Not implementing all the methods of an interface

Question

I tried reproducing the code below on eclipse. I get an error telling me that I have to implement all the inherited methods (because Comparator is an interface).

The type new Comparator(){} must implement the inherited abstract method Comparator.reversed().

There are many of these methods and the only one I want to overwrite is compare. Do I have to implement all the other methods or is there a way to specify I don't need to implement them? I understand that I will have to do it because of the contractual nature of an interface, but what if I just need to change one method?

static Map sortByValue(Map map) {
     List list = new LinkedList(map.entrySet());
     Collections.sort(list, new Comparator() {
          public int compare(Object o1, Object o2) {
               return ((Comparable) ((Map.Entry) (o1)).getValue())
              .compareTo(((Map.Entry) (o2)).getValue());
          }
     });

    Map result = new LinkedHashMap();
    for (Iterator it = list.iterator(); it.hasNext();) {
        Map.Entry entry = (Map.Entry)it.next();
        result.put(entry.getKey(), entry.getValue());
    }
    return result;
}

EDIT Solved by changing the compliance level to java8 in eclipse luna. Thanks!

possible duplicate of [not implementing all of the methods of interface. is it possible?](http://stackoverflow.com/questions/11437097/not-implementing-all-of-the-methods-of-interface-is-it-possible) — mbomb007, Jan 28 '15 at 22:07
Are you sure you should not use any kind of generics at all in your code? Is this for Java 1.4? — EpicPandaForce, Jan 28 '15 at 22:08
@mbomb007, you realize that OP have already linked to that question in his post? — aioobe, Jan 28 '15 at 22:08
@aioobe I get an error message on eclipse: The type new Comparator(){} must implement the inherited abstract method Comparator.reversed() then if I apply the fix, I have many functions added — giulio, Jan 28 '15 at 22:08
This is a wild guess: I think you are using Java8 in a pre-Java8-Eclipse (i.e. pre-Luna). So Eclipse does not know that all those new Comparator methods, like `thenComparing`, have a `default` implementation. — tobias_k, Jan 28 '15 at 22:09
@EpicPandaForce I don't know exactly, I am rather new to java. I am using java 8 — giulio, Jan 28 '15 at 22:10
@tobias_k actually I am using eclipse luna and java8 and the fix proposed gives me the default implementations — giulio, Jan 28 '15 at 22:10

score 17 · Accepted Answer · edited May 23 '17 at 11:54

17

The type new Comparator(){} must implement the inherited abstract method Comparator.reversed() then if I apply the fix, I have many functions added

Comparator.reversed was introduced in Java 1.8 and it's a default method i.e. a method that you don't have to override.

It seems like you have your compliance level set to pre Java 1.8 (since Eclipse asks you to override reversed), while using Java 1.8 API (since Comparator has a reversed method).

Make sure you either change your API to 1.7 or change your compliance level to 1.8. (The latter option requires Eclipse Luna or better.)

More on Eclipse compliance level: What is "compiler compliance level" in Eclipse?

edited May 23 '17 at 11:54

Community

1
1

answered Jan 28 '15 at 22:09

aioobe

413,195
112
811
826

Just tested this, and at least in Luna, a project with compliance level 1.7 builds fine with a 1.8 JDK and does not demand for all those methods to be implemented... – tobias_k Jan 28 '15 at 22:20
Ok. Well such setup seems broken to me anyway. – aioobe Jan 28 '15 at 22:21

score 5 · Answer 2 · answered Jan 28 '15 at 22:10

This is sort of a wild guess: I think you are using Java 8 in a pre-Java 8-Eclipse (i.e. pre-Luna). This way, Eclipse does not know that all those new Comparator methods, like thenComparing, have a default implementation.

In Java 7, Comparator has just the method compare, while in Java 8 in has a whole lot more methods, all of which have a default implementation directly in the interface and need not to be implemented in a subclass.

I suggest you switch to the newest version of Eclipse, which should be Luna. Alternatively, you can also install a patch for Eclipse Kepler, but switching to Luna is certainly better.

score 1 · Answer 3 · answered Jan 28 '15 at 22:13

The purpose of an interface is to obligate the implementer of said interface to have all of the methods listed there. Now there are a few ways to got about not implementing all of the methods.

For every method you don't wish to implement (support) just throw UnsupportedOperationException. An example of this sort of implementation would be the Collections API.

To keep the number of core collection interfaces manageable, the Java platform doesn't provide separate interfaces for each variant of each collection type. (Such variants might include immutable, fixed-size, and append-only.) Instead, the modification operations in each interface are designated optional — a given implementation may elect not to support all operations. If an unsupported operation is invoked, a collection throws an UnsupportedOperationException. Implementations are responsible for documenting which of the optional operations they support. All of the Java platform's general-purpose implementations support all of the optional operations.

Or you could follow what the Java developers did with some of the interfaces that declare a lot of methods eg: MouseAdapter. This way you'll implement every method, but they won't be doing anything useful.

score 0 · Answer 4 · answered Jan 28 '15 at 22:04

0

An abstract class may be better suited for what you are doing. Although you could just .super() everything.

answered Jan 28 '15 at 22:04

James Parsons

6,097
12
68
108

did not mean an abstract class that implements the interface ? – Kick Buttowski Jan 28 '15 at 22:09
no, I mean rewrite it as an abstract class, that way, you can choose what to implement. I believe java swing has a solution like this for event listeners, I just cant remember what they are called – James Parsons Jan 28 '15 at 22:11
Could you give an example please? (where shall I put the super?) – giulio Jan 28 '15 at 22:11
1

Adapter classes is what they are called – James Parsons Jan 28 '15 at 22:12
from what I read in a book. you should have an abstract class that implements the interface, but in long run, you have to implement all functions later somewhere hope it makes sense – Kick Buttowski Jan 28 '15 at 22:13

score 0 · Answer 5 · answered Jan 28 '15 at 22:16

You have run into an Eclipse bug: https://bugs.eclipse.org/bugs/show_bug.cgi?id=390889

Basically, JAVA 1.8 introduced a brand new method Comparator.reversed(). And since you have a JAVA 1.7 or earlier code, JAVA 1.8 doesn't find the method and fails to compile.

Not implementing all the methods of an interface

5 Answers5

Linked