Methods on the parameter, and constructors¶
☕ Java 17 · Passing Methods Around by Name · 2/3
Shapes 1 and 2 call a static method or a method on an object you already have. The last two shapes work differently.
Shape 3 — The parameter becomes the object¶
interface TitleTester {
boolean test(String title);
}
TitleTester blank = String::isEmpty; // t -> t.isEmpty()
blank.test(""); // true
isEmpty isn't static: you call it on a specific String. String::isEmpty means call isEmpty on whatever String arrives. A class name on the left doesn't mean the method is static.
With several parameters:
interface PrefixCheck {
boolean check(String title, String prefix);
}
PrefixCheck pc = String::startsWith; // (t, p) -> t.startsWith(p)
pc.check("Narcos", "Nar"); // true
- The first parameter is the object the method runs on.
- The other parameters are passed, in order, as arguments.
Shape 4 — A constructor¶
record Episode(String name) { }
interface EpisodeMaker {
Episode make(String name);
}
EpisodeMaker maker = Episode::new; // name -> new Episode(name)
Episode pilot = maker.make("Pilot"); // Episode[name=Pilot]
::new points at a constructor. There are no parentheses: Java matches the interface's parameters to a constructor, here the one taking a String. Each call to make() creates a new object.
The four shapes side by side¶
Math::round // p -> Math.round(p) 1 · static method
title::startsWith // s -> title.startsWith(s) 2 · method on an object
String::isEmpty // t -> t.isEmpty() 3 · method on the parameter
String::startsWith // (t, p) -> t.startsWith(p) 3 · method on the parameter
Episode::new // n -> new Episode(n) 4 · constructor
How to read a reference:
| Left side | Method | Shape |
|---|---|---|
| a class | static | 1: the parameters become the arguments |
| a variable | any instance method | 2: it always runs on that object |
| a class | instance method | 3: the first parameter becomes the object |
| a class | new |
4: a constructor |
When unsure, rewrite the reference as a lambda in your head: the parameter order becomes obvious, and so does whether the counts line up.
The lab uses three shapes at once to fill, filter and print a movie catalog.