There is no runtime impact, as compiler automatically replaces the * with concrete class names. If you decompile the .class file, you would never see import ...*. C# always uses * (implicitly) as you can only using package name. You can never specify the class name at all. Java introduces the feature after c#. (Java is so tricky in many aspects but it's beyond this topic). In Intellij Idea when you do "organize imports", it automatically replaces multiple imports of the same package with *. This is a mandantory feature as you can not turn it off (though you can increase the threshold). The case listed by the accepted reply is not valid. Without * you still got the same issue. You need specify the pakcage name in your code no matter you use * or not.

请参阅我的文章“按需进口是邪恶的”

简而言之，最大的问题是当一个类被添加到您导入的包中时，您的代码可能会中断。例如:

import java.awt.*;
import java.util.*;

// ...

List list;

在Java 1.1中，这很好;在java中找到列表。啊，没有冲突。

现在假设您检入了运行良好的代码，一年后，其他人使用Java 1.2编辑了它。

Java 1.2在Java .util中添加了一个名为List的接口。繁荣!冲突。完美工作的代码不再工作。

这是一个邪恶的语言特性。没有理由仅仅因为一个类型被添加到包中就停止编译……

此外，它使读者难以确定您使用的是哪个“Foo”。

It helps to identify classname conflicts: two classes in different packages that have the same name. This can be masked with the * import. It makes dependencies explicit, so that anyone who has to read your code later knows what you meant to import and what you didn't mean to import. It can make some compilation faster because the compiler doesn't have to search the whole package to identify depdencies, though this is usually not a huge deal with modern compilers. The inconvenient aspects of explicit imports are minimized with modern IDEs. Most IDEs allow you to collapse the import section so it's not in the way, automatically populate imports when needed, and automatically identify unused imports to help clean them up.

我工作过的大多数使用大量Java的地方都将显式导入作为编码标准的一部分。我有时仍然使用*来快速创建原型，然后在产品化代码时展开导入列表(一些ide也会为您这样做)。

忘掉混乱的命名空间……想想那些不得不在GitHub、vi、notepad++或其他非ide文本编辑器中阅读和理解你的代码的可怜人吧。

这个人必须煞费苦心地查找每个通配符作用域中所有类和引用中来自一个通配符的每个标记……只是想搞清楚到底发生了什么。

如果你只是为编译器编写代码——而且你知道你在做什么——我相信通配符没有问题。

但是，如果其他人(包括未来的您)希望一次阅读就能快速理解某个特定的代码文件，那么显式引用会有很大帮助。

在双方提出的所有有效观点中，我还没有找到避免通配符的主要原因:我喜欢能够阅读代码并直接知道每个类是什么，或者如果它的定义不在语言或文件中，那么在哪里可以找到它。如果用*导入了多个包，我必须搜索它们中的每一个，以找到我不认识的类。可读性是至高无上的，我同意代码不应该需要IDE来读取。

以下是我关于这个话题的一些发现。

During compilation, the compiler tries to find classes that are used in the code from the .* import and the corresponding byte code will be generated by selecting the used classes from .* import. So the byte code of using .* import or .class names import will be same and the runtime performance will also be the same because of the same byte code. In each compilation, the compiler has to scan all the classes of .* package to match the classes that are actually used in the code. So, code with .* import takes more time during the compilation process as compared to using .class name imports. Using .* import helps to make code more cleaner Using .* import can create ambiguity when we use two classes of the same name from two different packages. Eg, Date is available in both packages. import java.util.*; import java.sql.*; public class DateDemo { private Date utilDate; private Date sqlDate; }