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Why does Collections.sort(List<T>) have the signature :
public static <T extends Comparable<? super T>> void sort(List<T> list) and not :
public static <T extends Comparable<T>> void sort(List<? extends T> list) 
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super T denotes an unknown type that is a supertype of T (or T itself; remember that the supertype relation is reflexive). It is the dual of the bounded wildcards we've been using, where we use ? extends T to denote an unknown type that is a subtype of T .
By default, Collection. sort performs the sorting in ascending order. If we want to sort the elements in reverse order we could use following methods: reverseOrder() : Returns a Comparator that imposes the reverse of natural ordering of elements of the collection.
Collections class has a second sort() method and it takes Comparator. The sort() method invokes the compare() to sort objects.
Collections sort is a method of Java Collections class used to sort a list, which implements the List interface. All the elements in the list must be mutually comparable. If a list consists of string elements, then it will be sorted in alphabetical order.
Your proposed signature would probably work in Java-8. However in previous Java versions type inference was not so smart. Consider that you have List<java.sql.Date>. Note that java.sql.Date extends java.util.Date which implements Comparable<java.util.Date>. When you compile
List<java.sql.Date> list = new ArrayList<>(); Collections.sort(list); It perfectly works in Java-7. Here T is inferred to be java.sql.Date which is actually Comparable<java.util.Date> which is Comparable<? super java.sql.Date>. However let's try your signature:
public static <T extends Comparable<T>> void sort(List<? extends T> list) {} List<java.sql.Date> list = new ArrayList<>(); sort(list); Here T should be inferred as java.util.Date. However Java 7 specification does not allow such inference. Hence this code can be compiled with Java-8, but fails when compiled under Java-7:
Main.java:14: error: method sort in class Main cannot be applied to given types; sort(list); ^ required: List<? extends T> found: List<Date> reason: inferred type does not conform to declared bound(s) inferred: Date bound(s): Comparable<Date> where T is a type-variable: T extends Comparable<T> declared in method <T>sort(List<? extends T>) 1 error Type inference was greatly improved in Java-8. Separate JLS chapter 18 is dedicated to it now, while in Java-7 the rules were much simpler.
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