计数具有相同值的对象的频率数

Question

ArrayList<Observer> arrlist = new ArrayList<Observer>();

    public class  Observer{
    private String name;
}

Answer 1

您基本上需要迭代观察器，并计算每个键的发生次数（可以是单个属性，也可以是组合，这取决于您的需要）。

以observer.name为例，您可以执行以下操作：

Map<String, List<Observer>> observersByName = 
       arrlist.stream().collect(Collectors.groupingBy(Observer::getName));

然后迭代条目集，并在值列表上调用size()以获得频率。

Map<String, Long> nameFrequency = arrlist.stream().collect(
                                     Collectors.groupingBy(Observer::getName,
                                                           Collectors.counting()));

Map<String, Integer> nameFrequency = new HashMap<>();
for( Observer obs : arrlist ) {
   //use the name as the key, associate a value of 1 with each key
   //if an entry already existed merge existing and new value by summing them
   frequency.merge( obs.getName(), 1, Integer::sum);
}