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
