Java question / bad operand types for binary operator <

Can someone please help point out why I keep getting "bad operand types for binary operator <" ?
The method takes in Lists of any numeric inputs for some operations.
I am comparing two <? extends Number> objects here, and I guess error arises as not all Number subclasses impletement Comparable, so comparison cant be made. How can I solve the issue?

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public static void remerge(List<? extends Number> A1, List<? extends Number> A2, List<? extends Number> A3){
int A1index = 0, A2index = 0, A3index = 0;
while (A1index < A1.size() && A2index < A2.size()){ //while neither subarrays are exhausted
if ( (A1.get(A1index)) < (A2.get(A2index)) ){ //error occurs
A3.set(A3index,A1.get(A1index));

 

Can you show an example of how those lists A1 and A2 look like?

 

> can only be applied to primitive numeric types.
For comparables use .compareTo()

 

It is a merge sort method, below for Integer, trying to make it generic

------------------------------------------
public class MergeSort{ //sort a List or array in ascending order by merge sort
public static void mergeSort(List<Integer> A){
if (A.size() > 1){ //stopping case
List<Integer> A1 = new ArrayList<Integer>(A.subList(0, A.size()/2));
List<Integer> A2 = new ArrayList<Integer>(A.subList(A.size()/2, A.size()));

mergeSort(A1); //recursively sort A1
mergeSort(A2); //recursively sort A2
remerge(A1, A2, A); //re-combine A1 and A2 into A as sorted
}
}
public static void remerge(List<Integer> A1, List<Integer> A2, List<Integer> A3){
int A1index = 0, A2index = 0, A3index = 0;
while (A1index < A1.size() && A2index < A2.size()){ //while neither subarrays are exhausted
if (A1.get(A1index)<A2.get(A2index)){ //if A1 element is smaller than A2, move it to A3, else move A2 element to A3
A3.set(A3index,A1.get(A1index));
A3index++;
A1index++;}
else{
A3.set(A3index, A2.get(A2index));
A3index++;
A2index++;}
} //at this point either A1 or A2 is exhausted

if (A1index<A1.size()){ //A1 is not exhausted, so A2 is
while (A1index < A1.size()){
A3.set(A3index, A1.get(A1index));
A3index++;
A1index++;}}
else {
while (A2index < A2.size()) {
A3.set(A3index, A2.get(A2index));
A3index++;
A2index++;
}
}
}
}

 

Some subclasses of Number don't have primitives and are not Comparable, such that <? extends Number> can't be compared by > and compareTo. Working now with simply <T extends Comparable>.

------------------------------------
import java.util.*;

public class MergeSort{ //sort a List or array in ascending order by merge sort
public static <T extends Comparable<T>> void mergeSort(List<T> A){
if (A.size() > 1){ //stopping case
List<T> A1 = new ArrayList<T>(A.subList(0, A.size()/2));
List<T> A2 = new ArrayList<T>(A.subList(A.size()/2, A.size()));

mergeSort(A1); //recursively sort A1
mergeSort(A2); //recursively sort A2
remerge(A1, A2, A); //re-combine A1 and A2 into A as sorted
}
}
public static <T extends Comparable<T>> void remerge(List<T> A1, List<T> A2, List<T> A3){
int A1index = 0, A2index = 0, A3index = 0;
while (A1index < A1.size() && A2index < A2.size()){ //while neither subarrays are exhausted
if ( ((A1.get(A1index)).compareTo(A2.get(A2index))) < 0 ){ //if A1 element is smaller than A2, move it to A3, else move A2 element to A3
A3.set(A3index,A1.get(A1index));
A3index++;
A1index++;}
else{
A3.set(A3index, A2.get(A2index));
A3index++;
A2index++;}
} //at this point either A1 or A2 is exhausted

if (A1index<A1.size()){ //A1 is not exhausted, so A2 is
while (A1index < A1.size()){
A3.set(A3index, A1.get(A1index));
A3index++;
A1index++;}}
else {
while (A2index < A2.size()) {
A3.set(A3index, A2.get(A2index));
A3index++;
A2index++;
}
}
}
}

 

This is indeed one possibility. Another possibility is to consider whether you really need such a general approach, or whether you can use lists which are more limited (e.g. to integers, or doubles only).

 

I guess you're coming from C++ and trying to use operator overloading, there's no such thing in Java. Anyways, don't reinvent the wheel, use java.util.Collections.sort().

 

Can someone inform why I get Static Error: 'no method in Stream<Interger> with name 'collect' matches this invocation' ? https://docs.oracle.com/javase/8/do...ream.html#collect-java.util.stream.Collector-

---------------------------------------------------------------------
> import java.util.*;
> import java.util.stream.*;
> int[] number = new int[]{1, 2, 3};
List<Integer> list = Arrays.stream(number).boxed().collect(Collectors.toList()); //error
List<Integer> list = IntStream.of(number).boxed().collect(Collectors.toList()); //error too

 

Compiles and works for me.

 

Thanks. It works now after I compile. was testing things in Interactions pane and didnt work there. Seems Interactions doesn't have static stuff, hence the static call error.