Detailed introduction to the characteristics of static in Java

This article mainly introduces the characteristics of static in Java. Has very good reference value. Let’s take a look at it with the editor

I was asked about the characteristics of static during the interview two days ago. The answer was not very good at the time. Now let’s summarize it

When we understand something, we usually measure it from three aspects: what it is, why, and how. For static in Java, we will also discuss it in this way, and we must clarify the following points:

1. What does static mean in Java and why should we use it?

#2. How to use static in java?

#3. What are the characteristics and "limitations" of static use?

#4. When a member variable is statically modified, what is the difference between it and a non-static member variable?

#1. What does static mean in Java and why should we use it?

static——static——“specify location”

First, let’s take a look at java’s memory: java divides memory into stack memory and heap memory. Stack memory is used to Store some basic types of variables and reference variables of arrays and objects, while the heap memory is mainly used to place objects.

Variables and methods modified with static actually specify the "fixed location" of these variables and methods in memory - static storage. Since there is a "fixed position", their "size" seems to be fixed. With the characteristics of fixed position and fixed size, it is very convenient to open up space in the stack or heap. If a static variable or method does not go out of its scope, its reference handle will not change.

We often see: static variables are somewhat similar to the concept of global variables in C; static means the sharing of memory, that is, each instance of it points to the same memory address. Bringing static tells the JVM that it is static, and its references (including indirect references) point to the same location. If you change it in that place, it will not become the same as before. You clean it up. , it won't come back.

Note: The main() method itself in the main class of java is static, so the main method is executed when no new instance is generated.

2. How to use static in java?

• static is a modifier used to modify members (member variables and member functions).

• When a member is statically modified, there is an additional calling method. In addition to being called by the object, it can also be called directly by the class name: class name.static member.

#3. What are the characteristics and "limitations" of static use?

(1) Features

• Static members are loaded as the class is loaded;

• Static members take precedence over objects;

• Static members are shared by all objects;

• Static members There is an additional calling method, which can be called directly by the class name.

(2) Pros and Cons

Pros:

• Store the shared data of objects in a separate space to save space. There is no need to store a copy in each object;

• can be directly Class name call.

Disadvantages:

• life cycletoo long;

• Access is limited and only static access is available.

(3) Notes

• Static methods only Can access static members, non-static methods can access both static and non-static;

• this cannot be defined in static methods, super keyword; (because This represents an object, and when it exists statically, there may be no object, and static precedes the existence of objects. Therefore, when the static method is running, this is not represented by any object. Simply put, the data in the advanced memory cannot access the data in the backward memory. , but the backward memory data can access the advanced memory data)

• The main function is static

4. When the member variable is What is the difference between static modification and non-static member variables?

• Static variables are also called class variables, which are variables that can be called directly by the class name. This variable belongs to the class; non-static variables are called member variables, or instance variables, which are called What the object calls is the specific object it belongs to.

• Static variables are loaded with the loading of the class, which also means that they disappear with the disappearance of the class and have the longest life cycle; instance variables are loaded with the creation of the object and It disappears when the object disappears and exists according to the life cycle of the object.

• Static variables are stored in the static area of ​​​​the method area; instance variables exist in the heap memory to which the object belongs.

• Static variable data is shared by all objects; instance variables are unique data in the object.

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