {     public final void foo() {} }  class B extends C {     public void foo() {}  // error! } 
vote vote

61

static means there is only one copy of the variable in memory shared by all instances of the class.

The final keyword just means the value can't be changed. Without final, any object can change the value of the variable.

vote vote

54

final -

1)When we apply "final" keyword to a variable,the value of that variable remains constant. (or) Once we declare a variable as final.the value of that variable cannot be changed.

2)It is useful when a variable value does not change during the life time of a program

static -

1)when we apply "static" keyword to a variable ,it means it belongs to class.
2)When we apply "static" keyword to a method,it means the method can be accessed without creating any instance of the class

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