public class CustomOperation {
    public int customAdd(final String a, final String b) {
        return Integer.valueOf(a) + Integer.valueOf(b);
    }
}

public class Operation {
    private CustomOperation adaptee;

    Operation(final CustomOperation adaptee) {
        this.adaptee = adaptee;
    }

    public int add(final int a, final int b) {
        return adaptee.customAdd(String.valueOf(a), String.valueOf(b));
    }
}

public class AdapterPatternTest {
    public static void main(String[] args) {
        final CustomOperation adaptee = new CustomOperation();
        final Operation adapter = new Operation(adaptee);
        System.out.println(adapter.add(2, 3));
    }
}