What does the “static” modifier after “import” mea

问题:

When used like this:

import static com.showboy.Myclass;

public class Anotherclass{}

what\'s the difference between import static com.showboy.Myclass and import com.showboy.Myclass?

回答1:

See Documentation

The static import declaration is analogous to the normal import declaration. Where the normal import declaration imports classes from packages, allowing them to be used without package qualification, the static import declaration imports static members from classes, allowing them to be used without class qualification.

So when should you use static import? Very sparingly! Only use it when you\'d otherwise be tempted to declare local copies of constants, or to abuse inheritance (the Constant Interface Antipattern). In other words, use it when you require frequent access to static members from one or two classes. If you overuse the static import feature, it can make your program unreadable and unmaintainable, polluting its namespace with all the static members you import. Readers of your code (including you, a few months after you wrote it) will not know which class a static member comes from. Importing all of the static members from a class can be particularly harmful to readability; if you need only one or two members, import them individually. Used appropriately, static import can make your program more readable, by removing the boilerplate of repetition of class names.

回答2:

There is no difference between those two imports you state. You can, however, use the static import to allow unqualified access to static members of other classes. Where I used to have to do this:

import org.apache.commons.lang.StringUtils;
      .
      .
      .
if (StringUtils.isBlank(aString)) {
      .
      .
      .

I can do this:

import static org.apache.commons.lang.StringUtils.isBlank;
      .
      .
      .
if (isBlank(aString)) {
      .
      .
      .

回答3:

Static import is used to import static fields / method of a class instead of:

package test;

import org.example.Foo;

class A {

 B b = Foo.B_INSTANCE;

}

You can write :

package test;

import static org.example.Foo.B_INSTANCE;

class A {

 B b = B_INSTANCE;

}

It is useful if you are often used a constant from another class in your code and if the static import is not ambiguous.

Btw, in your example \"import static org.example.Myclass;\" won\'t work : import is for class, import static is for static members of a class.

回答4:

The basic idea of static import is that whenever you are using a static class,a static variable or an enum,you can import them and save yourself from some typing.

I will elaborate my point with example.

import java.lang.Math;

class WithoutStaticImports {

 public static void main(String [] args) {
  System.out.println(\"round \" + Math.round(1032.897));
  System.out.println(\"min \" + Math.min(60,102));
 }
}

Same code, with static imports:

import static java.lang.System.out;
import static java.lang.Math.*;

class WithStaticImports {
  public static void main(String [] args) {
    out.println(\"round \" + round(1032.897));
    out.println(\"min \" + min(60,102));
  }
}

Note: static import can make your code confusing to read.

回答5:

the difference between \"import static com.showboy.Myclass\" and \"import com.showboy.Myclass\"?

The first should generate a compiler error since the static import only works for importing fields or member types. (assuming MyClass is not an inner class or member from showboy)

I think you meant

import static com.showboy.MyClass.*;

which makes all static fields and members from MyClass available in the actual compilation unit without having to qualify them... as explained above

回答6:

The import allows the java programmer to access classes of a package without package qualification.

The static import feature allows to access the static members of a class without the class qualification.

The import provides accessibility to classes and interface whereas static import provides accessibility to static members of the class.

Example :

With import

import java.lang.System.*;    
class StaticImportExample{  
    public static void main(String args[]){  

       System.out.println(\"Hello\");
       System.out.println(\"Java\");  

  }   
} 

With static import

import static java.lang.System.*;    
class StaticImportExample{  
  public static void main(String args[]){  

   out.println(\"Hello\");//Now no need of System.out  
   out.println(\"Java\");  

 }   
} 

See also : What is static import in Java 5

回答7:

Say you have static fields and methods inside a class called MyClass inside a package called myPackage and you want to access them directly by typing myStaticField or myStaticMethod without typing each time MyClass.myStaticField or MyClass.myStaticMethod.

Note : you need to do an import myPackage.MyClass or myPackage.* for accessing the other resources

回答8:

The static modifier after import is for retrieving/using static fields of a class. One area in which I use import static is for retrieving constants from a class. We can also apply import static on static methods. Make sure to type import static because static import is wrong.

What is static import in Java - JavaRevisited - A very good resource to know more about import static.