Modifiers in the Java language

Source: Internet
Author: User
Tags modifiers

Available modifiers for classes, methods, member variables, and local variables

Modifier Class Member Methods Construction method Member variables Local variables
Abstract Y Y _ _ _
Static _ Y _ Y _
Public Y Y Y Y _
Protected _ Y Y Y _
Private _ Y Y Y _
Synchronized _ Y _ _ _
Native _ Y _ _ _
Transient _ _ _ Y _
Volatile _ _ _ Y _
Final Y Y _ Y Y

  1. Access control modifier:The Java language uses access control modifiers to control access to methods and variables for classes and classes, exposing interfaces only to the consumer, but hiding implementation details.
    • Public level: With public decoration, open to the outside
    • Protected level: exposed to subclasses and classes in the same package with protected adornments
    • Default level: No access control modifiers are exposed to classes in the same package
    • Private level: Decorated with private, only the class itself can be accessed, not publicly
the top-level class has only the default or public level, so it cannot be decorated with private and protected.  2. Abstract modifier:used to modify classes and member methods, abstract classes cannot be instantiated, and abstract methods do not provide specific implementations.
    • Abstract classes can have no abstract methods, but classes that contain abstract methods must be defined as abstract classes
    • If a subclass does not implement all of the abstract methods in the parent class, then the subclass must also be defined as an abstract class
    • No abstract construction method, no abstract static method
    • Abstract classes can have non-abstract construction methods that may be called when an instance of a subclass is created
    • Abstract classes and abstract methods cannot be modified by the final modifier (why?). )
 3. Final modifier:
    • With "immutable" meaning, it can modify non-abstract class fly, non-abstract member methods and variables.
    • Class with final decoration cannot be inherited, no subclasses
    • The method of final modification cannot overwrite the methods of the quilt class
    • A constant is represented by a final modified variable and can only be assigned one time value
    • Final cannot be used to modify the construction method, meaningless (why?) )
Final class:
    • Classes not specifically designed for inheritance
    • The implementation details of the class are not allowed to change for security reasons
    • When creating the object model, be sure that the class will no longer be extended
Final method:
    • For security reasons, the parent class does not allow subclasses to overwrite a method, and this method can be declared as final
final variable characteristics:
    • You can modify static variables, instance variables, and local variables, representing static constants, instance constants, and local constants, respectively.
    • The final variable must show initialization
    • Final variable can only be assigned one time value
    • If a variable of a reference type is final decorated, the variable can only always reference only one object, but it may change the content
final defines constants that have the following effects:
    • Improve program security and prohibit illegal modification of data that is fixed and not allowed to change
    • Improved maintainability of program code
    • Improve the readability of program code
 4. Static modifier:
    • Static variables are represented by statically modified variables, which can be accessed directly from the class name.
    • A static method is represented by a statically decorated member method, which can be accessed directly from the class name.
    • A static code block is represented by a statically decorated program code block, which executes the code when the Java Virtual machine loads the class.
static variable:
    • Static variables have only one copy in memory

    • Allocates memory once for each instance of an instance variable, and does not affect

    • Static variables can be shared by all instances of the class and can be used as shared data for communication between instances


static method (why)

    • The This keyword cannot be used in a static method, nor does it directly access the instance variables and instance methods of the owning class
    • You can directly access the static variables and static methods of the owning class
    • The Super keyword is also not available in static methods
    • A static method cannot be defined as an abstract method and must be implemented
Static code block

    • A class can also contain a static block of code that does not exist in any method body.
    • These static blocks of code are executed when the Java Virtual machine loads the class. The constructor of a class is used to initialize an instance of the class, whereas a static code block of a class can be used to initialize the class.
    • Static code blocks, like static methods, do not directly access instance variables and instance methods of a class, but must be accessed through the instance's reference

Modifiers in the Java language

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