How to Map Distinct Value Types Using Java Generics--reference

原文：http://www.codeaffine.com/2015/03/04/map-distinct-value-types-using-java-generics/

Occasionally the average developer runs into a situation where he has to map values of arbitrary types within a particular container. However the Java collection API provides container related parameterization only. Which limits the type safe usage of

HashMap

for example to a single value type. But what if you want to mix apples and pears?

Luckily there is an easy design pattern that allows to map distinct value types using Java generics, which Joshua Bloch has described as typesafe hetereogeneous container in his book Effective Java(second edition, Item 29).

Stumbling across some not altogether congenial solutions regarding this topic recently, gave me the idea to explain the problem domain and elaborate on some implementation aspects in this post.

Map Distinct Value Types Using Java Generics

Consider for the sake of example that you have to provide some kind of application context that allows to bind values of arbitrary types to certain keys. A simple non type safe implementation using

String

keys backed by a

HashMap

might look like this:

Context context = new Context();
Runnable runnable = ...
context.put( "key", runnable );

// several computation cycles later...
Runnable value = ( Runnable )context.get( "key" );

The drawback of this approach can be seen at line six where a down cast is needed. Obviously this can lead to a

ClassCastException

in case the key-value pair has been replaced by a different value type:

The cause of such problems can be difficult to trace as the related implementation steps might be spread wide apart in your application. To improve the situation it seems reasonable to bind the value not only to its key but also to its type.

Common mistakes I saw in several solutions following this approach boil down more or less to the following 
Context
 variant:

[code]public class Context {

private final <String, Object> values = new HashMap<>();

public <T> void put( String key, T value, Class<T> valueType ) {
values.put( key, value );
}

public <T> T get( String key, Class<T> valueType ) {
return ( T )values.get( key );
}

[...]
}

Again basic usage might look like this:

One first glance this code might give the illusion of being more type save as it avoids the down cast in line six. But running the following snippet gets us down to earth as we still run into the
ClassCastException
 scenario during the assignment in line ten:

Context context = new Context();
Runnable runnable = ...
context.put( "key", runnable, Runnable.class );

// several computation cycles later...
Executor executor = ...
context.put( "key", executor, Executor.class );

// even more computation cycles later...
Runnable value = context.get( "key", Runnable.class ); // runtime problem

So what went wrong?

First of all the down cast in

Context#get

of type

T

is ineffective as type erasure replaces unbounded parameters with a static cast to

Object

. But more important the implementation does not use the type information provided by

Context#put

as key. At most it serves as superfluous cosmetic effect.

Typesafe Hetereogeneous Container

Although the last

Context

variant did not work out very well it points into the right direction. The question is how to properly parameterize the key? To answer this take a look at a stripped-down implementation according to the typesafe hetereogenous container pattern described by Bloch.

The idea is to use the

class

type as key itself. Since

Class

is a parameterized type it enables us to make the methods of

Context

type safe without resorting to an unchecked cast to

T

. A

Class

object used in this fashion is called a type token.

Context context = new Context();
Runnable runnable ...
context.put( Runnable.class, runnable );

// several computation cycles later...
Executor executor = ...
context.put( Executor.class, executor );

// even more computation cycles later...
Runnable value = context.get( Runnable.class );

This time the client code will work without class cast problems, as it is impossible to exchange a certain key-value pair by one with a different value type.


Bloch mentions two limitations to this pattern. ‘First, a malicious client could easily corrupt the type safety [...] by using a class object in its raw form.’ To ensure the type invariant at runtime a dynamic cast can be used within

Context#put

.

public class Key<T> {

final String identifier;
final Class<T> type;

public Key( String identifier, Class<T> type ) {
this.identifier = identifier;
this.type = type;
}
}

Again we use the parameterized

Class

as hook to the type information. And the adjusted

Context

now uses the parameterized

Key

instead of

Class

:

Context context = new Context();

Runnable runnable1 = ...
Key<Runnable> key1 = new Key<>( "id1", Runnable.class );
context.put( key1, runnable1 );

Runnable runnable2 = ...
Key<Runnable> key2 = new Key<>( "id2", Runnable.class );
context.put( key2, runnable2 );

// several computation cycles later...
Runnable actual = context.get( key1 );

assertThat( actual ).isSameAs( runnable1 );

Although this snippet works, the implementation is still flawed. The

Key

implementation is used as lookup parameter in

Context#get

. Using two distinct instances of

Key

initialized with the same identifier and class – one instance used with put and the other used with get – would return

null

on

get

. Which is not what we want.

Luckily this can be solved easily with an appropriate

equals

and

hashCode

implementation of

Key

. That allows the

HashMap

lookup to work as expected. Finally one might provide a factory method for key creation to minimize boilerplate (useful in combination with static imports):

public static Key key( String identifier, Class type ) {
return new Key( identifier, type );
}
[/code]

Conclusion

‘The normal use of generics, exemplified by the collection APIs, restricts you to a fixed number of type parameters per container. You can get around this restriction by placing the type parameter on the key rather than the container. You can use

Class

objects as keys for such typesafe heterogeneous containers’ (Joshua Bloch, Item 29, Effective Java).

Given these closing remarks, there is nothing left to be added except for wishing you good luck mixing apples and pears successfully…