值受键限制的Java映射';s型参数
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:值受键限制的Java映射';s型参数,java,generics,Java,Generics,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>
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);
}
}
公共类Foo{
//这个声明不会编译-它应该是什么?
私有静态映射默认值;
//这两种方法都很好
公共静态void setDefaultValue(类clazz,T值){
defaultValues.put(clazz,value);
}
公共静态T getDefaultValue(类clazz){
返回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类型转换。不,您不能直接这样做。您需要围绕
Map
编写一个包装类,以强制该对象将是类的实例。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);
}
}
公共类测试{
私有映射值;
public void setDefaultValue(类clazz,T值){
defaultValues.put(clazz,value);
}
公共T getDefaultValue(类clazz){
返回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映射
,该对象将强制执行此类泛型。您不是在尝试实现Joshua Bloch的类型安全异构容器模式,是吗?基本上:
公共类收藏夹{
私有映射,对象>();
公共空间设置收藏夹(类klass,T thing){
收藏。放(克拉斯,东西);
}
公共T-getFavorite(klass类){
返回klass.cast(favorites.get(klass));
}
公共静态void main(字符串[]args){
收藏夹f=新收藏夹();
f、 setFavorite(String.class,“Java”);
f、 setFavorite(Integer.class,0xcafebabe);
String s=f.getFavorite(String.class);
inti=f.getFavorite(Integer.class);
}
}
从和。这个问题和答案让我想到了这个解决方案:。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 );
}
}
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();
}
}
import java.util.Collection;
导入java.util.HashMap;
导入java.util.Map;
导入java.util.Set;
公共类TypedMap实现映射{
私人地图代表;
公共类型映射(映射代理){
this.delegate=委托;
}
公共类型映射(){
this.delegate=new HashMap();
}
@抑制警告(“未选中”)
public T get(TypedMapKey键){
return(T)delegate.get(key);
}
@抑制警告(“未选中”)
公用T删除(类型DMAPKEY key){
返回(T)委托。删除(键);
}
公共无效集(TypedMapKey键,T值){
delegate.put(键、值);
}
//---只给下面的代表打电话
公共空间清除(){
delegate.clear();
}
公共布尔containsKey(对象键){
返回delegate.containsKey(键);
}
公共布尔包含值(对象值){
返回委托.containsValue(值);
}
公共集入口集(){
返回delegate.entrySet();
}
公共布尔等于(对象o){
返回delegate.equals(o);
}
公共对象获取(对象密钥){
返回delegate.get(key);
}
公共int hashCode(){
返回delegate.hashCode();
}
公共布尔值为空(){
返回delegate.isEmpty();
}
公共集密钥集(){
返回delegate.keySet();
}
公共对象put(对象键、对象值){
返回delegate.put(键、值);
}
public void putAll(Map可以创建一个类,该类将类型为safe key的映射存储到一个值,并在必要时强制转换。强制转换get
方法是安全的,就像使用new key()之后一样
,无法将其安全地强制转换为键
或键
,因此类型系统强制执行类的正确用法
键
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,尽管存在问题,也可以重写equals
使其成为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> {
}
}
公共最终类类型映射{
私有最终映射您可以使用以下2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);
}
//创建键includine类型定义
公共静态最终GenericKey请求=新的GenericKey(HttpServletRequest.class,“HttpRequestKey”);
公共作废示例(HttpServletRequestToSave)
{
GenericMap=新建GenericMap();
//储蓄价值
map.put(请求,请求保存);
//获得价值
HttpServletRequest=map.get(请求);
}
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(…)或get
- 没有打字错误,比如键是“字符串”类型
通用地图
公共类GenericMap
{
私有地图存储地图;
受保护的GenericMap()
{
storageMap=新的HashMap();
}
公共T获取(通用密钥)
{
对象值=storageMap.get(key.getKey());
如果(值==null)
{
返回null;
}
return key.getClassType().cast(v
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> {
}
}
// 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);
}
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();
}
}
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 + "]";
}
}