println with :: vs .

[jade moss]

Quick question about printing in Java. I'm watching this Spring Boot Tutorial and this guy did String.out::println where I was taught it was String.out.println. Difference is the :: vs . before the println. How did that :: work and when can I use this vs the other?

[vague finch]

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[jade moss]

Ping me when you reply. Gonna continue watching the Youtube Tutorial. Vid link for reference: https://youtu.be/UgX5lgv4uVM?si=49zi_RgMWt7_hyVr&t=2537

[golden schooner]

System.out.println() is a method call - it calls the println method which prints something

System.out::println is a method reference

it is equivalent to something -> System.out.println(something)

[jade moss]

Ok that's interesting

So its like map function in Python?

No, the forEach is the map() like function.

Just that to call the method reference is inside it

[golden schooner]

it's a higher order function,. yeah

you are passing the method System.out.println() to forEach

[jade moss]

But instead of System.out.println() I would use System.out::println, right?

[golden schooner]

you can do the following

Consumer<String> p = System.out::println;


p.accept("Hi");//This calls the System.out.println() since p references that

forEach expects a Consumer which it then calls multiple times

[jade moss]

If I made a class with a static method like World.greet() where it takes a string as arg to print out Hello {name}. Then I could use this in the forEach as World::greet?

[golden schooner]

see https://dev.java/learn/lambdas/

and https://dev.java/learn/lambdas/method-references/ specifically

[golden schooner]

yes

then it would print Hello <first string>, Hello <second string> etc

[jade moss]

Ah then I understood that correctly.

[golden schooner]

with lambdas, you can essentially store a method/code in a variable (or parameter)

but https://dev.java/learn/lambdas/ explains the topic in more detail

[jade moss]

Yeah I'm familiar with lambda expressions. They're like anonymous functions in Javascript and similar in Python

[jade moss]

That makes sense

[fresh summit]

In Java they're technically implementations of an interface, but yes

[golden schooner]

passing an implementation of an interface is essentially equivalent to passing the code

the difference is that it extends Object xd

[jade moss]

I find this to be more explicitly readable

IntBinaryOperator max = (a, b) -> Integer.max(a, b);

Compared to this

IntBinaryOperator max = Integer::max;

Is there a preference in Java community on Lambda expressions?

[fresh summit]

I use the method if it exists

Code that I didn't write, is less error prone in general, so I prefer "not my code"

[jade moss]

Yeah I guess. Same as how I would do it in Python.

[fresh summit]

In general, if it exists in the standard library I use it

[golden schooner]

it's a matter of preference

I would personally also use method references

Also there's a special edge case with generic checked exceptions where it's possible to do something with method reference that isn't possible with lambdas

[fresh summit]

sneakyThrow? Or something else still?

[jade moss]

Alright thanks all

[vague finch]

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