Related
I know what an interface is and what is a collection. But to be honest, I could not find any solid reason as why not to implement two collection interfaces in one class.
Got this question asked a few days back in an interview.
In some cases they are or can be implemented by the same object.
A Queue and a List are both implemented by LinkedList, TreeMap is both NavigableMap and SortedMap. There are a few other examples like this.
Each describes a trait or feature of the collection (exposed as a way to use it).
It just doesn't make sense all that often. For a Java collection to implement two interfaces it must be a near perfect implementation of both (and perhaps that's your answer).
A linkedlist COULD technically implement the methods of an ArrayList, however it would be a really bad idea.
Another point that hasn't been made already is that you can't implement the same interface twice with different type parameters.
As a result, if you tried to make a class implement both List<String> and Collection<Integer>, you would get a compiler error. This is because List<String> extends Collection<String> so we're trying to implement Collection twice with different type parameters.
If you did manage to implement two collection interfaces at once, it would have to be like the LinkedList example (where the generic type is the same when you think of it as a List and when you think of it as a Deque).
For starters, incompatible method contracts: e.g. List.add must always return true, and Set.add must always return false if the element is already present.
What are the reasons behind the decision to not have a fully generic get method
in the interface of java.util.Map<K, V>.
To clarify the question, the signature of the method is
V get(Object key)
instead of
V get(K key)
and I'm wondering why (same thing for remove, containsKey, containsValue).
As mentioned by others, the reason why get(), etc. is not generic because the key of the entry you are retrieving does not have to be the same type as the object that you pass in to get(); the specification of the method only requires that they be equal. This follows from how the equals() method takes in an Object as parameter, not just the same type as the object.
Although it may be commonly true that many classes have equals() defined so that its objects can only be equal to objects of its own class, there are many places in Java where this is not the case. For example, the specification for List.equals() says that two List objects are equal if they are both Lists and have the same contents, even if they are different implementations of List. So coming back to the example in this question, according to the specification of the method is possible to have a Map<ArrayList, Something> and for me to call get() with a LinkedList as argument, and it should retrieve the key which is a list with the same contents. This would not be possible if get() were generic and restricted its argument type.
An awesome Java coder at Google, Kevin Bourrillion, wrote about exactly this issue in a blog post a while ago (admittedly in the context of Set instead of Map). The most relevant sentence:
Uniformly, methods of the Java
Collections Framework (and the Google
Collections Library too) never
restrict the types of their parameters
except when it's necessary to prevent
the collection from getting broken.
I'm not entirely sure I agree with it as a principle - .NET seems to be fine requiring the right key type, for example - but it's worth following the reasoning in the blog post. (Having mentioned .NET, it's worth explaining that part of the reason why it's not a problem in .NET is that there's the bigger problem in .NET of more limited variance...)
The contract is expressed thus:
More formally, if this map contains a
mapping from a key k to a value v such
that (key==null ? k==null :
key.equals(k)), then this method
returns v; otherwise it returns null.
(There can be at most one such
mapping.)
(my emphasis)
and as such, a successful key lookup depends on the input key's implementation of the equality method. That is not necessarily dependent on the class of k.
It's an application of Postel's Law, "be conservative in what you do, be liberal in what you accept from others."
Equality checks can be performed regardless of type; the equals method is defined on the Object class and accepts any Object as a parameter. So, it makes sense for key equivalence, and operations based on key equivalence, to accept any Object type.
When a map returns key values, it conserves as much type information as it can, by using the type parameter.
I think this section of Generics Tutorial explains the situation (my emphasis):
"You need to make certain that the generic API is not unduly restrictive; it must
continue to support the original contract of the API. Consider again some examples
from java.util.Collection. The pre-generic API looks like:
interface Collection {
public boolean containsAll(Collection c);
...
}
A naive attempt to generify it is:
interface Collection<E> {
public boolean containsAll(Collection<E> c);
...
}
While this is certainly type safe, it doesn’t live up to the API’s original contract.
The containsAll() method works with any kind of incoming collection. It will only
succeed if the incoming collection really contains only instances of E, but:
The static type of the incoming
collection might differ, perhaps
because the caller doesn’t know the
precise type of the collection being
passed in, or perhaps because it is a
Collection<S>,where S is a
subtype of E.
It’s perfectly
legitimate to call containsAll() with
a collection of a different type. The
routine should work, returning false."
Compatibility.
