Sorting a generic array of elements in Java

Question

I have a generic class in my Java program, and the type can either be Double or Integer. I need to write a function that first needs to sort an array of values of the generic type. I am getting the following exception, and I am unsure how to fix it. I would appreciate any help.

Exception in thread "main" java.lang.NullPointerException
    at java.util.ComparableTimSort.countRunAndMakeAscending(ComparableTimSort.java:290)
    at java.util.ComparableTimSort.sort(ComparableTimSort.java:157)
    at java.util.ComparableTimSort.sort(ComparableTimSort.java:146)
    at java.util.Arrays.sort(Arrays.java:472)
    at datastructures.CircularBuffer.getMedian(CircularBuffer.java:98)
    at test.CircularBufferTests.test4(CircularBufferTests.java:67)
    at test.CircularBufferTests.main(CircularBufferTests.java:13)

Here is my code:

public class CircularBuffer<T extends Number> {

private T[] array;

public CircularBuffer(int n){
        array = (T[])new Number[n];
    }

public double getMedian(int size){


        ...
            Arrays.sort(array);
        ...

    }
}

Thanks!

An element of array is null. It cannot be. If you want to include null write a custom Comparator to handle it. — Boris the Spider
– Boris the Spider, Commented Sep 18, 2014 at 16:57
@AmirAfghani of ComparableTimSort.countRunAndMakeAscending? What's that going to tell the OP? — Boris the Spider
– Boris the Spider, Commented Sep 19, 2014 at 7:18

crazyPixel · Accepted Answer · 2014-09-18 17:04:27Z

1

Well obiviously there is not enough code here to understand but as for the ComparableTimSort (where the exception is actually comes from) class where you pass your T to, Not all the T types you pass (Number class) implement the Comparable interface in order to check if that is your problem try and pass you T with a Comparable cast

answered Sep 18, 2014 at 17:04

crazyPixel

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AlexITC · Accepted Answer · 2014-09-18 17:05:33Z

1

You have to write a custom Comparator in order to sort using Arrays.sort().

Arrays.sort(array, yourComparator);

answered Sep 18, 2014 at 17:05

AlexITC

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ChoChoPK · Accepted Answer · 2014-09-18 18:30:31Z

1

You can do

What vhax suggested. More specifically

Arrays.sort(array, new Comparator<T>{
    @Override
    public int compare(T arg0, T arg1)
    {
        ...
    }
});

What crazyPixel suggeseted, more spefically

public class CircularBuffer<T extends Number & Comparable<T>>

This way, you are restricting on T to be more specific. This is called "multiple bounds" http://docs.oracle.com/javase/tutorial/java/generics/bounded.html

answered Sep 18, 2014 at 18:30

ChoChoPK

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1 Comment

StormeHawke Over a year ago

When I first reviewed this I didn't understand why you were copying two different answers into a new answer (generally frowned on). Then I checked and you were actually providing significant additional information so you get a +1

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