Inheritance in Java and using generics

Question

I have two implementations of an interface

 EncryptionService
  **ABCEncryptionServiceImpl**
     encrypt(byte[] bytes)
     decrypt(byte[] bytes)
  **XYZEncryptionServiceImpl**
     encrypt(List<String> strings)
     decrypt(List<String> strings)

What I don't like is I have to:

Provide empty implementation in ABCEncryptionServiceImpl for methods:

encrypt(List<String> strings)
decrypt(List<String> strings)

Provide empty implementation in XYZEncryptionServiceImpl for methods:

encrypt(byte[] bytes)
decrypt(byte[] bytes)

Any ideas on how this issue should be handled in a better way i.e using Generics?

I don't understand. Why do they have to be empty? Do you want some default behavior? — Sotirios Delimanolis
– Sotirios Delimanolis, Commented Jul 23, 2015 at 23:16

rgettman · Accepted Answer · 2015-07-23 23:19:42Z

1

Instead of having different methods that take specific types such as byte[] and List<String>, you can declare a generic type parameter in EncryptionService.

public interface EncryptionService<T>
{
    public void encrypt(T obj);
    public void decrypt(T obj);
}

The implementation classes will define the type parameter to be a concrete type. This way, neither implementation class will need to provide implementations for encrypting/decrypting a type it doesn't cover.

public class ABCEncryptionServiceImpl implements EncryptionService<byte[]>
{
    @Override
    public void encrypt(byte[] obj) { /* implementation */ }
    @Override
    public void decrypt(byte[] obj) { /* implementation */ }
}

public class XYZEncryptionServiceImpl implements EncryptionService<List<String>>
{
    @Override
    public void encrypt(List<String> obj) { /* implementation */ }
    @Override
    public void decrypt(List<String> obj) { /* implementation */ }
}

answered Jul 23, 2015 at 23:19

rgettman

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3 Comments

Dici Over a year ago

Does T always have to be a mutable class, since you are returning no result ?

rgettman Over a year ago

@Dici It depends on the OP's actual return type, which hasn't been specified.

Dici Over a year ago

yep, which is why it is safer to makethe return type generic !

Vince · Accepted Answer · 2015-07-24 03:36:15Z

1

You could add a generic type argument to EncryptionService:

abstract class EncryptionService<T> {
    public abstract void encrypt(T item);
    public abstract void decrypt(T item);
}

You could then specify what data is to be encrypted/decrypted by using it as a type parameter when extending EncryptionService:

class ABCEncryptionServiceImpl extends EncryptionService<byte[]> {
    public void encrypt(byte[] bytes) {

    }

    public void decrypt(byte[] bytes) {

    }
}

class XYZEncryptionServiceImpl extends EncryptionService<List<String>> {
    public void encrypt(List<String> items) {

    }

   public void decrypt(List<String> items) {

   }
}

edited Jul 24, 2015 at 3:36

answered Jul 23, 2015 at 23:23

Vince

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Comments

Dici · Accepted Answer · 2015-07-23 23:22:14Z

0

Isn't it as simple as that ? If I missed something in your question, just tell me.

public interface EncryptionService<INPUT, OUTPUT> {
     OUTPUT encrypt(INPUT input);
     INTPUT decrypt(OUTPUT output);
}

public class ABCEncryptionServiceImpl extends EncryptionService<List<String>, List<String>> {
     ...
}

public class XYZEncryptionServiceImpl extends EncryptionService<byte[], byte[]> {
     ...
}

answered Jul 23, 2015 at 23:22

Dici

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3 Comments

Julie Over a year ago

what if input is a List and output is a Map for ABCEncryptionServiceImpl

Dici Over a year ago

@Julie Then just change the second type to Map : ABCEncryptionServiceImpl extends EncryptionService<List<String>, Map<String, String>> my answer is actually the only one hanle to handle this case. All the other answers have chosen a void return type, which won't work for every generic type

Dici Over a year ago

@Julie Pleased to help you ! You can accept the answer if it is helpful :D

crush · Accepted Answer · 2015-07-23 23:47:41Z

0

If I get you right, you want to make the parameters for encrypt and decrypt generics.

The naive solution would be

public interface EncryptionService<T> {
    void encrypt(T data);
    void decrypt(T data);
};

Then your implementations would be like

public class ABCEncryptionService implements EncryptionService<byte[]> {
    void encrypt(byte[] data){} 
    void decrypt(byte[] data){}
}

And

public class XYZEncryptionService implements EncryptionService<List<String>> {
    void encrypt(List<String> data){}
    void decrypt(List<String> data){}
}

edited Jul 23, 2015 at 23:47

answered Jul 23, 2015 at 23:25

crush

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4 Comments

Dici Over a year ago

byte[] is not a primitive type, it is an Array object .You can use it with generics

crush Over a year ago

@dici that's worth a -1? Tough crowd.

Dici Over a year ago

Well, maybe I downvote a bit too fast, but I remove it as the answer gets fixed. It also happens to me when I provide a wrong answer !

crush Over a year ago

I work with like 12 different languages. Can get confusing sometimes. My primary is the .net stack. Arrays are handled differently there than java. No problem though. It needed to be corrected for sure, but all it takes is a comment.

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