# LibraryMixin.java

    /*
     * Copyright 2009 Niclas Hedhman.
     * Copyright 2012 Paul Merlin.
     *
     * Licensed  under the  Apache License,  Version 2.0  (the "License");
     * you may not use  this file  except in  compliance with the License.
     * You may obtain a copy of the License at
     *
     *   http://www.apache.org/licenses/LICENSE-2.0
     *
     * Unless required by applicable law or agreed to in writing, software
     * distributed  under the  License is distributed on an "AS IS" BASIS,
     * WITHOUT  WARRANTIES OR CONDITIONS  OF ANY KIND, either  express  or
     * implied.
     *
     * See the License for the specific language governing permissions and
     * limitations under the License.
     */
    package org.qi4j.tutorials.services.step3;

    import java.util.ArrayList;
    import java.util.HashMap;
    import org.qi4j.api.injection.scope.Structure;
    import org.qi4j.api.value.ValueBuilder;
    import org.qi4j.api.value.ValueBuilderFactory;

    public class LibraryMixin
        implements Library
    {
        @Structure
        ValueBuilderFactory factory;
        private HashMap<String, ArrayList<Book>> books;

        public LibraryMixin()
        {
            books = new HashMap<String, ArrayList<Book>>();
        }

        @Override
        public void createInitialData()
        {
            createBook( "Eric Evans", "Domain Driven Design", 2 );
            createBook( "Andy Hunt", "Pragmatic Programmer", 3 );
            createBook( "Kent Beck", "Extreme Programming Explained", 5 );
        }

        @Override
        public Book borrowBook( String author, String title )
        {
            String key = constructKey( author, title );
            ArrayList<Book> copies = books.get( key );
            if( copies == null )
            {
                System.out.println( "Book not available." );
                return null; // Indicate that book is not available.
            }
            Book book = copies.remove( 0 );
            if( book == null )
            {
                System.out.println( "Book not available." );
                return null; // Indicate that book is not available.
            }
            System.out.println( "Book borrowed: " + book.title().get() + " by " + book.author().get() );
            return book;
        }

        @Override
        public void returnBook( Book book )
        {
            System.out.println( "Book returned: " + book.title().get() + " by " + book.author().get() );
            String key = constructKey( book.author().get(), book.title().get() );
            ArrayList<Book> copies = books.get( key );
            if( copies == null )
            {
                throw new IllegalStateException( "Book " + book + " was not borrowed here." );
            }
            copies.add( book );
        }

        private void createBook( String author, String title, int copies )
        {
            ArrayList<Book> bookCopies = new ArrayList<Book>();
            String key = constructKey( author, title );
            books.put( key, bookCopies );

            for( int i = 0; i < copies; i++ )
            {
                ValueBuilder<Book> builder = factory.newValueBuilder( Book.class );
                Book prototype = builder.prototype();
                prototype.author().set( author );
                prototype.title().set( title );

                Book book = builder.newInstance();
                bookCopies.add( book );
            }
        }

        private String constructKey( String author, String title )
        {
            return author + "::" + title;
        }
    }