使用Java 8流时的新对象实例化 [英] New object instantiation when using Java 8 streams

问题描述

使用以下构造除了在后者中的可读性稍好以外，是否还有其他区别?

Is there a differnce in using the following contstructs, other than slightly better readability in the latter?

someList.stream().map(item -> new NewClass(item)).collect(Collectors.toList());

someList.stream().map(NewClass::new).collect(Collectors.toList());

推荐答案

通常没有区别. NewClass::new生成的字节码更少，因为在lambda版本中，java编译器从lambda主体创建了自动生成的私有方法，而NewClass:new直接链接到构造函数方法句柄.因此，使用方法引用时，类文件的大小可能会略少一些.不过，预计不会有明显的性能差异.

Generally there's no difference. NewClass::new produces less bytecode as in lambda version an auto-generated private method is created by java compiler from the lambda body while NewClass:new directly 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.

另一个区别是方法解析过程.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:

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:

public NewClass(String a) {...}
public NewClass(String a, String b) {...}

并且您有一些接受功能接口的方法:

And you have some method which accepts functional interface:

public myMethod(Function<String, NewClass> fn) {...}

然后您可以使用lambda或功能接口来调用它:

Then you can call it both with lambda or functional interface:

myMethod(str -> new NewClass(str));
myMethod(NewClass::new);

但是假设以后您添加一个新的名称相同的方法:

But suppose that later you add a new method with the same name like this:

public myMethod(BiFunction<String, String, NewClass> fn) {...}

然后，方法引用调用将变得模棱两可，并且会导致编译错误，因为NewClass::new现在可以同时匹配两个构造函数，而lambda仍是模棱两可的.

Then method reference call will become ambiguous and will result in compilation error as NewClass::new now matches to both constructors, while lambda is still unambiguous.

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