问题
I'm trying to sort list of maps based on specific key lets say "tax" with int values in all the maps. However other keys like "id" and "price" have string and double type values respectively as shown below.
List<Map<String, Object>> taxes = Arrays.asList(Map.of("id", "3", "tax", 25, "price", 8.05),
Map.of("id", "9", "tax", 37, "price", 16.05), Map.of("id", "2", "tax", 19, "price", 9.25),
Map.of("id", "11", "tax", 28, "price", 16.05));
I want to sort list of maps with highest tax value to lowest tax value. I'm trying with below approach but getting error at m.get("tax").
list.stream()
.sorted(Comparator.comparing(map -> map.get("tax")).reversed())
.collect(Collectors.toList());`
回答1:
You are getting the error because compile could not interpret the type of object in a lambda expression, you can solve it but just interpreting map as Map<String, Integer>
list.stream()
.sorted(Comparator.comparing((Map<String,Integer> map) -> map.get("tax")).reversed())
.collect(Collectors.toList());
But this is not completely fixed and it will fail when tax is not presenting in any Map or having null as value, you can solve that using Comparator.nullsFirst or Comparator.nullsLast
List<Map<String,Integer>> taxes = List.of(Map.of("id",3, "tax",25),
Map.of("id",9, "tax",37),Map.of("id",2, "tax",19),
Map.of("id",11));
List<Map<String,Integer>> results = taxes.stream()
.sorted(Comparator.comparing((Map<String,Integer> map)->map.get("tax"),Comparator.nullsFirst(Comparator.naturalOrder())).reversed())
.collect(Collectors.toList());
System.out.println(results); //[{tax=37, id=9}, {tax=25, id=3}, {tax=19, id=2}, {id=11}]
来源：https://stackoverflow.com/questions/64975372/sorting-collection-of-maps-based-on-values