Java 8: pass method reference to another method

Question

I make myself familiar with Java 8's so called method references.

Two questions:

1) I want to println uppercased values. How can I pass the result of String::toUpperCase to println? For example this block of code doesn't compile:

List<String> food = Arrays.asList("apple", "banana", "mango", "orange", "ice");
food.forEach(System.out.println(String::toUpperCase));

2) Is there something similar to anonymous function parameters (_) like is Scala?

• Thorn G: What do you mean, "the result of a method reference?" println is a void method, so obviously you can't pass its result to forEach.
• Konstantin Milyutin: I want to print uppercased values. So I want to combine toUpperCase with println
• Konstantin Milyutin: "Nice" to get a vote down without explanation.

Answer 1

What you want to do is to combine a function with a consumer.

You can do it this way:

food.stream().map(String::toUpperCase).forEach(System.out::println);

Alternatively you can use a lambda expression:

food.forEach(x->System.out.println(x.toUpperCase()));

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Besides these straight-forward approaches, you can combine functions to create a new function but not a function with a consumer, however, with the following quirky code you can do it:

Function<String,String> f0=String::toUpperCase;
food.forEach(f0.andThen(" "::concat).andThen(System.out::append)::apply);

This get even uglier if you try to inline the first expression for a one-liner...

• Holger: The Stream is the way how to combine arbitrary operations.
• Holger: Well, _ is a reserved keyword and might be a placeholder for parameters you don’t want to use in a future Java version, however, that doesn’t apply here as here the parameters are used.