当方法返回null时处理Map返回类型

Question

下面的方法返回一个Map.我正在添加一个跳过DAO如何返回值映射的伪代码.

public Map<Intger,Integer> getIDsBasingonRanks(){
       Map<Integer,Integer> m = new HashMap<>();
       // Dao operation fetcing values from DB;
       if(dao==null){
           //logging some error or info
           return null;// In this cases Null Pointer will be thrown. How to handle it ?
       }
       m.put()// put the values inside the map from DAO
       return m;
}

现在我getIDsBasingonRanks()从另一个返回Map的类调用并从此映射中获取值

Map<Integer,Integer> m2 = getIDsBasingonRanks();
m2.getkey();//Incase the map is null we will have Null Pointer Exception
m2.getValue(); //Incase the map is null we will have Null Pointer Exception

在dao = null条件下处理上述方法的return语句我感觉很棘手.当返回null时如何提供返回方法以克服空指针,因为我们也无法处理它们.

Answer 1

一种可能性是可选的.

public Optional<Map<Integer, Integer>> getIDsBasingonRanks() {

    Map<Integer, Integer> m = new HashMap<>();
    ... // Dao operation fetcing values from DB;
        if (dao == null) {
            return Optional.empty();
        }

    ... // put the values inside the map from DAO
    return Optional.of(m);
}

getIDsBasingonRanks().ifPresent(m ->
        {
            ... m.get(42) ...
        });

int y = getIDsBasingonRanks().map(m -> m.get(42)).orElse(13);