Detailed explanation of the characteristics and usage of the static keyword in PHP

→static: static

Usage: It is a modifier used to modify members (member variables, member function)

class Person
{
    String country = "cn";
}

/*Each instantiation Person object, each object has a country

in the heap memory. After adding static, there is only one static country in the memory that can be directly called by other objects, which can save memory space.

When a member is statically modified, in addition to being called by the object, it can also be called directly by the class name. */

Special content is stored with objects

For example, in the person class, every Chinese person has his own name, and the nationality is the same Chinese, so contry can be defined as static and name Need not.

→static characteristics

1. Loading as the class is loaded

That is to say: static will disappear as the class disappears, indicating his Life cycleThe longest

2. Priority exists before the object

3. Shared by all objects

4. Can be called directly by the class name

→The difference between int age (member variable) instance variable and static int age (static member variable) class variable

1. Storage purpose

The instance variable is Exists in the heap memory as the object is created

Class variables exist in the method area as the class is loaded

2. Life cycle

Instance variables follow the object Disappear with the disappearance of

Class variables have the longest life cycle and disappear with the disappearance of the class

→Static useNotes

1. Static Methods can only access static members

2. This cannot be defined in static methods, and the super keyword

Because static precedes the existence of objects, this cannot appear in static methods

3. The main function is static

→The advantages and disadvantages of static

Benefits: Store the object’s data in a separate space, saving space, and there is no need to store it in each object A copy

can be called directly by the class name (Person.country)

Disadvantages: The life cycle is too long.

Access is limited. (Only static can be accessed)

→When to use static?

We need to start from two aspects

Because the content of static modification includes member variables and Member methods(function)

When to define static variables ( class variables)?

When shared data appears in the object, the data is modified statically

The unique data in the object must be defined to be non-static and exist in the heap memory.

When should we define static functions?

When the function does not access non-static data (object-specific data), the function can be defined as static.

→Static application

Every application has common functions

These functions can be extracted and independently packaged

for reuse.

Define all member methods as static and call them directly with the class name.

There will be a default constructor in a class. The permissions of this constructor are consistent with the class to which it belongs.

Static code block

Format:

static

{

Execution statements in static code blocks;

}

//Features: As the class is loaded And execution is only executed once. Used to initialize classes.

The following example:

class StaticCode
{
    static 
    {
        System,out.println("a");
    }
}

Loading sequence static code block-->Construction code block-->Constructor function

Object initialization process

Person p = new Person("zhangsan",20);

1. Because new uses Person.class. So the Person.class file will be found first and loaded into memory.

2. Execute the static code block (static code block) in the class. If there is one, initialize the Person.class class.

3. Open up space in the heap memory. Assign the memory address

4. Create the object’s uniqueproperties in the heap memory and perform default initialization

5. Initialize the properties for display

6. Initialize the object with the construction code block

7. Initialize the object with the corresponding constructor

8. Pay the memory address to the p variable in the stack memory

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