Detailed explanation of Java scope and access modifiers

Detailed explanation of Java scope and access modifiers

In Java,Scope(Scope) refers to the accessibility and life cycle of variables, methods, or classes, that is, their visible range in code. Scope byThe location of the declarationandModifierJoint decision。

1. The scope of variables

There are five scope types for variables

Scope Type	Declare location	life cycle	Access scope
Instance variables	Inside the class, outside the method	Object creation to destruction	All non-static methods in the class
Static variables	Inside the class, outside the method + static	Class loading to the end of the program	All methods in the class, other classes pass the class name
Local variables	Method/Construction Method/In-block	During method/block execution	The declaration is at the end of the method/block
Block variables	Inside the code block	During block execution	Inside the block
Method parameters	Method parameter list	During method call	The whole method body

2. The scope of the method

Method Type	Accessible members	Call method	Modifier limit
Example method	Instance variables, static variables, other instances/static variables	Object name.Method name()	No special restrictions
Static method	Static variables, other static variables	Class name. Method name() or object name. Method name()	Cannot directly access instance members

Access Modifiers:

It is used to control the visibility and access scope of classes, variables, methods and constructors. It is the core mechanism of Encapsulation in object-oriented programming.

Java provides four access modifiers:

According to the access range, from wide to narrow:public > protected > Default (no modifier) > private

Modifier	Visibility range	Typical uses
public	Globally visible	External interfaces, tool classes, constants
protected	Same package, word category	Protected methods or variables that allow subclasses to extend
default	Same pack	Implementation details of in-package sharing
private	Current class only	Hide internal state, force access through method

Things to note：

1. When the subclass rewrites the parent class method,Access permissions cannot be smaller than parent class scope

2. When the construction method is modified with private, it is one of the requirements of the singleton pattern, that is, external instantiation is not allowed.

Summarize

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