Summary of java modifier knowledge points

Modifiers in Java are divided into class modifiers, field modifiers, and method modifiers. According to different functions, it is mainly divided into the following categories:

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Permission access modifier (can be used to modify classes and methods and fields)

#public: Available to anyone.

protect: Inherited classes can access and have the same permissions as private.

default: Package access permissions, that is, it can be accessed in the entire package.

private: An element that is not accessible to anyone except the type's creator and the type's internal methods.

Usage summary:

Modified class

Top-level classes can only be modified with public, and top-level classes cannot be modified with private and protected.

External classes can be modified by public or not written by default. Private and protected cannot be used.

Inner classes can be static and can be modified with protected and private.

Modified method

Usually methods can be modified by four access modifiers, and constructors can also be modified by four access modifiers.

Abstract methods in abstract classes cannot be modified by private, but can be modified by the other three.

The methods in the interface can only be modified by public (the default is also public).

Modified attributes

Attributes can be modified by four modifiers, and attributes in interfaces can only be modified by public (the default is also public).

Modify code block

The code block cannot be modified by any modifier (because there are properties and methods in the code block, these properties and methods have their own modifiers ).

The role of the construction code block: non-static properties can be initialized. The construction code block takes precedence over the constructor function and can be placed in the constructor function.

The role of static code blocks: static properties can be initialized, loaded when the class is loaded, and cannot be placed in the constructor.

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##final

Modified class: final class, the final class cannot be inherit. The final class is usually a class that completes certain standard functions, such as the mathematics class Math in the Java library. Modified methods: Methods modified with final cannot be inherited or overridden (such as wait() in Object), but they can be overloaded. Constants: Use final to define local constants, and static final (or final static) to define global constants.

static

Modify class: You can modify internal classes. Modify attributes: static can modify any attribute. Modified methods: Abstract methods cannot be modified, and constructor methods cannot be modified. Modified code block: The modified code block is called a static code block

abstract

Modified class (abstract class): Abstract class cannot be instantiated. Abstract classes are meant to be inherited. Modified method (abstract method): A method that can be modified is called an abstract method. It only has the declaration of the method but no implementation of the method. It ends with a semicolon. Cannot be modified: Properties, private methods, constructors, static methods, and final methods cannot be modified.

What keywords cannot abstract coexist with?

final: If the method is abstracted, it needs to be overridden, and the final modification cannot be overridden.

private: If the function is private, subclasses cannot directly access it and cannot overwrite it.

static: Class method, it makes no sense to call abstract methods directly.

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