Java按颜色分组Widget并汇总属性生成Map的实现方法

步骤1：修正Widget类的getter/setter #

原示例代码中存在属性名与setter方法名不匹配的问题（比如colour属性对应setColor，legs属性对应setLeg），先修正为标准的getter/setter：

class Widget {
    private String colour;
    private Double legs;
    private Double arms;

    public String getColour() {
        return colour;
    }

    public void setColour(String colour) {
        this.colour = colour;
    }

    public Double getLegs() {
        return legs;
    }

    public void setLegs(Double legs) {
        this.legs = legs;
    }

    public Double getArms() {
        return arms;
    }

    public void setArms(Double arms) {
        this.arms = arms;
    }
}

步骤2：定义汇总结果类 #

创建一个WidgetSummary类来存储每组的legs和arms总和，同时提供合并方法：

class WidgetSummary {
    private Double totalLegs = 0.0;
    private Double totalArms = 0.0;

    // 合并单个Widget到当前汇总结果
    public void merge(Widget widget) {
        this.totalLegs += widget.getLegs();
        this.totalArms += widget.getArms();
    }

    // 合并两个汇总结果（支持并行流）
    public WidgetSummary combine(WidgetSummary other) {
        this.totalLegs += other.totalLegs;
        this.totalArms += other.totalArms;
        return this;
    }

    @Override
    public String toString() {
        return String.format("Legs(%.0f), Arms(%.0f)", totalLegs, totalArms);
    }
}

步骤3：Stream分组汇总实现 #

使用Collectors.groupingBy按colour分组，下游收集器用Collector.of自定义汇总逻辑：

public class WidgetGroupingDemo {
    public static void main(String[] args) {
        List<Widget> widList = new ArrayList<>();
        
        Widget wid1 = new Widget();
        wid1.setColour("Yello");
        wid1.setLegs(10.0);
        wid1.setArms(10.0);
        
        Widget wid2 = new Widget();
        wid2.setColour("Yello");
        wid2.setLegs(20.0);
        wid2.setArms(30.0);
        
        Widget wid3 = new Widget();
        wid3.setColour("White");
        wid3.setLegs(20.0);
        wid3.setArms(30.0);

        // 分组并汇总
        Map<String, WidgetSummary> result = widList.stream()
                .collect(Collectors.groupingBy(
                        Widget::getColour,
                        Collector.of(
                                WidgetSummary::new,
                                WidgetSummary::merge,
                                WidgetSummary::combine
                        )
                ));

        // 打印结果
        result.forEach((colour, summary) -> System.out.println("\"" + colour + "\" : " + summary));
    }
}

运行结果 #

"Yello" : Legs(30), Arms(40)
"White" : Legs(20), Arms(30)

替代方案：使用reducing收集器 #

如果不想自定义收集器，也可以用Collectors.reducing实现：

Map<String, WidgetSummary> result = widList.stream()
        .collect(Collectors.groupingBy(
                Widget::getColour,
                Collectors.reducing(
                        new WidgetSummary(),
                        widget -> {
                            WidgetSummary summary = new WidgetSummary();
                            summary.merge(widget);
                            return summary;
                        },
                        WidgetSummary::combine
                )
        ));

内容的提问来源于stack exchange，提问作者Mohit H