值受键的类型参数限制的泛型Java映射

Question

Java中有没有一种方法可以将值的类型参数绑定到键的类型参数？我想写的内容如下：

public class Foo {
    // This declaration won't compile - what should it be?
    private static Map<Class<T>, T> defaultValues;

    // These two methods are just fine
    public static <T> void setDefaultValue(Class<T> clazz, T value) {
        defaultValues.put(clazz, value);
    }

    public static <T> T getDefaultValue(Class<T> clazz) {
        return defaultValues.get(clazz);
    }
}

也就是说，只要值的类型与类对象的类型匹配，我就可以针对类对象存储任何默认值。我不明白为什么不允许这样做，因为我可以确保在设置/获取值时类型是正确的

编辑：谢谢克莱特斯的回答。我实际上不需要映射本身的类型参数，因为我可以确保获取/设置值的方法的一致性，即使这意味着使用一些稍微难看的强制转换

Answer 1

您可以使用以下两个类，映射类：GenericMap，映射键类：GenericKey

例如：

// Create a key includine Type definition
public static final GenericKey<HttpServletRequest> REQUEST = new GenericKey<>(HttpServletRequest.class, "HttpRequestKey");

public void example(HttpServletRequest requestToSave)
{
    GenericMap map = new GenericMap();

    // Saving value
    map.put(REQUEST, requestToSave);

    // Getting value
    HttpServletRequest request = map.get(REQUEST);
}

优势

• 它通过编译错误强制用户输入和获取正确的类型
• 它在里面为你做外壳
• 泛型键有助于避免每次调用put（..）时编写类类型或获取
• 没有打字错误，比如键是“字符串”类型

通用地图

public class GenericMap
{  
    private Map<String, Object> storageMap;

    protected GenericMap()
    {
        storageMap = new HashMap<String, Object>();
    }

    public <T> T get(GenericKey<T> key)
    {
        Object value = storageMap.get(key.getKey());
        if (value == null)
        {
            return null;
        }

        return key.getClassType().cast(value);
    }

    /**
     * @param key    GenericKey object with generic type - T (it can be any type)
     * @param object value to put in the map, the type of 'object' mast be - T
     */
    public <T> void put(GenericKey<T> key, T object)
    {
        T castedObject = key.getClassType().cast(object);
        storageMap.put(key.getKey(), castedObject);
    }

    @Override
    public String toString()
    {
        return storageMap.toString();
    }
}

通用密钥

public class GenericKey<T>
{
    private Class<T> classType;
    private String key;

    @SuppressWarnings("unused")
    private GenericKey()
    {
    }

    public GenericKey(Class<T> iClassType, String iKey)
    {
        this.classType = iClassType;
        this.key = iKey;
    }

    public Class<T> getClassType()
    {
        return classType;
    }

    public String getKey()
    {
        return key;
    }

    @Override
    public String toString()
    {
        return "[classType=" + classType + ", key=" + key + "]";
    }
}

Answer 2

不，你不能直接做。您需要围绕Map<Class, Object>编写一个包装器类，以强制该对象为instanceof类

Answer 3

可以创建一个类来存储类型为safe key的映射到值，并在必要时强制转换。强制转换get方法是安全的，因为在使用new Key<CharSequence>()之后，不可能将其安全地强制转换为Key<String>或Key<Object>，因此类型系统强制正确使用类

Key类必须是final，否则，如果两个不同类型的元素相等，用户可能会重写equals并导致类型不安全。或者，如果您想使用继承，尽管它存在问题，也可以将equals重写为final

public final class TypeMap {
    private final Map<Key<?>, Object> m = new HashMap<>();

    public <T> T get(Key<? extends T> key) {
        // Safe, as it's not possible to safely change the Key generic type,
        // hash map cannot be accessed by an user, and this class being final
        // to prevent serialization attacks.
        @SuppressWarnings("unchecked")
        T value = (T) m.get(key);
        return value;
    }

    public <T> void put(Key<? super T> key, T value) {
        m.put(key, value);
    }

    public static final class Key<T> {
    }
}

Answer 4

你不是在尝试实现Joshua Bloch的类型安全异构容器模式吧？基本上：

public class Favorites {
  private Map<Class<?>, Object> favorites =
    new HashMap<Class<?>, Object>();

  public <T> void setFavorite(Class<T> klass, T thing) {
    favorites.put(klass, thing);
  }

  public <T> T getFavorite(Class<T> klass) {
    return klass.cast(favorites.get(klass));
  }

  public static void main(String[] args) {
    Favorites f = new Favorites();
    f.setFavorite(String.class, "Java");
    f.setFavorite(Integer.class, 0xcafebabe);
    String s = f.getFavorite(String.class);
    int i = f.getFavorite(Integer.class);
  }
}

