Why does the errors shows up when using filters in lambda expressions?

Question

I am new to lambda expressions. I have the below code:

List<String> someNumbers = Arrays.asList("N40", "N36", "B12", "B6", "G53", "G49", "G60", "G50", "G53", "I26", "I17", "I29", "O71");

someNumbers
            .stream()
            .filter(startsWith("G"))

In the above code, the 'filter' should act as a predicate and return a boolean value. But why does it show a compile error? I don't get an error when I use the below line:

.filter(s->s.startsWith("G"))

Above, the stream get passed to the filter. so what is the need for the argument s? for instance, '.map' processes it without any errors if used as

.map(String::toUppercase).

Answer 1

basic knowledge on method references I guess.

String::toUppercase

is equivalent to:

s -> s.toUppercase()

While:

startsWith("G")

would theoretically be equivalent to:

s -> s.startsWith("G")

This is simply not allowed by the language.