Before generics were available, there was just get(Object o).
Had they changed this method to get(<K> o) it would have potentially forced massive code maintenance onto java users just to make working code compile again.
They could have introduced an additional method, say get_checked(<K> o) and deprecate the old get() method so there was a gentler transition path. But for some reason, this was not done. (The situation we are in now is that you need to install tools like findBugs to check for type compatibility between the get() argument and the declared key type <K> of the map.)
The arguments relating to the semantics of .equals() are bogus, I think. (Technically they're correct, but I still think they're bogus. No designer in his right mind is ever going to make o1.equals(o2) true if o1 and o2 do not have any common superclass.)
The reason is that containment is determined by equals and hashCode which are methods on Object and both take an Object parameter. This was an early design flaw in Java's standard libraries. Coupled with limitations in Java's type system, it forces anything that relies on equals and hashCode to take Object.
The only way to have type-safe hash tables and equality in Java is to eschew Object.equals and Object.hashCode and use a generic substitute. Functional Java comes with type classes for just this purpose: Hash<A> and Equal<A>. A wrapper for HashMap<K, V> is provided that takes Hash<K> and Equal<K> in its constructor. This class's get and contains methods therefore take a generic argument of type K.
Example:
HashMap<String, Integer> h =
new HashMap<String, Integer>(Equal.stringEqual, Hash.stringHash);
h.add("one", 1);
h.get("one"); // All good
h.get(Integer.valueOf(1)); // Compiler error
There is one more weighty reason, it can not be done technically, because it brokes Map.
Java has polymorphic generic construction like <? extends SomeClass>. Marked such reference can point to type signed with <AnySubclassOfSomeClass>. But polymorphic generic makes that reference readonly. The compiler allows you to use generic types only as returning type of method (like simple getters), but blocks using of methods where generic type is argument (like ordinary setters).
It means if you write Map<? extends KeyType, ValueType>, the compiler does not allow you to call method get(<? extends KeyType>), and the map will be useless. The only solution is to make this method not generic: get(Object).
Backwards compatibility, I guess. Map (or HashMap) still needs to support get(Object).
I was looking at this and thinking why they did it this way. I don't think any of the existing answers explains why they couldn't just make the new generic interface accept only the proper type for the key. The actual reason is that even though they introduced generics they did NOT create a new interface. The Map interface is the same old non-generic Map it just serves as both generic and non-generic version. This way if you have a method that accepts non-generic Map you can pass it a Map<String, Customer> and it would still work. At the same time the contract for get accepts Object so the new interface should support this contract too.
In my opinion they should have added a new interface and implemented both on existing collection but they decided in favor of compatible interfaces even if it means worse design for the get method. Note that the collections themselves would be compatible with existing methods only the interfaces wouldn't.
We are doing big refactoring just now and we were missing this strongly typed get() to check that we did not missed some get() with old type.
But I found workaround/ugly trick for compilation time check: create Map interface with strongly typed get, containsKey, remove... and put it to java.util package of your project.
You will get compilation errors just for calling get(), ... with wrong types, everything others seems ok for compiler (at least inside eclipse kepler).
Do not forget to delete this interface after check of your build as this is not what you want in runtime.
What are the reasons behind the decision to not have a fully generic get method
in the interface of java.util.Map<K, V>.
To clarify the question, the signature of the method is
V get(Object key)
instead of
V get(K key)
and I'm wondering why (same thing for remove, containsKey, containsValue).
As mentioned by others, the reason why get(), etc. is not generic because the key of the entry you are retrieving does not have to be the same type as the object that you pass in to get(); the specification of the method only requires that they be equal. This follows from how the equals() method takes in an Object as parameter, not just the same type as the object.
Although it may be commonly true that many classes have equals() defined so that its objects can only be equal to objects of its own class, there are many places in Java where this is not the case. For example, the specification for List.equals() says that two List objects are equal if they are both Lists and have the same contents, even if they are different implementations of List. So coming back to the example in this question, according to the specification of the method is possible to have a Map<ArrayList, Something> and for me to call get() with a LinkedList as argument, and it should retrieve the key which is a list with the same contents. This would not be possible if get() were generic and restricted its argument type.
An awesome Java coder at Google, Kevin Bourrillion, wrote about exactly this issue in a blog post a while ago (admittedly in the context of Set instead of Map). The most relevant sentence:
Uniformly, methods of the Java
Collections Framework (and the Google
Collections Library too) never
restrict the types of their parameters
except when it's necessary to prevent
the collection from getting broken.
