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The signature of java.util.Collections.max looks like this:
public static <T extends Object & Comparable<? super T>> T max(Collection collection);
From what I understand, it basically means that T must be both a java.lang.Object and a java.lang.Comparable<? super T>>,
However, since every java.lang.Comparable is also an java.lang.Object, what is the difference between the signature above and this below? :
public static <T extends Comparable<? super T>> T max(Collection collection);
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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 .
The extends keyword extends a class (indicates that a class is inherited from another class). In Java, it is possible to inherit attributes and methods from one class to another. We group the "inheritance concept" into two categories: subclass (child) - the class that inherits from another class.
There is no difference whatsoever. extends Object is redundant and assumed otherwise, just like writing class F extends Object . I would bet they are equivalent, since all classes in Java implicitly extend Object . No difference at all, it is the consequence of language design/grammar.
extends Object> means you can pass an Object or a sub-class that extends Object class. ArrayList<? extends Number> numberList = new ArrayList<Number>(); //Number of subclass numberList = new ArrayList<Integer>(); //Integer extends Number numberList = new ArrayList<Float>(); // Float extends Number.
To preserve binary compatibility: It's completely described here. The second signature actually changes the return type of the method to Comparable and it loses the generality of returning an Object. The original signature preserves both.
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