I found that function interface and getMethod seem to be replaceable, What makes it work?
public class App {
public static void main(String[] args) {
Map<String, String> collect = Stream.of(new App(), new App(), new App())
.collect(Collectors.toMap(App::getString, (app) -> "aaa"));
}
public String getString() {
return "str";
}
}
But when I use lambda to replace getMethod, it fails . Why this does not work
Map<String, String> collect = Stream.of(new App(), new App(), new App())
.collect(Collectors.toMap(() -> "str", (app) -> "aaa"));
Collectors.toMap requires a Function<? super T, ? extends K> as its first parameter, where T is the type of elements in the stream, and K is the key type of the map you want.
In this case, you have a stream of Apps and you want a Map<String, String>, so T is App and K is String. In other words, you need a function that accepts an App, and returns a String.
App::getString is such a Function<? super T, ? extends K>. You might be wondering why it accepts a App when getString accepts no parameters. Notice how getString is an instance method, and you are referring to it without an instance! A method reference of the form ClassName::instanceMethodName implicitly accepts an extra parameter of type ClassName, because you need an instance of that class to call it!
On the other hand, your lambda is not such a function. It accepts no parameters, as indicated by the empty brackets at the start (()). Your lambda expression would be represented by the Supplier<String> functional interface, not the Function<App, String> that you need.
To use a lambda expression here, simply do what you did to the second parameter of toMap, and add a lambda parameter:
.collect(Collectors.toMap((app) -> "str", (app) -> "aaa"));
// ^^^
Note that this is required even if you don't use app in the lambda expression.
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