How to find the minimum and maximum in a ArrayList<Entry> using Java

Question

I'm trying to find the minimum and the maximum of a

ArrayList<Entry>

For example my ArrayList looks like this:

ArrayList<Entry> test = new ArrayList<Entry>();
test.add(new Entry(20, 0));
test.add(new Entry(5, 0));
test.add(new Entry(15, 0));

now I want the minimum(5) and the maximum(20) of this list.

I tried it with:

Collections.min(test);

But it says:

Bound mismatch: The generic method min(Collection<? extends T>) of type Collections is not applicable for the arguments (ArrayList<Entry>). The inferred type Entry is not a valid substitute for the bounded parameter <T extends Object & Comparable<? super T>>

I also tried:

test.length()

so I could do a for loop. But it also failed with this kind of ArrayList.

Use test.size() instead of test.length(). And don't hesitate to read docs — AlexW
– AlexW, Commented Nov 25, 2015 at 11:31

AlexW · Accepted Answer · 2015-11-25 11:55:39Z

1

First, define a Comparator<Entry> which defines an ordering for Entry:

class EntryComparator implements Comparator<Entry> {
  @Override public int compare(Entry a, Entry b) {
    // ... whatever logic to compare entries.
    // Must return a negative number if a is "less than" b
    // Must return zero if a is "equal to" b
    // Must return a positive number if a is "greater than" b
  }
}

Then just iterate through the list, comparing each element to the current minimum and maximum elements:

Comparator<Entry> comparator = new EntryComparator();
Iterator<Entry> it = list.iterator();
Entry min, max;
// Assumes that the list is not empty
// (in which case min and max aren't defined anyway).

// Any element in the list is an upper bound on the min
// and a lower bound on the max.
min = max = it.next();

// Go through all of the other elements...
while (it.hasNext()) {
  Entry next = it.next();
  if (comparator.compare(next, min) < 0) {
    // Next is "less than" the current min, so take it as the new min.
    min = next;
  }
  if (comparator.compare(next, max) > 0) {
    // Next is "greater than" the current max, so take it as the new max.
    max = next;
  }
}

edited Nov 25, 2015 at 11:55

AlexW

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answered Nov 25, 2015 at 11:17

Andy Turner

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4 Comments

AlexW Over a year ago

What's the point of creating a custom comparator and explicit iterating through collection and calling it? At first, you're having tough time trying not to mess all these -1, 0 and 1, and then... you're doing a reverse conversion from -1,0,1 to "smaller" or "larger" relations. Isn't it better to just call Collections.max(list, comparator) and Collections.max(list, comparator)?

Andy Turner Over a year ago

@VladimirS. (I'm guessing you mean min and max) Well, sure you can. This solution is a) more flexible, in that it works on Iterable as well as Collection; b) potentially more efficient since it only iterates once; c) more illustrative, since it shows how to actually implement it without using library methods. <shrug> YMMV :) You are right that using Collections methods would be "better", if conciseness is your metric.

roux. Over a year ago

I tried to implement the Class EntryComparator but I can't use the less than and greater than operators in combination with a Entry.

public class  EntryComparator implements Comparator<Entry> {     @Override     public int compare(Entry a, Entry b) { 		 		if(a == b){				 			return 0; 		} 		else if(a < b){ 			return -1; 		} 		else if(a > b){ 			return 1; 		} 		else{ 			return 0; 		}	  	  	  } 	}

Andy Turner Over a year ago

@roux. You can only compare primitive values with <, > etc. If you could write comparisons like this, you'd have no need for the comparator :) Your code should look more like if (a.getField() == b.getField()) { return 0; } ... Please add the definition of Entry to the question, then I can suggest an implementation.

Blackbelt · Accepted Answer · 2015-11-25 11:12:45Z

0

Entry has to implement the Comparator interface and provide an implementation for compare(T o1, T o2).

compare returns 0 if o1 and o2 are equals, a positive value if o1 is less then o2, a negative value otherwise

answered Nov 25, 2015 at 11:12

Blackbelt

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1 Comment

Andy Turner Over a year ago

It is not usual for a class to implement Comparator for itself; it might implement Comparable, however, if there is a natural ordering for elements of that class.

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