对一个 List 里面相同属性的内容进行分类，如：对相同部门的员工进行分类。
思路是使可以区分类型的属性进行分类，以部门类型为例。

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List<Devloper> devlopers = new ArrayList<>();
devlopers.add(new Devloper(1, "嗯哼", 1));
devlopers.add(new Devloper(2, "啊哈", 1));
devlopers.add(new Devloper(3, "哦吼", 1));
devlopers.add(new Devloper(4, "呃呵", 2));
devlopers.add(new Devloper(5, "呜唿", 2));

Map<Integer, List<Devloper>> devloperMap = new HashMap<>();
for (Devloper devloper: devlopers) {
	if (devloperMap.containsKey(devloper.getDept())) {
		devloperMap.get(devloper.getDept()).add(devloper);
	} else {
		List<Devloper> devs = new ArrayList<>();
		devs.add(devloper);
		devloperMap.put(devloper.getDept(), devs);
	}
}

# 结果：

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{
1=[
	Devloper{id=1, name='嗯哼', dept=1}, 
	Devloper{id=2, name='啊哈', dept=1}, 
	Devloper{id=3, name='哦吼', dept=1}
	], 
2=[
	Devloper{id=4, name='呃呵', dept=2}, 
	Devloper{id=5, name='呜唿', dept=2}
	]
}