Constraint Streams分组用BiFunction的max报错，如何获取每组最大值？

问题描述 #

在Constraint Streams的groupBy中使用max方法时出现错误，需要实现获取每个分组的最大值。

原代码示例 #

Constraint checkScrubNurseSkill(ConstraintFactory constraintFactory) {
    return constraintFactory.forEach(NurseAssignment.class)
            .join(NurseOpeSkill.class,
                    Joiners.filtering((nurseAssignment, nurseOpeSkill) -> nurseAssignment.getOpeID() == nurseOpeSkill.getOpeID() &&
                    nurseAssignment.getNurseID() == nurseOpeSkill.getNurseID()))
            .groupBy((nurseAssignment, nurseOpeSkill) -> nurseAssignment.getOpe(),
                     (nurseAssignment, nurseOpeSkill) -> nurseAssignment.getDutyID(),
                    max((nurseAssignment, nurseOpeSkill) -> nurseOpeSkill.getScurbOpeLevel() )
                    )
            .filter((ope, dutyid,  max) -> max < 3)
            .penalize("Check Scrub Nurse Skill", HardSoftScore.ofHard(10));
}

报错信息（中文翻译） #

• error1：方法max(Comparator)对于类型○○ConstraintProvider不明确
• error2：方法getScurbOpeLevel()对于类型Object未定义

解决方案 #

OptaPlanner的Constraint Streams中，groupBy的聚合逻辑需要通过Collectors工具类提供的聚合器实现，不能直接调用max方法。同时需要处理类型推断和maxBy返回的Optional对象，具体修改如下：

修改后的代码 #

Constraint checkScrubNurseSkill(ConstraintFactory constraintFactory) {
    return constraintFactory.forEach(NurseAssignment.class)
            .join(NurseOpeSkill.class,
                    Joiners.filtering((nurseAssignment, nurseOpeSkill) -> 
                            nurseAssignment.getOpeID() == nurseOpeSkill.getOpeID() &&
                            nurseAssignment.getNurseID() == nurseOpeSkill.getNurseID()))
            .groupBy(
                    (nurseAssignment, nurseOpeSkill) -> nurseAssignment.getOpe(),
                    (nurseAssignment, nurseOpeSkill) -> nurseAssignment.getDutyID(),
                    // 使用Collectors.maxBy定义最大值聚合逻辑，明确指定比较的是技能等级
                    Collectors.maxBy(Comparator.comparingInt((nurseAssignment, nurseOpeSkill) -> 
                            nurseOpeSkill.getScurbOpeLevel()))
            )
            // 处理Optional，先判断是否存在最大值，再比较数值
            .filter((ope, dutyId, maxLevelOptional) -> 
                    maxLevelOptional.isPresent() && maxLevelOptional.get().getSecond().getScurbOpeLevel() < 3)
            .penalize("Check Scrub Nurse Skill", HardSoftScore.ofHard(10));
}

关键修改点说明 #

1. 聚合器替换：用Collectors.maxBy(Comparator.comparingInt(...))替代原代码中的max方法，这是Constraint Streams支持的聚合方式，明确告诉编译器如何计算分组内的最大值。
2. 类型推断修正：通过comparingInt指定比较的是整数类型，避免编译器将参数推断为Object，解决getScurbOpeLevel()未定义的错误。
3. Optional处理：maxBy返回Optional类型（分组可能为空），因此在filter中需要先调用isPresent()判断是否存在有效值，再获取数值进行比较。

内容的提问来源于stack exchange，提问作者tsushimaru