# BalanceData.java

    /*
     * Copyright (c) 2010, Rickard Öberg. All Rights Reserved.
     *
     * 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.dci.moneytransfer.domain.data;

    import org.qi4j.api.common.UseDefaults;
    import org.qi4j.api.injection.scope.State;
    import org.qi4j.api.mixin.Mixins;
    import org.qi4j.api.property.Property;
    import org.qi4j.library.constraints.annotation.GreaterThan;

    /**
     * Maintain a balance for an account, of any type. Methods are in past tense,
     * and are considered events. Only minimal sanity checks are performed.
     * The roles invoking these methods decide if it's ok to perform the change or not.
     */
    @Mixins( BalanceData.Mixin.class )
    public interface BalanceData
    {
        void increasedBalance( @GreaterThan( 0 ) Integer amount );

        void decreasedBalance( @GreaterThan( 0 ) Integer amount );

        Integer getBalance();

        // Default implementation

        class Mixin
            implements BalanceData
        {
            @State
            @UseDefaults
            Property<Integer> balance;

            public void increasedBalance( Integer amount )
            {
                balance.set( balance.get() + amount );
            }

            public void decreasedBalance( Integer amount )
            {
                balance.set( balance.get() - amount );
            }

            public Integer getBalance()
            {
                return balance.get();
            }
        }
    }