why does this:
import java.util.*;
public class my {
public static void main(String[] a) {
TreeMap<TreeSet<Integer>, String> map = new TreeMap<TreeSet<Integer>, String>();
TreeSet<Integer> set = new TreeSet<Integer>();
map.put(set, "lol");
}
}
work in Java 6? I mean that by specification putting TreeSet as a key of TreeMap without proper Comparator should lead to ClassCastException but it doesn't when running under Java 6. Was it a bug or there were some specification changes in Java 7 that made it work correctly (i.e. throw ClassCastException)?
This is allowed in Java 7 too, not just Java 6.
The put method will throw a ClassCastException if the key is not of a type that implements Comparable. The class could have been designed to throw the exception in the constructor, but it doesn't because
(a) generic parameter types are subjected to type erasure, so the actual TreeMap sees everything as a plain Object and can't apply any checks in the constructor;
(b) this way allows you to use any object as a key, as long as it is of a subclass that implements Comparable, even if you don't initially declare the TreeMap to take a comparable key type.
Related
In a legacy code, I'm working with, I found the following thing:
#Autowired
final lateinit var controller: CustomController
what does this final keyword mean here?
In a Kotlin documentation I found a short description about final keyword that is blocking overriding of the methods in open classes but no information about fields. Also - the class within which I found the line is not open
A final property or a method in Kotlin prevents overriding of the field / method. That being said, Kotlin by default considers a property or a method/function to be final unless specified by the keyword open. In your case, the final keyword is redundant.
Here's a small demo test case to illustrate the same.
open class Parent {
open val someValue = 0
final val otherValue = 13 // redundant modifier 'final' warning in Android Studio
}
class Child : Parent() {
override val someValue = 5
// override val otherValue = 19 // compile error
}
There is an interesting problem called Fragile Base Class in OOP and why some languages like Kotlin prefer final by default.
What you have there is a property, not a field.
It looks just like a field, as it would in Java; but in Kotlin, it actually defines a public getter method, a public setter method, and a private backing field*.
So the final modifier applies to the accessor methods, preventing those from being overridden in a subclass. (As you say, the backing field itself can't be overridden anyway.)
As Siddharth says, final is the default in Kotlin, so you usually wouldn't need to specify it, though there are a few situations in which it would be needed — e.g. if it were already overriding something, or you were using the all-open or kotlin-spring compiler plug-ins. (The use of #Autowired suggests that this is a Spring module, which probably explains why final is needed here.) In any case, your IDE would probably indicate where it's not needed, e.g. by showing it greyed-out.
(* Only the getter is necessary; the setter isn't generated for a val, and the backing field isn't generated if you override the accessor(s) and they don't refer to it.)
I was playing with the following question: Using Java 8's Optional with Stream::flatMap and wanted to add a method to a custom Optional<T> and then check if it worked.
More precise, I wanted to add a stream() to my CustomOptional<T> that returns an empty stream if no value is present, or a stream with a single element if it is present.
However, I came to the conclusion that Optional<T> is declared as final.
Why is this so? There are loads of classes that are not declared as final, and I personally do not see a reason here to declare Optional<T> final.
As a second question, why can not all methods be final, if the worry is that they would be overridden, and leave the class non-final?
According to this page of the Java SE 8 API docs, Optional<T> is a value based class. According to this page of the API docs, value-based classes have to be immutable.
Declaring all the methods in Optional<T> as final will prevent the methods from being overridden, but that will not prevent an extending class from adding fields and methods. Extending the class and adding a field together with a method that changes the value of that field would make that subclass mutable and hence would allow the creation of a mutable Optional<T>. The following is an example of such a subclass that could be created if Optional<T> would not be declared final.
//Example created by #assylias
public class Sub<T> extends Optional<T> {
private T t;
public void set(T t) {
this.t = t;
}
}
Declaring Optional<T> final prevents the creation of subclasses like the one above and hence guarantees Optional<T> to be always immutable.
As others have stated Optional is a value based class and since it is a value based class it should be immutable which needs it to be final.
But we missed the point for this. One of the main reason why value based classes are immutable is to guarantee thread safety. Making it immutable makes it thread safe. Take for eg String or primitive wrappers like Integer or Float. They are declared final for similar reasons.
Probably, the reason is the same as why String is final; that is, so that all users of the Optional class can be assured that the methods on the instance they receive keep to their contract of always returning the same value.
