How to implement the sorting of custom class key values in TreeMap

Updated to 19 days ago

In TreeMap, if the key value is the basic type in Java, TreeMap will sort it for us. If it is a custom type, if we define a Student class as the key value, if we do not inform the sorting rules, we will report an error when running. So, how to implement custom sorting? There are 2 solutions, just choose one of them:

Implement Comparable in Student class and override the compareTo method
In the constructor, new Comparator, anonymous internal class, override the compare method

TreeMap<String,List<ZTreeDTO>> retultMap = new TreeMap<String,List<ZTreeDTO>>(new Comparator<String>(){
			@Override
			public int compare(String o1, String o2) {
				o1 = getTypeByProjectName(o1);
            	o2 = getTypeByProjectName(o2);
                return  (o2);
			}
        });