Modifiers for the Java language

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

Modifiers	Class	Member Methods	Construction method	Member variable	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 the access rights of classes and classes ' methods and variables, exposing only the interface to the consumer, but hiding the implementation details.

Public level: To be publicly decorated, externally

Protected level: Protected decorated, exposing to subclasses and classes in the same package

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 public

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 class 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 the 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 invoked when instances of subclasses are created

Abstract classes and abstract methods cannot be decorated with the final modifier (why?) ）