Editing Adapter pattern (section)

===A further form of runtime adapter pattern===
====Motivation from compile time solution====
It is desired for {{Java|classA}} to supply {{Java|classB}} with some data, let us suppose some {{Java|String}} data. A compile time solution is:

<syntaxhighlight lang=Java>
classB.setStringData(classA.getStringData());
</syntaxhighlight>

However, suppose that the format of the string data must be varied. A compile time solution is to use inheritance:

<syntaxhighlight lang=Java>
public class Format1ClassA extends ClassA {
    @Override
    public String getStringData() {
        return format(toString());
    }
}
</syntaxhighlight>

and perhaps create the correctly "formatting" object at runtime by means of the [[factory pattern]].

====Run-time adapter solution====
A solution using "adapters" proceeds as follows:
{{ordered list|list-style-type=lower-roman
|Define an intermediary "provider" interface, and write an implementation of that provider interface that wraps the source of the data, {{Java|ClassA}} in this example, and outputs the data formatted as appropriate:

<syntaxhighlight lang=Java>
public interface StringProvider {
    public String getStringData();
}

public class ClassAFormat1 implements StringProvider {
    private ClassA classA = null;

    public ClassAFormat1(final ClassA a) {
        classA = a;
    }

    public String getStringData() {
        return format(classA.getStringData());
    }

    private String format(final String sourceValue) {
        // Manipulate the source string into a format required 
        // by the object needing the source object's data
        return sourceValue.trim();
    }
}
</syntaxhighlight>

|Write an adapter class that returns the specific implementation of the provider:

<syntaxhighlight lang=Java>
public class ClassAFormat1Adapter extends Adapter {
    public Object adapt(final Object anObject) {
        return new ClassAFormat1((ClassA) anObject);
    }
}
</syntaxhighlight>

|Register the {{Java|adapter}} with a global registry, so that the {{Java|adapter}} can be looked up at runtime:

<syntaxhighlight lang=Java>
AdapterFactory.getInstance().registerAdapter(ClassA.class, ClassAFormat1Adapter.class, "format1");
</syntaxhighlight>

|In code, when wishing to transfer data from {{Java|ClassA}} to {{Java|ClassB}}, write:

<syntaxhighlight lang=Java>
Adapter adapter =
    AdapterFactory.getInstance()
        .getAdapterFromTo(ClassA.class, StringProvider.class, "format1");
StringProvider provider = (StringProvider) adapter.adapt(classA);
String string = provider.getStringData();
classB.setStringData(string);
</syntaxhighlight>

or more concisely:

<syntaxhighlight lang=Java>
classB.setStringData(
    ((StringProvider)
            AdapterFactory.getInstance()
                .getAdapterFromTo(ClassA.class, StringProvider.class, "format1")
                .adapt(classA))
        .getStringData());
</syntaxhighlight>

|The advantage can be seen in that, if it is desired to transfer the data in a second format, then look up the different adapter/provider:

<syntaxhighlight lang=Java>
Adapter adapter =
    AdapterFactory.getInstance()
        .getAdapterFromTo(ClassA.class, StringProvider.class, "format2");
</syntaxhighlight>

|And if it is desired to output the data from {{Java|ClassA}} as, say, image data in {{Java|Class C}}:
<syntaxhighlight lang=Java>
Adapter adapter =
    AdapterFactory.getInstance()
        .getAdapterFromTo(ClassA.class, ImageProvider.class, "format2");
ImageProvider provider = (ImageProvider) adapter.adapt(classA);
classC.setImage(provider.getImage());
</syntaxhighlight>

|In this way, the use of adapters and providers allows multiple "views" by {{Java|ClassB}} and {{Java|ClassC}} into {{Java|ClassA}} without having to alter the class hierarchy. In general, it permits a mechanism for arbitrary data flows between objects that can be retrofitted to an existing object hierarchy.
}}
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