How do I create some variable type alias in Java

Question

let say I have this code

Map<String, String> list = new HashMap<String, String>();
list.put("number1", "one");
list.put("number2", "two");

how can I make some "alias" the type

Map<String, String>

to something that easier to be rewritten like

// may be something like this
theNewType = HashMap<String, String>;

theNewType list = new theNewType();
list.put("number1", "one");
list.put("number2", "two");

basically my question is, how to create "alias" to some "type", so i can make it easier to write and easier when need to change the whole program code.

Thanks, and sorry if this is silly question. I'm kinda new in Java.

Solution 1

There are no aliases in Java. You can extend the HashMap class with your class like this:

public class TheNewType extends HashMap<String, String> {
    // default constructor
    public TheNewType() {
        super();
    }
    // you need to implement the other constructors if you need
}

But keep in mind that this will be a class it won't be the same as you type HashMap<String, String>

Solution 2

There is no typedef equivalent in Java, and there is no common idiom for aliasing types. I suppose you could do something like

class StringMap extends HashMap<String, String> {}

but this is not common and would not be obvious to a program maintainer.

Solution 3

Although Java doesn't support this, you can use a generics trick to simulate it.

class Test<I extends Integer> {
    <L extends Long> void x(I i, L l) {
        System.out.println(
            i.intValue() + ", " + 
            l.longValue()
        );
    }
}

Source: http://blog.jooq.org/2014/11/03/10-things-you-didnt-know-about-java/

Solution 4

The closest one could think of is to make a wrapper class like so

class NewType extends HashMap<String, String> {
     public NewType() { }
}

I really wish Java had a sound type aliasing feature.

Solution 5

Nothing like that exists in Java. You might be able to do something with IDE templates or autocompletion, and look forward to (limited) generics type inference in Java 7.

View more solutions

Comments

[McDowell]

@Lee - KARASZI István's caveat about types is important - see ibm.com/developerworks/java/library/j-jtp02216/index.html

[Andrey]

There is an alternative trick/hack you can do with generics: stackoverflow.com/questions/5604390/…

[nasukkin]

Bear in mind that you should implement the missing details of the Serializable interface for your subclass, should you do this. Else, you may encounter curiosities if you ever run into a situation where you need to (de)serialize your object.

[shabunc]

I'm not sure why exactly you hate OP and want him to be bashed to deadh by coworkers

[Niccolò]

But because this is a specialization of a more general type, you can't use TheNewType in place of HashMap<String, String> as much as you can't use the latter where a Map<String, String> is returned.

[ThomasH]

IMO this is the same idea as in the other answers, in that you use the concept of derivation to get a new type name. Here it is I from Integer and L from Long by constraining type variables, where the other answers propose NewType from HashMap<String,String> by constructing a new class. The drawback of the type variable approach is that you can only use it within the scope of the type variables, e.g. the Test class or the x method.