I'm not entirely sure I agree with it as a principle - .NET seems to be fine requiring the right key type, for example - but it's worth following the reasoning in the blog post. (Having mentioned .NET, it's worth explaining that part of the reason why it's not a problem in .NET is that there's the bigger problem in .NET of more limited variance...)
The contract is expressed thus:
More formally, if this map contains a
mapping from a key k to a value v such
that (key==null ? k==null :
key.equals(k)), then this method
returns v; otherwise it returns null.
(There can be at most one such
mapping.)
(my emphasis)
and as such, a successful key lookup depends on the input key's implementation of the equality method. That is not necessarily dependent on the class of k.
It's an application of Postel's Law, "be conservative in what you do, be liberal in what you accept from others."
Equality checks can be performed regardless of type; the equals method is defined on the Object class and accepts any Object as a parameter. So, it makes sense for key equivalence, and operations based on key equivalence, to accept any Object type.
When a map returns key values, it conserves as much type information as it can, by using the type parameter.
I think this section of Generics Tutorial explains the situation (my emphasis):
"You need to make certain that the generic API is not unduly restrictive; it must
continue to support the original contract of the API. Consider again some examples
from java.util.Collection. The pre-generic API looks like:
interface Collection {
public boolean containsAll(Collection c);
...
}
A naive attempt to generify it is:
interface Collection<E> {
public boolean containsAll(Collection<E> c);
...
}
While this is certainly type safe, it doesn’t live up to the API’s original contract.
The containsAll() method works with any kind of incoming collection. It will only
succeed if the incoming collection really contains only instances of E, but:
The static type of the incoming
collection might differ, perhaps
because the caller doesn’t know the
precise type of the collection being
passed in, or perhaps because it is a
Collection<S>,where S is a
subtype of E.
It’s perfectly
legitimate to call containsAll() with
a collection of a different type. The
routine should work, returning false."
Compatibility.
Before generics were available, there was just get(Object o).
Had they changed this method to get(<K> o) it would have potentially forced massive code maintenance onto java users just to make working code compile again.
They could have introduced an additional method, say get_checked(<K> o) and deprecate the old get() method so there was a gentler transition path. But for some reason, this was not done. (The situation we are in now is that you need to install tools like findBugs to check for type compatibility between the get() argument and the declared key type <K> of the map.)
The arguments relating to the semantics of .equals() are bogus, I think. (Technically they're correct, but I still think they're bogus. No designer in his right mind is ever going to make o1.equals(o2) true if o1 and o2 do not have any common superclass.)
The reason is that containment is determined by equals and hashCode which are methods on Object and both take an Object parameter. This was an early design flaw in Java's standard libraries. Coupled with limitations in Java's type system, it forces anything that relies on equals and hashCode to take Object.
The only way to have type-safe hash tables and equality in Java is to eschew Object.equals and Object.hashCode and use a generic substitute. Functional Java comes with type classes for just this purpose: Hash<A> and Equal<A>. A wrapper for HashMap<K, V> is provided that takes Hash<K> and Equal<K> in its constructor. This class's get and contains methods therefore take a generic argument of type K.
Example:
HashMap<String, Integer> h =
new HashMap<String, Integer>(Equal.stringEqual, Hash.stringHash);
h.add("one", 1);
h.get("one"); // All good
h.get(Integer.valueOf(1)); // Compiler error
There is one more weighty reason, it can not be done technically, because it brokes Map.
Java has polymorphic generic construction like <? extends SomeClass>. Marked such reference can point to type signed with <AnySubclassOfSomeClass>. But polymorphic generic makes that reference readonly. The compiler allows you to use generic types only as returning type of method (like simple getters), but blocks using of methods where generic type is argument (like ordinary setters).
It means if you write Map<? extends KeyType, ValueType>, the compiler does not allow you to call method get(<? extends KeyType>), and the map will be useless. The only solution is to make this method not generic: get(Object).
Backwards compatibility, I guess. Map (or HashMap) still needs to support get(Object).
I was looking at this and thinking why they did it this way. I don't think any of the existing answers explains why they couldn't just make the new generic interface accept only the proper type for the key. The actual reason is that even though they introduced generics they did NOT create a new interface. The Map interface is the same old non-generic Map it just serves as both generic and non-generic version. This way if you have a method that accepts non-generic Map you can pass it a Map<String, Customer> and it would still work. At the same time the contract for get accepts Object so the new interface should support this contract too.
