Same method for multiple classes that implement the same interface

Question

Sorry I really didn't know how to title the question, here's the problem...

I have an interface and multiple classes that implement the interface. The implementation of some of the methods in the interface are exactly the same in every implementing class. I feel like there should be a way to simplify this so I don't have to write the same code every time. Example:

public interface Foo {
    String getName();
}

public class FooImpl1 implements Foo {
    private String name = "foo name1";

    public String getName() {
        return name;
    }
}

public class FooImpl2 implements Foo {
    private String name = "foo name2";

    public String getName() {
        return name;
    }
}

So to break down..

is there a way to put the code for getName in one place and each class has it's own name variable?
is there a way to make getName static so I don't have to create a new instance

Have better ideas?

You could define a abstract class that is the parent of those two and that contains the name field and the get name method. Child classes will not need to implement it. — Edwin Dalorzo
– Edwin Dalorzo, Commented Aug 31, 2014 at 21:16

Community · Accepted Answer · 2017-05-23 12:09:05Z

12

Use an abstract class and initialize the field in the constructor:

public abstract class Foo {

 protected String name;

 String getName() {
  return name;
 }
}

public class FooImpl1 extends Foo {

 public FooImpl1 () {
  name = "foo name1";
 }
}

public class FooImpl2 extends Foo {

 public FooImpl1 () {
  name = "foo name2";
 }
}

JB Nizlet pointed out that it would be cleaner to do something like this:

public abstract class Foo {

protected String name;

public Foo(String newName) {
    name = newName;
}

 String getName() {
  return name;
 }
}

And then call super("foo name 1") in the subclasses.

As @Peter Lawrey said, if you already have an interface or want one for some other reason:

public interface IFoo {
 String getName();
}

public abstract class Foo implements IFoo {

 protected String name;

 String getName() {
  return name;
 }
}

edited May 23, 2017 at 12:09

CommunityBot

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answered Aug 31, 2014 at 21:18

Anubian Noob

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2 Comments

Peter Lawrey Over a year ago

+1 If you need an interface, the abstract class can implement the interface.

JB Nizet Over a year ago

Would be cleaner to take the name in the superclass constructor and to call super(name)

OldCurmudgeon · Accepted Answer · 2014-08-31 22:23:25Z

1

You could take a very simple approach:

public interface Foo {

    String getName();
}

public class Named implements Foo {

    String name;

    public String getName() {
        return name;
    }
}

public class FooImpl1 extends Named {
    {name = "foo name1";}
}

public class FooImpl2 extends Named {
    {name = "foo name2";}
}

answered Aug 31, 2014 at 22:23

OldCurmudgeon

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2 Comments

DaoWen Over a year ago

@AnubianNoob - Assuming you're talking about the {name = ...} parts, those are initialization blocks. On another note, I always find questions/comments asking "is this valid" pointless because you can usually just run the code through a compiler/interpreter as easily as ask if it's valid, and you get an immediate answer.

Anubian Noob Over a year ago

Well I know it's valid. What I'm asking is what it's called (thanks) and maybe even a link (thanks a lot).

Janick Bernet · Accepted Answer · 2014-09-02 10:30:17Z

It doesn't really work for getters like in your example, but since the question is more generic I think it's worth mentioning that with Java 8 you can define default methods in interfaces. So if you had something a bit more complex like:

public interface ComplexFoo {
    String getFirstName();
    String getLastName();

    String getFullName();
}

You do not have to repeat the getFullName implementation in every subclass, but can specify it in the interface:

public interface ComplexFoo {
    String getFirstName();
    String getLastName();

    default String getFullName() {
        return getFirstName() + " " + getLastName();
    }
}

The getters though still need repeating, since they access members that are not yet available in the interface:

public class FooImpl1 implements ComplexFoo {
    private String firstName = "foo";
    private String lastName = "name1;

    private String getFirstName() { return firstName; }
    private String getLastName() { return lastName; }
}

public class FooImpl2 implements ComplexFoo {
    private String firstName = "foo";
    private String lastName = "name2;

    private String getFirstName() { return firstName; }
    private String getLastName() { return lastName; }
}

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