Why does invokeLater execute in the main thread?

To me it seems like a misunderstanding on your side

The first line is like saying: "Ok, Swing, what I want you to invokeLater is someMethod().toString()". So Swing executes it

The second line is like saying: "Ok, Swing, what I want you to invokeLater is the method toString() of the object returned by the method someMethod()". A someMethod() method that I am executing right now

So the result is completely logical to me

Just keep in mind that before evaluating a function (in this case invokeLater) Java needs to evaluate all arguments. So in the first case Java evaluate a lambda function (no need to execute it) and in the second case it encounters a method invocation so it needs to execute it

This is not related to Swing, it's what happens when using method references and lambdas behind the scenes.

A simpler example:

public static void main(String[] args) {    Stream.of(1, 2, 3).map(initMapper()::inc);    Stream.of(1, 2, 3).map(x -> initMapper().inc(x));}private static Mapper initMapper() {    System.out.println("init");    return new Mapper();}static class Mapper {    public int inc(int x) {        return x + 1;    }}

You will get a single init output here; notice that there is no terminal operation for the stream.