Class that generates a valid Salesforce ID given a object type.

IDGenerator.cls

/**
 * The MIT License (MIT)
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to
 * deal in the Software without restriction, including without limitation the
 * rights to use, copy, modify, merge, publish, distribute, sublicense, and/or
 * sell copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
 * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
 * IN THE SOFTWARE.
 */
public without sharing class IDGenerator {
    private static Integer s_num = 1;

    /**
     * Generates an Id for the object type specified.
     * @param  sot Objetct Type
     * @return     Id generated
     */
    public static String generate(Schema.SObjectType sot) {
        String result = String.valueOf(s_num++);
        String result_id = sot.getDescribe().getKeyPrefix() + '0'.repeat(12-result.length()) + result;
        return result_id;
    }

    public static String generateByPrefix(String prefix) {
        String result = String.valueOf(s_num++);
        String result_id = prefix + '0'.repeat(12-result.length()) + result;
        return result_id;
    }
}