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A common support library.
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/*
 * Copyright 2016 UsefulMilk
 *
 * 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 com.usefulmilk.support.domain;

import com.usefulmilk.support.Validate;

import java.util.Objects;

/**
 * It is a base class for domain objects.
 * {@link DomainObject} is a super class of {@link Entity} class and {@link ValueObject} class.
 * The idea behind this class is supply a default way to create identities by using generics.
 */
abstract class DomainObject {
    private T id;

    /**
     * Set an identity for this object.
     * @param id The identity of this object.
     */
    public void setId(T id){
        if(this.id != null && !this.id.equals(id)){
            throw new IllegalStateException("Id was already set. You cannot change it.");
        }

        this.id = Validate.nonNullArgument(id);
    }

    /**
     * Get an identity of this object.
     * @return The identity of this object.
     */
    public T getId() {
        return id;
    }

    @Override
    public abstract int hashCode();

    @Override
    public abstract boolean equals(Object obj);

    /**
     * Check if two {@link DomainObject} instances are equals.
     * @param domainObject1 A {@link DomainObject} to be compared.
     * @param domainObject2 A {@link DomainObject} to be compared.
     * @return Return true it they are equals.
     */
    static boolean idEquals(DomainObject domainObject1, DomainObject domainObject2) {
        if (domainObject1 == null && domainObject2 == null){
            return false;
        }

        if (domainObject1 == domainObject2){
            return true;
        }

        if (domainObject1 == null ||
                domainObject2 == null ||
                domainObject1.getClass() != domainObject2.getClass()){
            return false;
        }

        return Objects.equals(domainObject1.getId(), domainObject2.getId());
    }

    /**
     * Get the hashCode of Id.
     * @param domainObject The {@link DomainObject} owner of Id.
     * @return Return the hashCode of Id.
     */
    static int getIdHashCode(DomainObject domainObject) {
        return Objects.hash(domainObject.getId());
    }
}