In my opinion they should have added a new interface and implemented both on existing collection but they decided in favor of compatible interfaces even if it means worse design for the get method. Note that the collections themselves would be compatible with existing methods only the interfaces wouldn't.
We are doing big refactoring just now and we were missing this strongly typed get() to check that we did not missed some get() with old type.
But I found workaround/ugly trick for compilation time check: create Map interface with strongly typed get, containsKey, remove... and put it to java.util package of your project.
You will get compilation errors just for calling get(), ... with wrong types, everything others seems ok for compiler (at least inside eclipse kepler).
Do not forget to delete this interface after check of your build as this is not what you want in runtime.
Suppose I have an Employee class. How can I implement an ArrayList only containing Employee elements without using generics? That is, without Arraylist<Employee>, how can I restrict the ArrayList to add only Employee objects?
Extend ArrayList and customize add() and addAll() method to check the object being added is instanceof Employee
You could use a wrapper class that holds a private ArrayList field, say called employeeList, has a
public void add(Employee employee) {
employeeList.add(employee);
}
as well as any other necessary methods that would allow outside classes to interact with the ArrayList in a controlled fashion.
I find it much better to use composition for this than inheritance. That way if you wanted to change from an ArrayList to something else, say a LinkedList, or even something completely different, you would have an easier time.
You could use Collections.checkedList() - but why would you want to not use generics?
Subclass the ArrayList class and name it something like EmployeeArrayList.
If you're wanting to avoid generics for their own sake, e.g. for compatibility with very old versions of Java, then extending or wrapping ArrayList won't help - you probably want to find or make another array implementation that has the same functionality.
Basically, ArrayList is just a wrapper for a primitive array that copies and pastes its data into a larger array when necessary, so this isn't especially difficult to write from scratch.
What exactly do you want when you "restrict"? There are two possible places where one could place a restriction: at compile-time or runtime.
Generics is a purely compile-time thing. It helps you write correct code but you can still bypass it and put the wrong type in the array and it won't complain at runtime.
On the other hand, something like Collections.checkedList()is a runtime restrictions. It throws an error at runtime when an object of the wrong type comes. But it does not help you at compile-time if you do not have generics.
So the two things are orthogonal, and neither is a replacement for the other. What exactly do you want?
What are the reasons behind the decision to not have a fully generic get method
in the interface of java.util.Map<K, V>.
To clarify the question, the signature of the method is
V get(Object key)
instead of
V get(K key)
and I'm wondering why (same thing for remove, containsKey, containsValue).
As mentioned by others, the reason why get(), etc. is not generic because the key of the entry you are retrieving does not have to be the same type as the object that you pass in to get(); the specification of the method only requires that they be equal. This follows from how the equals() method takes in an Object as parameter, not just the same type as the object.
Although it may be commonly true that many classes have equals() defined so that its objects can only be equal to objects of its own class, there are many places in Java where this is not the case. For example, the specification for List.equals() says that two List objects are equal if they are both Lists and have the same contents, even if they are different implementations of List. So coming back to the example in this question, according to the specification of the method is possible to have a Map<ArrayList, Something> and for me to call get() with a LinkedList as argument, and it should retrieve the key which is a list with the same contents. This would not be possible if get() were generic and restricted its argument type.
An awesome Java coder at Google, Kevin Bourrillion, wrote about exactly this issue in a blog post a while ago (admittedly in the context of Set instead of Map). The most relevant sentence:
Uniformly, methods of the Java
Collections Framework (and the Google
Collections Library too) never
restrict the types of their parameters
except when it's necessary to prevent
the collection from getting broken.
I'm not entirely sure I agree with it as a principle - .NET seems to be fine requiring the right key type, for example - but it's worth following the reasoning in the blog post. (Having mentioned .NET, it's worth explaining that part of the reason why it's not a problem in .NET is that there's the bigger problem in .NET of more limited variance...)
The contract is expressed thus:
More formally, if this map contains a
mapping from a key k to a value v such
that (key==null ? k==null :
key.equals(k)), then this method
returns v; otherwise it returns null.
(There can be at most one such
mapping.)
(my emphasis)
and as such, a successful key lookup depends on the input key's implementation of the equality method. That is not necessarily dependent on the class of k.
