Java static domain

If you define a domainStatic, Then eachClass has only oneSuch a domain. For comparison, each object has its own copy for all instance domains.

For example, assume that each employee must be assigned a unique ID code. Here we add an instance domain id and a static domain nextId to the Employee class:

class Employee{    ......    private int id;    private static int nextId = 1;}

Now, each employee object has its own id domain, but all instances of this class will share a nextId domain. In other words, if there are 1000 Employee class objects, there are 1000 instance domain IDs. If any instance uses the setId () method, the static fields of the Employee class are changed to the set values. However, there is only one static domain nextId. Even if there is no employee object, the static domain nextId also exists. It belongs to the classAnd does not belong to any independent object. It can also be proved that the static domain is called directly through the class name and the instance domain is called through the (class) instance.

Let's take a look.Static Method:

Static methods cannot perform operations on objects.

For example, Math. pow (x, a) does not use any Math object During computation. In other words, there is no implicit parameter (this ).

It can be considered that the static method does not have the this parameter. (In a non-static method, this parameter indicates the implicit parameter of the method .) For comparison: non-static method call (this.) pow ();

Because static methods cannot operate on objects, they cannot access instance domains in static methods. However, static methods can be accessed.Static fields in its own class.

public static int getNextId(){    return nextId;//return static field}

Call this method using the Class Name: int n = Employee. getNextId ();

Note: The static domain cannot directly access the instance domain (because it is related to the class rather than an object ), however, the instance domain can directly access the static domain (because the instance domain is derived from the class ).

If harry is an Employee object, use harry. getNextId () to replace Employee. getNextId (). However, this method is confusing. BecauseThe calculation result of the getNextId method is irrelevant to harry.. We recommend that you use a class name instead of an object to call static methods.

Use static methods in the following two cases:

1.When a method does not need to access the object state, all the required parameters are provided explicitly (such as Math. pow ).

2.When a method only needs static fields of the category class (for example, Employee. getNextId ).