我可以使用变量类型在Java中声明另一个变量吗？

Question

Can I do something like this in Java?

HashMap<String, Child> childMap=new HashMap<String, Child>();
HashMap<String, childMap.typeName> parentMap=new HashMap<String, childMap.typeName>();
//instead of
HashMap<String, HashMap<String, Child>> parentMap=new HashMap<String, HashMap<String, Child>>();

or something like this

HashMap<String, HashMap<String, Child>> parent1=new HashMap<String, HashMap<String, Child>>();
parent1.typeName parent2=new parent1.typeName;

Because some time, if the map level is too deep or too complex, it is very hard to write and read.

Answer 1

Abbreviations are possible by defining a subclass:

class Str2Child extends HashMap<String, Child>>{}
class Str2Map extends HashMap<String,Str2Child>{}

Str2Map parent1 = new Str2Map();

Answer 2

No but you could shorten it if you're using Java 7 or higher. The compiler can infer the type parameters from the left side of the assignment and you can skip them altogether while creating the object HashMap<String, HashMap<String, Child>> parentMap = new HashMap<>();

In older versions of Java, you could resord to Guava's Maps class and its newHashMap method. HashMap<String, HashMap<String, Child>> parentMap = Maps.newHashMap();

Another thing you could possibly do is create a type that implements a certain specification of the generic HashMap .

public class HashMapStringChild extends HashMap<String, Child> {
}

and then use it as a type parameter

HashMap<String, HashMapStringChild> parent2 = new HashMap<>();

but personally, I find this a bit of a stretch. I certainly wouldn't overuse it and I'd be careful extending the collection classes.

Addendum

You should also note that you're effectively binding your API to a specific implementation of the Map interface ( HashMap ), or even worse, in case of introducing the new class ( HashMapStringChild ), to a specific, non-standard implementation.

What if at some point, you decide to keep your Child objects sorted at all times? You could do this by switching to a TreeMap but that would mean a big deal of refactoring.

You would be better off basing your API on a more general interface. This way you could switch from

Map<String, Map<String, Child>> map = new HashMap<String, HashMap<String, Child>>();

to

Map<String, Map<String, Child>> map = new HashMap<String, TreeMap<String, Child>>();

or

Map<String, Map<String, Child>> map = new TreeMap<String, TreeMap<String, Child>>();

or any other implementation without a hassle.

If you really want to make the map of String to Child a specific type, you could introduce an interface

public interface MapStringToChild extends Map<String, Child> {
}

Then you could keep your reference types general and use HashMap<String, Child> , TreeMap<String, Child> , HashMapStringChild or literally any other implementation mapping a String to a Child interchangeably, while keeping the code short.