de.lmu.ifi.dbs.elki.utilities

Class ClassGenericsUtil

java.lang.Object

de.lmu.ifi.dbs.elki.utilities.ClassGenericsUtil

public final class ClassGenericsUtil
extends java.lang.Object

Utilities for handling class instantiation, especially with respect to Java generics.

Due to the way generics are implemented - via erasure - type safety cannot be guaranteed properly at compile time here. These classes collect such cases using helper functions, so that we have to suppress these warnings only in one place.

Note that many of these situations are still type safe, i.e. an empty array of List<List<?>> can indeed be cast into a List<List<Whatever>>.

The only one potentially unsafe is instantiateGenerics(java.lang.Class<?>, java.lang.String), since we can't verify that the runtime type 'type' adhers to the compile time restriction T. When T is not generic, such a check is possible, and then the developer should use instantiate(java.lang.Class<T>, java.lang.String) instead.

Since:

0.2

Author:

Erich Schubert

Field Summary

Fields

Modifier and Type	Field and Description
(package private) static java.lang.ClassLoader	CLASSLOADER Class loader.
static java.lang.String	FACTORY_METHOD_NAME Name for a static "parameterize" factory method.
private static Logging	LOG Static logger to use.

Constructor Summary

Constructors

Modifier	Constructor and Description
private	ClassGenericsUtil() Fake Constructor.

Method Summary

Modifier and Type	Method and Description
static Parameterizer	getParameterizer(java.lang.Class<?> c) Get a parameterizer for the given class.
static <T> T	instantiate(java.lang.Class<T> type, java.lang.String className) Returns a new instance of the given type for the specified className.
static <T> T	instantiateGenerics(java.lang.Class<?> type, java.lang.String className) Returns a new instance of the given type for the specified className.
static <T> T	instantiateLowlevel(java.lang.Class<? extends T> clz) Instantiate the first available implementation of an interface.
static <C> C	parameterizeOrAbort(java.lang.Class<?> c, Parameterization config) Force parameterization method.
static <C> C	tryInstantiate(java.lang.Class<C> r, java.lang.Class<?> c, Parameterization config) Instantiate a parameterizable class.
static <D,T extends D> java.lang.Class<T>	uglyCastIntoSubclass(java.lang.Class<D> cls) Cast the (erased) generics onto a class.
static <BASE,FROM extends BASE,TO extends BASE> java.lang.Class<TO>	uglyCrossCast(java.lang.Class<FROM> cls, java.lang.Class<BASE> base) This class performs an ugly cast, from Class<F> to Class<T>, where both F and T need to extend B.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Field Detail

LOG

private static final Logging LOG

Static logger to use.

CLASSLOADER

static final java.lang.ClassLoader CLASSLOADER

Class loader.

FACTORY_METHOD_NAME

public static final java.lang.String FACTORY_METHOD_NAME

Name for a static "parameterize" factory method.

See Also:

Constant Field Values

Constructor Detail

ClassGenericsUtil

private ClassGenericsUtil()

Fake Constructor. Use static methods.

Method Detail

instantiate

public static <T> T instantiate(java.lang.Class<T> type,
                                java.lang.String className)
                         throws ClassInstantiationException

Returns a new instance of the given type for the specified className.

If the Class for className is not found, the instantiation is tried using the package of the given type as package of the given className.

Type Parameters:

T - Class type for compile time type checking

Parameters:

type - desired Class type of the Object to retrieve

className - name of the class to instantiate

Returns:

a new instance of the given type for the specified className

Throws:

ClassInstantiationException - When a class cannot be instantiated.

instantiateGenerics

public static <T> T instantiateGenerics(java.lang.Class<?> type,
                                        java.lang.String className)
                                 throws ClassInstantiationException

Returns a new instance of the given type for the specified className.

If the class for className is not found, the instantiation is tried using the package of the given type as package of the given className.

This is a weaker type checked version of "instantiate(java.lang.Class<T>, java.lang.String)" for use with generics.

Type Parameters:

T - Class type for compile time type checking

Parameters:

type - desired Class type of the Object to retrieve

className - name of the class to instantiate

Returns:

a new instance of the given type for the specified className

Throws:

ClassInstantiationException - When a class cannot be instantiated.

getParameterizer

public static Parameterizer getParameterizer(java.lang.Class<?> c)

Get a parameterizer for the given class.

Parameters:

c - Class

Returns:

Parameterizer or null.

tryInstantiate

public static <C> C tryInstantiate(java.lang.Class<C> r,
                                   java.lang.Class<?> c,
                                   Parameterization config)
                            throws ClassInstantiationException

Instantiate a parameterizable class. When using this, consider using Parameterization.descend(java.lang.Object)!

Type Parameters:

C - base type

Parameters:

r - Base (restriction) class

c - Class to instantiate

config - Configuration to use for instantiation.

Returns:

Instance

Throws:

ClassInstantiationException - When a class cannot be instantiated.

parameterizeOrAbort

public static <C> C parameterizeOrAbort(java.lang.Class<?> c,
                                        Parameterization config)

Force parameterization method.

Please use this only in "runner" classes such as unit tests, since the error handling is not very flexible.

Type Parameters:

C - Type

Parameters:

c - Class to instantiate

config - Parameters

Returns:

Instance or throw an AbortException

uglyCastIntoSubclass

public static <D,T extends D> java.lang.Class<T> uglyCastIntoSubclass(java.lang.Class<D> cls)

Cast the (erased) generics onto a class.

Note: this function is a hack - notice that it would allow you to up-cast any class! Still it is preferable to have this cast in one place than in dozens without any explanation.

The reason this is needed is the following: There is no Class<Set<String>>.class. This method allows you to do Class<Set<String>> setclass = uglyCastIntoSubclass(Set.class);

We can't type check at runtime, since we don't have T.

Type Parameters:

D - Base type

T - Supertype

Parameters:

cls - Class type

Returns:

cls parameter, but cast to Class<T>

uglyCrossCast

public static <BASE,FROM extends BASE,TO extends BASE> java.lang.Class<TO> uglyCrossCast(java.lang.Class<FROM> cls,
                                                                                         java.lang.Class<BASE> base)

This class performs an ugly cast, from Class<F> to Class<T>, where both F and T need to extend B.

The restrictions are there to avoid misuse of this cast helper.

While this sounds really ugly, the common use case will be something like

 BASE = Class<Database>
 FROM = Class<Database>
 TO = Class<Database<V>>
 

i.e. the main goal is to add missing Generics to the compile time type.

Type Parameters:

BASE - Base type

TO - Destination type

FROM - Source type

Parameters:

cls - Class to be cast

base - Base class for type checking.

Returns:

Casted class.

instantiateLowlevel

public static <T> T instantiateLowlevel(java.lang.Class<? extends T> clz)

Instantiate the first available implementation of an interface.

Only supports public and parameterless constructors.

Parameters:

clz - Class to instantiate.

Returns:

Instance