Focus on detailed explanation of Java classes and objects

Classes and objects

• • The relationship between classes and objects.
  • Introduction to classes
  • • Class variables (static variables)
  ##public && private
  • Some suggestions And summary
  • Written at the end

Related free learning recommendations:java basic tutorial

class Person { public int age;//成员属性 实例变量 public String name; public String sex; public void eat() {//成员方法 System.out.println("吃饭!"); } public void sleep() { System.out.println("睡觉!"); }}

class Person { public String name; public String sex; public int age; //....人的属性}

., see the picture below

Person person = new Person();//实例化一个对象

You can see that before there is no assignment, the value of 0 corresponding to the type will be assigned by default.

Note: The reason why there is a default value of 0 here is because they are all member variables and do not belong to local variables, so they have default values. Local variables need to be assigned their own values before they can be used.

Class variables (static variables)

The so-called class variables are static variables, modified by the keyword static, and stored in the method area.Space will be allocated when the class is loaded., so class variables are separated from objects. That is to say, class variables can be used even if there is no instantiated object. See the picture below. If there is no object, you can still use class variables throughclass name.variable name

Similarly, the method modified by static is also a class method, also called a static method. It has the same properties as class variables. Space is allocated when the class is loaded and does not depend on the instantiated object. It can be directly Called throughclass name.function name().
If the variables and methods are not static, then we need to instantiate an object through the new keyword before we can use it.

public && private

There are many access modifiers in the definition of a class, and public and private are the two most commonly used. As the name implies, variables modified by public are public. The so-called public means that it can be accessed directly. For example, when we use public to modify member variables, it can be used normally and the members of the object are accessed through dots. But when we use private to modify, An error will be reported because private is private and can only be used within the
current classand cannot be referenced externally.
If you want to use it in other classes, you need to provide a public interface and call it to the class that needs to be used. To put it simply, I write a public method and you pass the data through the method. I, I operate it myself, such as the following methods. You can also know the functions of these methods by looking at their names. Let’s use these methods


You can see that through the public interface We can perform a series of operations on private member variables. Careful friends may find that
this appears in setName and other methods. Variable nameSo what is this? This is actually a Keyword, which represents
the reference of the current method, so why do we need this keyword? That’s because when the variable names are the same, local variables have limited use, which may lead to situations such as failure to assign values. See the figure below. When this is removed, the name is obviously the initial value. This is because local variables are used first, resulting in the member variables in the class not being assigned a value.
Some Suggestions and Summary

If you add this.name, you don’t need setname. You only need to pass the parameters directly when creating the object. Other bloggers will not go into details. .

Written at the end

The above is what the blogger has learned during this period. That’s it for today’s sharing. If there are any unclear or incorrect expressions in the article, please comment or send a private message to point it out. The blogger will deal with it in time. Well, this blog ends here. See you in the next blog.

This article is reproduced from: https://blog.csdn.net/JunFengYiHan/article/details/116066556

Author: JunFengYiHan

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