Though we could not extend the Optional class, we could create our own wrapper class.
public final class Opt {
private Opt() {
}
public static final <T> Stream<T> filledOrEmpty(T t) {
return Optional.ofNullable(t).isPresent() ? Stream.of(t) : Stream.empty();
}
}
Hope it might helps you. Glad to see the reaction!
Putting extra elements property in the class to support backward/forward compatibility and implement ISupportInitialize seems ugly for me and it is also OCP violation.
I want to handle extra elements outside of deserialized class and run some migration logic in external classes.
I mean that Bson serializer will put extra elements data somewhere not on the deserialized class and than after finishing all deserialization staff call some migrator for loaded object.
That way I can can support compatibility between fetched document (that may be in older or newest version) and currently running code.
Something like:
public interface IMigrate<T>
{
void Migrate(T obj, BsonDocument extraElements)
}
public class MigrateClazzA : IMigrate<ClazzA>
{
public void Migrate(ClazzA obj, BsonDocument extraElements)
{
...
}
}
How can I do it?
I'm facing a problem with Spring and restTemplate. I want to send an object (ListResponse) that contains a generic array. The defenition is as follow:
public class ListResponse<T> implements Serializable {
private long total;
private int page;
private int pageSize;
private T[] objects;
I send a request whith restTemplate.getForObject(). As a result I get an object of type ListResponse but the objects array contains an array of LinkedHashMaps instead of an array with objects of type T.
It seems like restTemplate can not convert the elements in the array to their correct type.
How can I make sure that I get an array of objects of type T back ?
I had this problem today and here is the solution that I came up with (actually, that one of my co-workers suggested). We use it with an interface that returns List<MyDto>.
When you call the RestTemplate, don't pass in the generic type.
Define: public class MyDtoListTemplate extends ListTemplate<MyDto>
Then, call
MyDtoListTemplate template = restTemplate.getForObject("url", MyDtoListTemplate .class, mapOfPathVariables);
It's a bummer that you have to define a concrete class that extends/implements the generic type, but then the generic information is available to the jackson deserializer.
I remember I was able to deserialize generic classes with Jackson 2. I had to add MappingJackson2HttpMessageConverter converter to RestTemplate before making any Http calls with it.
RestTemplate template = new RestTemplate();
template.getMessageConverters().add(new MappingJackson2HttpMessageConverter());
Unfortunately there's no easy way of doing it that I know of. The problem is that the RestTemplate is told which object type to expect. As long as all the fields in this object has a corresponding element in the json/xml, everything works fine. In the case of generics, the serializer doesn't know which class to expect so it just turns the map it gets to a java Map.
You will have the same problem if you tried to getForObject for a generic return type.
As Bloch states in Item 3 ("Enforce the singleton property with a private constructor or an enum type") of Effective Java 2nd Edition, a single-element enum type is the best way to implement a singleton. Unfortunately the old private constructor pattern is still very widespread and entrenched, to the point that many developers don't understand what I'm doing when I create enum singletons.
A simple // Enum Singleton comment above the class declaration helps, but it still leaves open the possibility that another programmer could come along later and add a second constant to the enum, breaking the singleton property. For all the problems that the private constructor approach has, in my opinion it is somewhat more self-documenting than an enum singleton.
I think what I need is an annotation which both states that the enum type is a singleton and ensures at compile-time that only one constant is ever added to the enum. Something like this:
#EnumSingleton // Annotation complains if > 1 enum element on EnumSingleton
public enum EnumSingleton {
INSTANCE;
}
Has anyone run across such an annotation for standard Java in public libraries anywhere? Or is what I'm asking for impossible under Java's current annotation system?
UPDATE
One workaround I'm using, at least until I decide to actually bother with rolling my own annotations, is to put #SuppressWarnings("UnusedDeclaration") directly in front of the INSTANCE field. It does a decent job of making the code look distinct from a straightforward enum type.
You can use something like this -
public class SingletonClass {
private SingletonClass() {
// block external instantiation
}
public static enum SingletonFactory {
INSTANCE {
public SingletonClass getInstance() {
return instance;
}
};
private static SingletonClass instance = new SingletonClass();
private SingletonFactory() {
}
public abstract SingletonClass getInstance();
}
}
And you can access in some other class as -
SingletonClass.SingletonFactory.INSTANCE.getInstance();
I'm not aware of such an annotation in public java libraries, but you can define yourself such a compile time annotation to be used for your projects. Of course, you need to write an annotation processor for it and invoke somehow APT (with ant or maven) to check your #EnumSingleton annoted enums at compile time for the intended structure.
Here is a resource on how to write and use compile time annotations.