Adapter

Adapter lets classes work together that could not otherwise because of incompatible interfaces.

Example - User adaptation

Lets say we work for eBay company and there are other users we need to integrate with our systems. Our, eBay, user is defined by EbayUser interface. External users are represented by class ExternalUser, but we do not want to mess with this class in our code and that is why we create an adapter that will adapt external user into our system.

interface EbayUser {
    String getUserName();
}

class EbayUserImpl implements EbayUser {

    private String userName;

    public EbayUserImpl(String userName) {
        this.userName = userName;
    }

    @Override
    public String getUserName() {
        return userName;
    }
}

class ExternalUserToEbayUserAdapter implements EbayUser {

    private String userName;

    public ExternalUserToEbayUserAdapter(ExternalUser payPalUser) {
        userName = payPalUser.getUsername();
    }

    @Override
    public String getUserName() {
        return userName;
    }
}

class ExternalUser {

    private String username;

    public ExternalUser(String username) {
        this.username = username;
    }

    public String getUsername() {
        return username;
    }
}

Here is an example how we could adapt an external user into our system.

ExternalUser externalUser = new ExternalUser("john");

EbayUser ebayUser = new ExternalUserToEbayUserAdapter(externalUser);