Introduction to final and static keywords in Java

1. final

The final class cannot be inherited. There is no subclass. The methods in the final class are final by default

The final method cannot be overridden by the method of the subclass, but it can be inherited

final member variables Represents a constant, which can only be assigned once and cannot be changed after assignment.

final cannot be used to modify constructors.

private cannot be overridden by subclass methods. Private type methods are final by default.

There are three types of variables modified by final. : Static variables, instance variables and local variables represent three types of constants respectively.

Note: When defining a final variable, you can declare it first without giving an initial value. This variable is also called final blank. No matter what the situation, the compiler ensures that the blank final must be initialized before use.

2. static

Static means "global" or "static". It is used to modify member variables and member methods, and can also form static static code blocks, but there are no global variables in the Java language. the concept of.

                                                                                                                                                                                          belonging out out to be statically modified member variables and member methods independent of any object of the class. That is, it does not depend on a specific instance of the class and is shared by all instances of the class. As long as this class is loaded, the Java virtual machine can find them by default in the method area of ​​the runtime data area based on the class name. Therefore, a static object can be accessed before any of its objects are created, without referencing any objects.

                                                                                                                                                                                                                          的 ‑ 五月 东西 [ this point is very important. In fact, you need to understand that private means access permission restriction, and static means it can be used without instantiation, which is much easier to understand. The effect of adding other access permission keywords before static is also the same. C i Static modified member variables and member methods are used to static variables and static methods. You can directly access it through the class name. The access grammar is:

class name. Static method name (parameter list ...) Class name. Static variable name

          The code block modified with static represents a static code block. When the Java Virtual Machine (JVM) loads the class, the code block will be executed.

1. Static variables

Class member variables can be classified into two types according to whether they are static: one is a variable modified by static, called a static variable or a class variable; the other is a variable that is not modified by static, called a variable. Instance variables. The difference between the two is:

There is only one copy of the static variable in the memory (to save memory). The JVM only allocates memory once for the static variable. The memory allocation of the static variable is completed during the class loading process and can be directly accessed by the class name (convenient) , of course it can also be accessed through objects (but this is not recommended).没 For instance variables, without creating an instance, it will allocate a memory for instance variables. The instance variables can be copied multiple in memory, which does not affect each other (flexible).态 静2, static method

Static method can be called directly through class names, and any instance can be called. Therefore, the keywords of This and Super in the static method cannot be used. Member variables and member methods with static) can only access static member variables and member methods of the class to which they belong. Because instance members are associated with specific objects! This needs to be understood and understood, not memorized! ! !
Because static methods are independent of any instance, static methods must be implemented and cannot be abstract. At t t3, Static code block
Static code block is also called static code blocks. It is a Static sentence block independent in class members in the class. It can be placed in the position. It is not in any method. These static code blocks will be executed. If there are multiple static code blocks, the JVM will execute them in the order in which they appear in the class. Each code block will only be executed once. At4, Static, and FINAL are used to indicate that Static Final is used to modify member variables and membership methods, which can be simply understood as "global constant"!
​​​​For variables, it means that once a value is given, it cannot be modified and can be accessed through the class name.

For methods, it means they cannot be overridden and can be accessed directly through the class name.

                                                                                                                                                                               For instance constants modified by static and final, the instance itself cannot be changed, but for instance variables of some container types (such as ArrayList, HashMap), the container variable itself cannot be changed. But the objects stored in the container can be modified, which is used a lot in programming.

　The object "installed" in the container type variable modified by static final can be changed.

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