What is the purpose of * in java.io.*

Question

What is the purpose of the * in java.io.*?

import java.io.*;
class Trial{
     public static void main(String[]args){
         System.out.println("Hello,World!");
     }
}

Answer 1

The * tells the compiler to import all top-level classes in java.io on demand. The construct is called a type-import-on-demand declaration.

From JLS §7.5.2 :

A type-import-on-demand declaration allows all accessible types of a named package or type to be imported as needed.

\nTypeImportOnDemandDeclaration: \n    import PackageOrTypeName .  * ; \n

So, for example, since you've included that import statement, you can use a class like java.io.File without having to prefix the type name with java.io ; you can use the simple name File .

Answer 2

星号表示应该导入java.io包中的所有类。

Answer 3

A wildcard in a package name import is used to include all the classes contained in that specific package. Check official documentation .

In addition you are able to import inner static classes to be able to refer to them without a fully qualified name, eg:

import org.package.MyClass;

//MyClass.InnerClass inner; not needed
InnerClass inner;