Java 8 streams “ifPresent”

Question

I am trying to learn about streams and encountered a problem: I want to get the minimal value of a list and assign it to an int variable. For that I did the following:

List<Integer> list = new ArrayList<>();
    list.add(1);
    list.add(2);
    list.add(3);
    int smallest = list.stream().min(Integer::compareTo).get();
    System.out.println(smallest);

This works well and i get 1 as a result. The issue is that the IDE gives the warning that Optional.get is called before checking for .isPresent . To fix that i used the slightly different ifPresent method and tried the following:

int smallest = list.stream().min(Integer::compareTo).ifPresent(integer -> integer);

Unfortunately this doesn't work since I get the warning: Bad return type in Lambda, Integer cannot be converted to void. My question finally is: How can I assign the min value to the int smallest variable WITH checking ifPresent?

Answer 1

Here's how I'd do it:

package lambdas;

import java.util.ArrayList;
import java.util.List;

/**
 * Created by Michael
 * Creation date 7/31/2016.
 * @link https://stackoverflow.com/questions/38688119/java-8-streams-ifpresent
 */
public class MinimumExample {

    public static void main(String[] args) {
        List<Integer> list = new ArrayList<>();
        int smallest = list.stream().min(Integer::compareTo).orElse(Integer.MIN_VALUE);
        System.out.println(smallest);
    }
}

Answer 2

Stream#min return Optional of result because in general stream can be empty so there can be no minimal value.

 Optional<Integer> minimal = list.stream().min(Integer::compareTo);

To get value from Optional you need to have some fallback value

Integer absent = Integer.MIN_VALUE;

The easiest would be to use orElse

Integer smallest = minimal.orElse(absent);

Little bit longer would be isPresent

Integer smallest = minimal.isPresent() ? minimal.get() : absent;

Answer 3

The ifPresent method takes Consumer<? super T> Consumer<? super T> as a parameter. Simply speaking, it should be an action without return statement. You could print the value if it's present, like

[...].ifPresent(System.out::print);

But it is not what about IDEA says. I think, you simply need to save an Option<Integer> instance and then check it by isPresent :

Optional<Integer> o = list.stream().min(Integer::compareTo);
if (o.isPresent()) {
    smallest = o.get();
}

Of course, there are more convenient ways with orElse :

smallest = o.orElse(Integer.MIN_VALUE);

or with the ternary operator:

smallest = o.isPresent() ? o.get() : Integer.MIN_VALUE;