Java中的多层哈希

Question

In Perl if I want to have a multi-layered hash, I would write:

$hash_ref->{'key1'}->{'key2'}='value';

Where 'key1' might be a person's name, 'key2' might be "Savings Account" (vs. "Checking Account") and 'value' might be the amount of money in the account.

Is there an equivalent to this in Java, ie access values via hash references? What's the syntax like for this? Any examples or other resource references would be greatly appreciated. Thanks!

Answer 1

In Java, we use objects. We would have a Person object, with a name property of type String , and a savingsAccount of type Account . This Account object would have a value property, of type BigDecimal .

Java is an OO language. It's not Perl. You should use Java idioms in Java, and not Perl idioms.

Answer 2

You can have a Map<Map<..>> , where you'll be able to call map.get("key1").get("key2")

But note that Java is a statically-typed, object-oriented language. So you'd better creat classes: Person , SavingsAccount , and a Person has a field private SavingsAccount savingsAcount . Then you'll be able to do a compile-time safe: map.get("John").getSavingsAccount()

Answer 3

您可以使用equals和hashCode方法的正确实现来创建表示密钥的类：

Answer 4

我不确定为什么这个问题被低估了，但回答你的问题，在Java中你可以使用嵌套的Map来实现同样的目的。

Answer 5

For instance, you can have a

HashMap<String, HashMap<String, BigDecimal>>. 

I would not call it OOD, but you can.

It would probably be a bit more readable if you had a class for representing Person, a class to represent PersonalAccounts, which has Account instances as attributes, one for each account type (I assume those would be very few, otherwise a list would be better).

Then a single

HashMap<Person, PersonalAccounts> 

is enough, if you want to use an HashMap.

Actually you don't even need a map if an instance of PersonalAccounts is an attribute of a Person.