将null值保存为多个键，其中null具有单个含义

Question

I need a multiple key map and I want to use null values as keys. I am using HashBasedTable which does not allow null keys.

Table<String, String, JsonObject> table = HashBasedTable.create();
String key1= null, key2= null;
table.put(key1, key2, value);

How can I achieve the same. Please help. Thanks

Answer 1

You can use a null object , in that case a string which you know is not part of the possible set of strings (eg "Unknown" ):

Table<String, String, JsonObject> table = HashBasedTable.create();
String key1 = MoreObjects.firstNonNull(realKey1, "Unknown"),
        key2 = MoreObjects.firstNonNull(realKey2, "Unknown");
table.put(key1, key2, value);

You can also use Optional<String> to wrap your keys instead, and then your null object becomes Optional.empty() (or Optional.absent() if you use Guava's Optional ):

Table<Optional<String>, Optional<String>, JsonObject> table = HashBasedTable.create();
Optional<String> key1 = Optional.ofNullable(realKey1),
        key2 = Optional.ofNullable(realKey2);
table.put(key1, key2, value);