来自Effective Java (2nd edition)和this presentation

Answer 5

这个问题和答案让我想到了这个解决方案：Type-safe object map。这是代码。测试用例：

import static org.junit.Assert.*;

import java.util.ArrayList;
import java.util.List;

import org.junit.Test;


public class TypedMapTest {
    private final static TypedMapKey<String> KEY1 = new TypedMapKey<String>( "key1" );
    private final static TypedMapKey<List<String>> KEY2 = new TypedMapKey<List<String>>( "key2" );

    @Test
    public void testGet() throws Exception {

        TypedMap map = new TypedMap();
        map.set( KEY1, null );
        assertNull( map.get( KEY1 ) );

        String expected = "Hallo";
        map.set( KEY1, expected );
        String value = map.get( KEY1 );
        assertEquals( expected, value );

        map.set( KEY2, null );
        assertNull( map.get( KEY2 ) );

        List<String> list = new ArrayList<String> ();
        map.set( KEY2, list );
        List<String> valueList = map.get( KEY2 );
        assertEquals( list, valueList );
    }
}

这是重点课程。注意，类型T从未在这个类中使用过！这纯粹是为了在从映射中读取值时进行类型转换。字段key只给出键的名称

public class TypedMapKey<T> {
    private String key;

    public TypedMapKey( String key ) {
        this.key = key;
    }

    @Override
    public int hashCode() {
        final int prime = 31;
        int result = 1;
        result = prime * result + ( ( key == null ) ? 0 : key.hashCode() );
        return result;
    }

    @Override
    public boolean equals( Object obj ) {
        if( this == obj ) {
            return true;
        }
        if( obj == null ) {
            return false;
        }
        if( getClass() != obj.getClass() ) {
            return false;
        }
        TypedMapKey<?> other = (TypedMapKey<?>) obj;
        if( key == null ) {
            if( other.key != null ) {
                return false;
            }
        } else if( !key.equals( other.key ) ) {
            return false;
        }
        return true;
    }

    @Override
    public String toString() {
        return key;
    }
}

类型映射。爪哇：

import java.util.Collection;
import java.util.HashMap;
import java.util.Map;
import java.util.Set;

public class TypedMap implements Map<Object, Object> {
    private Map<Object, Object> delegate;

    public TypedMap( Map<Object, Object> delegate ) {
        this.delegate = delegate;
    }

    public TypedMap() {
        this.delegate = new HashMap<Object, Object>();
    }

    @SuppressWarnings( "unchecked" )
    public <T> T get( TypedMapKey<T> key ) {
        return (T) delegate.get( key );
    }

    @SuppressWarnings( "unchecked" )
    public <T> T remove( TypedMapKey<T> key ) {
        return (T) delegate.remove( key );
    }

    public <T> void set( TypedMapKey<T> key, T value ) {
        delegate.put( key, value );
    }

    // --- Only calls to delegates below

    public void clear() {
        delegate.clear();
    }

    public boolean containsKey( Object key ) {
        return delegate.containsKey( key );
    }

    public boolean containsValue( Object value ) {
        return delegate.containsValue( value );
    }

    public Set<java.util.Map.Entry<Object, Object>> entrySet() {
        return delegate.entrySet();
    }

    public boolean equals( Object o ) {
        return delegate.equals( o );
    }

    public Object get( Object key ) {
        return delegate.get( key );
    }

    public int hashCode() {
        return delegate.hashCode();
    }

    public boolean isEmpty() {
        return delegate.isEmpty();
    }

    public Set<Object> keySet() {
        return delegate.keySet();
    }

    public Object put( Object key, Object value ) {
        return delegate.put( key, value );
    }

    public void putAll( Map<? extends Object, ? extends Object> m ) {
        delegate.putAll( m );
    }

    public Object remove( Object key ) {
        return delegate.remove( key );
    }

    public int size() {
        return delegate.size();
    }

    public Collection<Object> values() {
        return delegate.values();
    }

}

Answer 6

T作为类型必须在类实例中进行常规定义。以下示例有效：

public class Test<T> {

    private Map<Class<T>, T> defaultValues;

    public void setDefaultValue(Class<T> clazz, T value) {
        defaultValues.put(clazz, value);
    }

    public T getDefaultValue(Class<T> clazz) {
        return defaultValues.get(clazz);
    }

}

或者，您可以使用paultomblin的答案，并用您自己的对象包装Map，这将强制执行这种类型的泛型