使用 Java 8 流时的新对象实例化
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New object instantiation when using Java 8 streams
提问by ShellDragon
Is there a differnce in using the following contstructs, other than slightly better readability in the latter?
除了后者的可读性稍好之外，使用以下结构是否有区别？
someList.stream().map(item -> new NewClass(item)).collect(Collectors.toList());
someList.stream().map(NewClass::new).collect(Collectors.toList());
采纳答案by Tagir Valeev
Generally there's no difference. NewClass::newproduces less bytecode as in lambda version an auto-generated private method is created by java compiler from the lambda body while NewClass:newdirectly links to the constructor method handle. So using method references you may have slightly less class file size. No significant performance difference is expected though.
一般没有什么区别。NewClass::new产生更少的字节码，因为在 lambda 版本中，自动生成的私有方法由 java 编译器从 lambda 主体创建，同时NewClass:new直接链接到构造函数方法句柄。因此，使用方法引用，您的类文件大小可能会稍小一些。不过，预计不会有显着的性能差异。
Another difference is method resolution procedure. It's not applicable in your particular example, but may be applicable in other code. For example, you have two constructors:
另一个区别是方法解析程序。它不适用于您的特定示例，但可能适用于其他代码。例如，您有两个构造函数：
public NewClass(String a) {...}
public NewClass(String a, String b) {...}
And you have some method which accepts functional interface:
你有一些接受功能接口的方法：
public myMethod(Function<String, NewClass> fn) {...}
Then you can call it both with lambda or functional interface:
然后你可以用 lambda 或函数接口调用它：
myMethod(str -> new NewClass(str));
myMethod(NewClass::new);
But suppose that later you add a new method with the same name like this:
但是假设稍后您添加了一个具有相同名称的新方法，如下所示：
public myMethod(BiFunction<String, String, NewClass> fn) {...}
Then method reference call will become ambiguous and will result in compilation error as NewClass::newnow matches to both constructors, while lambda is still unambiguous.
然后方法引用调用将变得不明确，并且会导致编译错误，因为NewClass::new现在与两个构造函数都匹配，而 lambda 仍然是明确的。