It's an application of Postel's Law, "be conservative in what you do, be liberal in what you accept from others."
Equality checks can be performed regardless of type; the equals method is defined on the Object class and accepts any Object as a parameter. So, it makes sense for key equivalence, and operations based on key equivalence, to accept any Object type.
When a map returns key values, it conserves as much type information as it can, by using the type parameter.
I think this section of Generics Tutorial explains the situation (my emphasis):
"You need to make certain that the generic API is not unduly restrictive; it must
continue to support the original contract of the API. Consider again some examples
from java.util.Collection. The pre-generic API looks like:
interface Collection {
public boolean containsAll(Collection c);
...
}
A naive attempt to generify it is:
interface Collection<E> {
public boolean containsAll(Collection<E> c);
...
}
While this is certainly type safe, it doesn’t live up to the API’s original contract.
The containsAll() method works with any kind of incoming collection. It will only
succeed if the incoming collection really contains only instances of E, but:
The static type of the incoming
collection might differ, perhaps
because the caller doesn’t know the
precise type of the collection being
passed in, or perhaps because it is a
Collection<S>,where S is a
subtype of E.
It’s perfectly
legitimate to call containsAll() with
a collection of a different type. The
routine should work, returning false."
Compatibility.
Before generics were available, there was just get(Object o).
Had they changed this method to get(<K> o) it would have potentially forced massive code maintenance onto java users just to make working code compile again.
They could have introduced an additional method, say get_checked(<K> o) and deprecate the old get() method so there was a gentler transition path. But for some reason, this was not done. (The situation we are in now is that you need to install tools like findBugs to check for type compatibility between the get() argument and the declared key type <K> of the map.)
The arguments relating to the semantics of .equals() are bogus, I think. (Technically they're correct, but I still think they're bogus. No designer in his right mind is ever going to make o1.equals(o2) true if o1 and o2 do not have any common superclass.)
The reason is that containment is determined by equals and hashCode which are methods on Object and both take an Object parameter. This was an early design flaw in Java's standard libraries. Coupled with limitations in Java's type system, it forces anything that relies on equals and hashCode to take Object.
The only way to have type-safe hash tables and equality in Java is to eschew Object.equals and Object.hashCode and use a generic substitute. Functional Java comes with type classes for just this purpose: Hash<A> and Equal<A>. A wrapper for HashMap<K, V> is provided that takes Hash<K> and Equal<K> in its constructor. This class's get and contains methods therefore take a generic argument of type K.
Example:
HashMap<String, Integer> h =
new HashMap<String, Integer>(Equal.stringEqual, Hash.stringHash);
h.add("one", 1);
h.get("one"); // All good
h.get(Integer.valueOf(1)); // Compiler error
There is one more weighty reason, it can not be done technically, because it brokes Map.
Java has polymorphic generic construction like <? extends SomeClass>. Marked such reference can point to type signed with <AnySubclassOfSomeClass>. But polymorphic generic makes that reference readonly. The compiler allows you to use generic types only as returning type of method (like simple getters), but blocks using of methods where generic type is argument (like ordinary setters).
It means if you write Map<? extends KeyType, ValueType>, the compiler does not allow you to call method get(<? extends KeyType>), and the map will be useless. The only solution is to make this method not generic: get(Object).
Backwards compatibility, I guess. Map (or HashMap) still needs to support get(Object).
I was looking at this and thinking why they did it this way. I don't think any of the existing answers explains why they couldn't just make the new generic interface accept only the proper type for the key. The actual reason is that even though they introduced generics they did NOT create a new interface. The Map interface is the same old non-generic Map it just serves as both generic and non-generic version. This way if you have a method that accepts non-generic Map you can pass it a Map<String, Customer> and it would still work. At the same time the contract for get accepts Object so the new interface should support this contract too.
In my opinion they should have added a new interface and implemented both on existing collection but they decided in favor of compatible interfaces even if it means worse design for the get method. Note that the collections themselves would be compatible with existing methods only the interfaces wouldn't.
We are doing big refactoring just now and we were missing this strongly typed get() to check that we did not missed some get() with old type.
But I found workaround/ugly trick for compilation time check: create Map interface with strongly typed get, containsKey, remove... and put it to java.util package of your project.
You will get compilation errors just for calling get(), ... with wrong types, everything others seems ok for compiler (at least inside eclipse kepler).
Do not forget to delete this interface after check of your build as this is not what you want in runtime.