Develop Reference

Java - generic class parameter with a generic parameter

how do I correctly write following class declaration in Java?
public class BinarySearchTree<T extends BinarySearchNode<E implements Comparable<E>>> implements Iterable<E>
Basically, I am trying to create a BinarySearchTree of any class T that inherits BinarySearchNode of a Comparable datatype E (And my BinarySearchTree should also be Iterable).
How can I properly declare that class in Java?
Thanks!

public class BinarySearchTree<T extends BinarySearchNode<E>, E extends Comparable<E>> implements Iterable<E>
The type parameter E needs to be defined separately from the node type, even if it ends up looking redundant: BinarySearchTree<BinarySearchNode<String>, String>. Java won't let you directly access a generic parameter of a generic type.
Also, note that generic parameters always use "extends" even with interfaces.

With Generics in Java, you will only use extends. So, essentially your method decoration would become:
public class BinarySearchTree<T extends BinarySearchNode<E extends Comparable<E>>> extends Iterable<T>
Even for those classes that implement an interface, generics will use extends. The only other syntax for generics is super if you are looking at the inheritance from the opposite direction

Java Generics Comparator

I am coding with Java Generics. I want to define a Binary Tree class generically that will be able to take in any class , and guarantee that that class has the Comparator method compare(T o1, T o2) to see whether I need to follow the right or left subtree for insert into my binary tree.
public class treeDB <T implements Comparator> {
//define my binary tree methods
}
That is my best estimation of how to force to implement Comparator method, but the compile throws an error and I don't know enough to know what it wants.

Everyone has provided the correct syntax, but you might want to consider using Comparable as in
class treeDB <T extends Comparable<T>>
The differences are subtle, and maybe it isn't the better choice. But it never hurts to look.

try this
class treeDB <T extends Comparator<T>> {
...

Firstly, implements should be replaced with extends. In generics extends is the keyword which is used even if the generic type implements an interface.
And secondly, using only Comparator will result in a warning it being a raw type. You must parameterize it. This is your solution:
public class treeDB <T extends Comparator<T>> {
}

This should be public class treeDB <T extends Comparator>, not public class treeDB <T implements Comparator>.

java generic comparable idiom

I encountered the following piece of a definition for a generic class:
public class binarysearchnode<T extends Comparable<T>> implements Comparable<binarysearchnode<T>>{
.............
}
Please help explaining why a class would specify itself as a Type parameter to comparable while implementing the comparable interface?
How would it be different from the following:
public class binarysearchnode<T extends Comparable<T>> implements Comparable<? super (or extends)T>{
.............
}

This makes it possible to compare binarysearchnodes to each other. If it implemented Comparable<T>, that would instead mean that the node could be compared to the value of the node, which would be odd.
Inside the class you will probably find something like this:
T value;
public int compareTo(binarysearchnode<T> other) {
return value.compareTo(other.value);
}
In order to be able to implement compareTo() like this, the value class (T) needs to be comparable to other objects of its class - hence the declaration of <T extends Comparable<T>> in the class definition.

It is because what the class author wants is to be able to write:
b1.compareTo(b2)
where b1 and b2 are binarysearchnode instances. The developer also adds a constraint to T so that T extends Comparable<T>. Probably so that the implementation of Comparable for binarysearchnode can just rely on T instances being Comparable themselves.
More generally, while it is possible for a class C1 to implement Comparable<C2>, ultimately, it makes no sense to do so: this does not mean that an instance of C2 could be comparable to an instance of C1. And due to type erasure, it would not be possible, for instance, for class C1 to implement both Comparable<C1> and Comparable<C2>.
Also, if binarysearchnode<T> were to implement Comparable<T> directly, you would have at least two problems:
you would not be able to compare one binarysearchnode<T> to another;
given a binarysearchnote<T> instance b and a T instance t, you would be able to write b.compareTo(t) but not t.compareTo(b) (since T does not, and cannot, implement Comparable<binarysearchnode<T>>), and that breaks the Comparable contract.

