CODEWITHSUNDEEP
java
Shirohige
Shirohige

Reputation: 1

Using the Collections.max for a Period (java)

So I have a List<Period> periods = new Arraylist<>(); which stores my dates which it gets from the database. (Period being my own class, forgot to mention)

The List kinda looks like this: 2012-02-03, 2012-02-04, 2012-03-05, 2012-03-07, 2012-03-08, 2012-03-09, 2012-03-10 and I need to find the biggest period out of the list. So the outcome would have to be period: 2012-03-07 to 2012-03-10.

Now I thought I would use the collection method like this: Period biggestperiod = Collections.max(periods); except it doesnt work and I get the next error: "max(java.util.Collection) in Collections cannot be applied to (java.util.List) reason; no instance of type variables T exist so that Period conforms to Comparable .

Im a complete noob when it comes to programming so could someone point me in the right direction?

Upvotes: 0

Views: 333

Answers (1)

maio290
maio290

Reputation: 6742

I'm guessing Period is one of your own classes, if it's Java's Period, you have to use Collections.max with the additional Comparator you cannot use the Collections.max function without it. To use the Collections.max method without an additional Comparator, Period has to implement Comparable - java.time.Period doesn't.

See https://docs.oracle.com/javase/7/docs/api/java/util/Collections.html#max(java.util.Collection) for reference and https://docs.oracle.com/javase/8/docs/api/java/time/Period.html for the Period class.

I just wrote a simple example of how this works:

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

public class Number implements Comparable
{
public int number;

public Number(int number)
{
    this.number = number;
}

@Override
public int compareTo(Object arg0)
{
    if (number > ((Number) arg0).number) {
        return 1;
    }
    if (number == ((Number) arg0).number) {
        return 0;
    }
    if (number < ((Number) arg0).number) {
        return -1;
    }
    return -2;

}

public static void main(String[] args)
{
    Number one = new Number(1);
    Number three = new Number(3);
    Number two = new Number(2);

    List<Number> numberList = new ArrayList<>();
    numberList.add(one);
    numberList.add(three);
    numberList.add(two);

    System.err.println(Collections.max(numberList).number);
}
}

Upvotes: 1

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