How to create a class in Java

What is a class? What is an object?

Class - is abstract. It is an abstraction (or collection) of objects with common attributes and services, which includes two main parts: attributes and services;

Object - is concrete. It is an entity used to describe objective things in the system. It is a basic unit that constitutes the system.

The relationship between classes and objects is like the relationship between molds and castings. The instantiation result of a class is an object, and the abstraction of a type of object is a class.

Definition of class

The general format of defining a class in Java:

修饰符 class 类名{成员}

Understanding: The modifier is optional, there is public (public) Or do not add;

class - keyword, must have;

class name - capitalize the first letter, and name according to camel case naming rules, must have;

members ——There are member attributes and member methods.

Member definition

(1) Member attribute definition

Format:

[ public | private | protected ]　[ static ]　[ final ]　类型　成员名

Understanding: type and member name It is required, others are optional;

public public members - can also be accessed by different packages;

private private members - can only be accessed in this class;

protected Protected members—accessible to the same package and subclasses;

None of the above three defaults—accessible to the same package;

static Static members—belonging to the class, It does not belong to a specific object, all objects of the class share this member. All objects that are not static objects must be instantiated before they can be accessed.

Static members or methods are accessed directly through the class name (via: class name. reference);

final member - the final member, its value cannot be changed;

final Class - the final class, cannot be inherited;

final method - the final method, this method cannot be overridden;

Note: final and public can be loaded on the class, and static cannot.

(2) Member method definition

Format:

[public|private|protected] [static] [final] 类型 方法名（形参）{
　　　　方法体；
　　　　return表达式；
}

Understanding:

Type (first letter is capitalized) (required): refers to return The type of value (can be a basic type or an object type);

Method name (required): what to do;

Method body: defines the specific content of the method, usually starting from It has two functions: one is to perform various operations around the attributes of the class, and the other is to perform data exchange and message passing operations with other classes and objects;

Formal parameters (optional): There are only two types : Basic types and object types. Parameters in method calls are used to pass values ​​and references, and methods can also be nested and called recursively;

return can not only return a value, but also end the method body.

It is worth noting: If a non-void return type is specified in the method body, the method must include a return statement to ensure that a value is returned under any circumstances. The return statement cannot be followed by any expression.

Recommended tutorial: Java tutorial

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