Let's say you have a superclass A and a subclass B. Imagine the superclass implements Comparable<A>, then B will also implement Comparable<A> through inheritance.
Your binarysearchnode class declared as such :
public class binarysearchnode<T extends Comparable<T>>
will not be able to take B as a type parameter for T (B does not implement Comparable<B>)
But when defined as such :
public class binarysearchnode<T extends Comparable<? super T>>
it will be able to take B as a type parameter for T, since B implements Comparable<A> which fulfills Comparable<? super T>.

The syntax <T extends Class<T>> in Java

I have couple of thoughts regarding the following:
public interface MaxStack<T extends Comparable <T>>
1-Why does the class that implements MaxStack should be written like this:
public class MaxStackclass<T extends Comparable <T>> implements MaxStack<T>
and not public class MaxStackclass<T extends Comparable <T>> implements MaxStack<T extends Comparable <T>>?
2- why do the private variables of this class, when I use generics, should be written only with <T> and not with <T extnds Comparable<T>>? For example, private List<T> stack= new ArrayList<T>();
3-What is the difference between <T extends Comparable<T>> and <T extends Comparable>- if I need to compare bewteen elements in my class, both will be O.K, no?
Edit: I think that thee problem with 3 is that maybe it allows to insert of a list that was defined in the second way to have different elements which all extends from comparable and then when I want to compare them, it won't be possible, since we can't compare String to Integer, both extend from Comparable.

In the declaration maxStackclass<T extends Comparable <T>> you have already expressed the bounds on T. So you do not need it again.
Reason same as above. No need to specify bounds on the same type parameter again.
<T extends Comparable<T>> means that T must implement the Comparable interface that can compare two T instances. While <T extends Comparable> means T implements Comparable such that it can compare two Objects in general. The former is more specific.
if I need to compare bewteen elements in my class, both will be O.K,
no?
Well, technically you can achieve the same result using both. But for the declaration <T extends Comparable> it will involve unnecessary casts which you can avoid using the type safe <T extends Comparable<T>>

1) the class has a type parameter T with a bound (extends Comparable <T>), this parameter is passed to the interface (which need the same bound here). When passing a type parameter, you must not repeat its bound - why you should do so?
2) like 1), the type parameter has its bound declared, no repeat neccessary.
To clarify:
The first type parameter occurence (here behind the interface or class name) is its declaration. Any following occurence is a usage. You even never would write a variables type declaration each time you use it, right?

"3-What is the difference between <T extends Comparable<T>> and <T extends Comparable>- if I need to compare bewteen elements in my class, both will be O.K, no?"
No, both will not be okay. Suppose I have a class Foo which implements Comparable<Bar> but classes Foo and Bar have no relation to each other. Then Foo cannot compare to other objects of type Foo. <T extends Comparable<T>> will catch this as a problem. <T extends Comparable> will not.

Java: Is it possible to have a generic class that only takes types that can be compared?

I wanted to do something along the lines of:
public class MyClass<T implements Comparable> {
....
}
But I can't, since, apparently, generics only accept restrictions with subclasses, and not interfaces.
It's important that I'm able to compare the types inside the class, so how should I go about doing this? Ideally I'd be able to keep the type safety of Generics and not have to convert the T's to Object as well, or just not write a lot of code overall. In other words, the simplest the better.

The implements is wrong. It only accepts extends or super. You can use here extends:
public class MyClass<T extends Comparable<T>> {
// ...
}
To learn more about Generics, you may find this tutorial (PDF) useful.

The best way to do it is:
public class MyClass<T extends Comparable<? super T>> {
....
}
If you just do <T extends Comparable<T>>, then it won't work for subclasses of comparable classes.

Also for interfaces you have to use extends. So in your case it would be:
public class MyClass<T extends Comparable<T>> {
....
}