Generic Methods in Java: "Object does not take parameters"

Question

I want to create a generic method inside a class which takes an array of given type objects and sorts them to return a pair object containing the minimum array item and the maximum array item.

At the moment I get 3 compiler errors:

type Object does not take parameters: public <FirstType> Pair sortMinMax(Object<FirstType>[] array)
type Object does not take parameters: Object<FirstType> minimum = array[0]
type Object does not take parameters: Object<FirstType> maximum = array[0]

Here is my Class:

public class MinMaxArray {

  // takes an array of a given object and returns a pair 
  // of the min and max array elements
  public <FirstType> Pair sortMinMax(Object<FirstType>[] array) {
    Object<FirstType> minimum = array[0];
    Object<FirstType> maximum = array[0];

    // loop through all array items and perform sort
    for (int counter = 1; counter < array.length; counter++) {
      // if this element is less than current min, set as min
      // else if this element is greater than current max, set as max
      if (array[counter].compareTo(minimum) < 0) {
        minimum = array[counter];
      } else if (array[counter].comparTo(maximum) > 0) {
        maximum = array[counter];
      } // end if else new min, max
    } // end for (all array items)

    return new Pair<FirstType, FirstType>(minimum, maximum);
  } // end compare()

} // end MinMaxArray

java.lang.Object is not generic

Stefan Warminski
– Stefan Warminski

2017-03-29 10:21:39 +00:00
Commented Mar 29, 2017 at 10:21 — Stefan Warminski
– Stefan Warminski, Commented Mar 29, 2017 at 10:21

Tom Hawtin - tackline · Accepted Answer · 2017-03-29 10:30:14Z

3

Firstly, avoid reference arrays. They don't play well with generics. Use List instead.

It's not clear to me if you intend FirstType to be a type or a generic parameter (which would usually be given a single letter name).

If FirstType is a type

public static Pair<FirstType,FirstType> sortMinMax(List<FirstType> things) {

If you mean a generic parameter, look at java.util.Collections.sort.

public static <T extends Comparable<? super T>> void sort(List<T> list) {

So

public static <T extends Comparable<? super T>> Pair<T,T> sortMinMax(List<T> list) {

edited Mar 29, 2017 at 10:30

answered Mar 29, 2017 at 10:24

Tom Hawtin - tackline

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Comments

Haris Osmanagić · Accepted Answer · 2017-03-29 10:23:46Z

1

type Object does not take parameters

This is what tells you most. Simply, you can write things like Object<T>. The class Object is not parameterized. Lists for example, are.

Also, in order to use the compareTo() method, you need to have something like this:

public <FirstType extends Comparable> Pair sortMinMax(FirstType[] array)

answered Mar 29, 2017 at 10:23

Haris Osmanagić

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2 Comments

Tom Hawtin - tackline Over a year ago

Comparable and Pair are generic so will need arguments. The array is going to make things tricky.

Haris Osmanagić Over a year ago

Definitely true.

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