Java 8 get a field of object when another field is minimum from a list of objects

Question

I have list of objects objList which has two fields age and id. age is long and id is String. I want to iterate over the list and get the id of object that has minimum age in the list. I can do it with a traditional for loop, but I am sure there would be a compact solution in java 8.

If I do objList.stream() and then check map functions on that stream, I just have mapToInt, mapToLong, and mapToDouble. With these I get the min age, but I need id of the object that has minimum age.

Its guaranteed that the list will not be empty and will have at least one object.

Ohh in my case its guaranteed that the list will not be empty. Will add that in the description. — krackoder
– krackoder, Commented Jun 23, 2017 at 0:26

erickson · Accepted Answer · 2017-06-23 00:28:00Z

5

Assuming the elements in the list are of type MyObject, and offer accessors getAge() and getId(), the pipeline would look like this:

String id = objList.stream()
  .min(Comparator.comparingLong(MyObject::getAge))
  .map(MyObject::getId)
  .orElseThrow(IllegalArgumentException::new);

If your assumption is invalid, and objList is empty, this will throw an IllegalArgumentException—but that will "never happen," right? Alternatively, you could return a default value or generate one.

answered Jun 23, 2017 at 0:28

erickson

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shmosel · Accepted Answer · 2017-06-23 00:27:37Z

3

objList.stream()
        .min(Comparator.comparingLong(MyObj::getAge))
        .orElseThrow(AssertionError::new)
        .getId()

answered Jun 23, 2017 at 0:27

shmosel

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2 Comments

krackoder Over a year ago

The min function returns Optional<T> right? So in this case it would return Optional<MyObj>. The optional will be empty if the list is empty.

Holger Over a year ago

Or without streams: Collections.min(objList, Comparator.comparingLong(MyObj::getAge)) .getId()

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