一些Lambda表达式的学习

1. Collectors.toMap(Department::getId, Department::getName)这样是形成一个Map<Long, String>，Department的ID为值，Name为值

2. Collectors.toMap(Department::getId, Function.identity())这样是形成一个Map<Long, Department>，Department的ID为键，Department为值

3. 当要转化成一个Map的时候，怎么判断是用Collectors.toMap还是Collectors.groupingBy呢？很简单，如果Map的值是一个List就用groupingBy，如果不是List就用toMap

Map<Long, List<Animal>> id2AnimalsMap = animals.stream().collect(Collectors.groupingBy(Animal::getId()));
Map<Long, Animal> id2AnimalMap = animals.stream().collect(Collectors.toMap(Animal::getId(), Function.identity());

4. 用forEach来循环

for (Animal animal : animalList){
/*
* process
* */
5 }

可改写为

animalList.forEach(animal -> {
/*
* process
* */
});