Question

在Java中有没有办法让地图中某个值的类型参数与某个键的类型参数相关联？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，如果值的类型与Class对象的类型匹配，我可以对Class对象存储任何默认值。我不明白为什么不允许这样做，因为我可以确保设置/获取类型正确的值。

编辑：感谢cletus的回答。我实际上并不需要地图本身的类型参数，因为我可以确保获取/设置值的方法的一致性，即使它意味着使用一些稍微丑陋的演员表。

Answer 1

你不是在试图实施Joshua Bloch的类型安全的异构容器模式吗？基本上是：

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 2

问题和答案让我想出了这个解决方案：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 );
    }
}

关键课程：

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;
    }
}

TypedMap.java：

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 3

不，你不能直接这样做。您需要在Map<Class, Object>周围编写一个包装类，以强制该对象为instanceof Class。

Answer 4

可以创建一个类，该类存储安全键类型到值的映射，并在必要时进行强制转换。使用get进行强制转换是安全的，因为使用new Key<CharSequence>()后，无法安全地将其强制转换为Key<String>或Key<Object>，因此类型系统会强制正确使用一个班级。

Key类必须是最终类，否则，如果两个具有不同类型的元素相等，则用户可能会覆盖equals并导致类型不安全。另外，如果您想使用继承，尽管有问题，也可以覆盖equals作为最终版本。

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 5

您可以使用以下2个类，Map类： GenericMap ，Map-Key类： GenericKey < / p>

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

它会强制用户因编译错误而放置并获取正确的类型
它在里面为您做包装
通用密钥有助于避免在每次调用 put（..） 或获取

没有输入错误，例如键是否为“字符串”类型

GenericMap

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(); } }

GenericKey

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 6

T作为类型必须在类实例中一般定义。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Paul Tomblin的答案，并使用您自己的对象包装Map，这将强制执行此类泛型。

Java映射的值受键的类型参数限制

6 个答案:

GenericMap

GenericKey