I'm having difficulty using Comparators in Constructors. When I try the following code:
InsertionSorter is = new InsertionSorter(bookList.toArray(new Comparable[bookList.size()]), new GenreComparator(), SortType.ASC);
I get this error:
no suitable constructor found for InsertionSorter(java.lang.Comparable[],sort.GenreComparator,sort.SortType)
constructor sort.InsertionSorter.InsertionSorter(java.lang.Comparable[],java.util.Comparator<java.lang.Object>,sort.SortType) is not applicable
(actual argument sort.GenreComparator cannot be converted to java.util.Comparator<java.lang.Object> by method invocation conversion)
constructor sort.InsertionSorter.InsertionSorter(java.lang.Comparable[],sort.SortType) is not applicable
(actual and formal argument lists differ in length)
Here are the useful code snippets:
AbstractSorter.java:
abstract class AbstractSorter {
protected Comparable[] values;
protected Comparator<Object> cmp;
protected SortType st;
protected abstract void doSort();
protected AbstractSorter(Comparable[] init, SortType type) {
values = new Comparable[init.length];
System.arraycopy(init, 0, values, 0, init.length);
cmp = null;
st = type;
}
protected AbstractSorter(Comparable[] init, Comparator<Object> comp, SortType type) {
values = new Comparable[init.length];
System.arraycopy(init, 0, values, 0, init.length);
cmp = comp;
st = type;
}
public Comparable[] getValues() {
return values;
}
}
class InsertionSorter extends AbstractSorter {
public InsertionSorter(Comparable[] init, SortType type) {
super(init, type);
}
public InsertionSorter(Comparable[] init, Comparator<Object> comp, SortType type) {
super(init, comp, type);
}
@Override
public void doSort() {
// sorts values
}
}
enum SortType {ASC, DSC};
Book.java:
public class Book implements Cloneable, Comparable<Book> {
private Person author; // implementation details of Person don't matter here
private String title, isbn;
private int year;
private BookGenre genre;
public Book(Person authorInit, String titleInit, String isbnInit,
int yearInit, BookGenre genreInit) {
author = authorInit;
title = titleInit;
isbn = isbnInit;
year = yearInit;
genre = genreInit;
}
@Override
public String toString() {
return author.toString() + " " + title + " " + isbn + " " + year
+ " " + genre;
}
@Override
public Object clone() throws CloneNotSupportedException {
return super.clone();
}
@Override
public int compareTo(Book other) {
return author.compareTo(other.author);
}
public Person getAuthor() {
return author;
}
public String getTitle() {
return title;
}
public String getIsbn() {
return isbn;
}
public int getYear() {
return year;
}
public BookGenre getGenre() {
return genre;
}
}
class TitleComparator implements Comparator<Book> {
@Override
public int compare(Book a, Book b) {
return a.getTitle().compareToIgnoreCase(b.getTitle());
}
}
class GenreComparator implements Comparator<Book> {
@Override
public int compare(Book a, Book b) {
return a.getGenre().compareTo(b.getGenre());
}
}
enum BookGenre { COMIC, MYSTERY, SCIENCE, TRAVEL };
EDIT Thanks to Rohit for a solution to this problem, but now I'm having a related problem. In the method doSort()
, I have the code snippet
cmp.compare(values[j-1], values[j])
(don't worry, I have checks in place to make sure this isn't called if cmp == null
. This gives the following error:
method compare in interface java.util.Comparator<T> cannot be applied to given types;
required: capture#1 of ? extends java.lang.Object,capture#1 of ? extends java.lang.Object
found: java.lang.Comparable,java.lang.Comparable
reason: actual argument java.lang.Comparable cannot be converted to capture#1 of ? extends java.lang.Object by method invocation conversion
Note that I have changed the above in AbstractSort.java to replace Comparable<Object>
with Comparable<? extends Object>
Comparable<? extends Object>
GenreComparator
class implements Comparator<Book>
. So you can't pass an instance of that class where a Comparator<Object>
is required. Both are non-compatible.
You can however change Comparator<Object>
to Comparator<? extends Object>
Comparator<? extends Object>
in your constructor, and also the type of field cmp
in AbstractSorter
class.
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