受键类型参数限制的值的 Java 映射

你不是在尝试实现Joshua Bloch的typeafe hetereogene容器模式，是吗？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（第 2 版）和本演示文稿。

问题和答案使我想出了这个解决方案：类型安全对象映射。下面是代码。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 Key 类。请注意，此类中从不使用该类型！这纯粹是为了在从地图中读取值时进行类型转换。该字段仅为密钥指定一个名称。Tkey

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;
    }
}

类型地图.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();
    }

}