一、概述
Java泛型（generics）是JDK 5中引入的一个新特性，泛型提供了编译时类型安全监测机制，该机制允许程序员在编译时监测非法的类型。使用泛型机制编写的程序代码要比那些杂乱地使用Object变量，然后再进行强制类型转换的代码具有更好的安全性和可读性。泛型对于集合类尤其有用，例如，ArrayList就是一个无处不在的集合类。
泛型的本质是参数化类型，也就是所操作的数据类型被指定为一个参数。
定义泛型的规则：
只能是类类型，不能是简单数据类型 泛型参数可以有多个 可以用使用 extends 语句或者 super 语句 如表示类型的上界，T 只能是 superClass 或其子类， 表示类型的下界，K 只能是 childClass 或其父类。 可以是通配符类型，比如常见的 Class<?>
二、泛型的使用
泛型有三种常用的使用方式：泛型类，泛型接口和泛型方法。
下面讲一一简单介绍这三种使用方法：
- 泛型类（generic class）就是具有一个或多个类型变量的类。
下面是Pair类的代码：
public class Pair<F, S> {
public final F first;
public final S second;
/**
* Constructor for a Pair.
*
* @param first the first object in the Pair
* @param second the second object in the pair
*/
public Pair(F first, S second) {
this.first = first;
this.second = second;
}
/**
* Checks the two objects for equality by delegating to their respective
* {@link Object#equals(Object)} methods.
*
* @param o the {@link Pair} to which this one is to be checked for equality
* @return true if the underlying objects of the Pair are both considered
* equal
*/
@Override
public boolean equals(Object o) {
if (!(o instanceof Pair)) {
return false;
}
Pair<?, ?> p = (Pair<?, ?>) o;
return Objects.equals(p.first, first) && Objects.equals(p.second, second);
}
/**
* Compute a hash code using the hash codes of the underlying objects
*
* @return a hashcode of the Pair
*/
@Override
public int hashCode() {
return (first == null ? 0 : first.hashCode()) ^ (second == null ? 0 : second.hashCode());
}
@Override
public String toString() {
return "Pair{" + String.valueOf(first) + " " + String.valueOf(second) + "}";
}
/**
* Convenience method for creating an appropriately typed pair.
* @param a the first object in the Pair
* @param b the second object in the pair
* @return a Pair that is templatized with the types of a and b
*/
public static <A, B> Pair <A, B> create(A a, B b) {
return new Pair<A, B>(a, b);
}
}
Pair类定义了两个泛型参数F,S
public final F first;
public final S second;
- 泛型接口
public interface Map<K, V> {
// Query Operations
/**
* Returns the number of key-value mappings in this map. If the
* map contains more than {@code Integer.MAX_VALUE} elements, returns
* {@code Integer.MAX_VALUE}.
*
* @return the number of key-value mappings in this map
*/
int size();
/**
* Returns {@code true} if this map contains no key-value mappings.
*
* @return {@code true} if this map contains no key-value mappings
*/
boolean isEmpty();
/**
* Returns {@code true} if this map contains a mapping for the specified
* key. More formally, returns {@code true} if and only if
* this map contains a mapping for a key {@code k} such that
* {@code Objects.equals(key, k)}. (There can be
* at most one such mapping.)
*
* @param key key whose presence in this map is to be tested
* @return {@code true} if this map contains a mapping for the specified
* key
* @throws ClassCastException if the key is of an inappropriate type for
* this map
* (<a href="Collection.html#optional-restrictions">optional</a>)
* @throws NullPointerException if the specified key is null and this map
* does not permit null keys
* (<a href="Collection.html#optional-restrictions">optional</a>)
*/
boolean containsKey(Object key);
/**
* Returns {@code true} if this map maps one or more keys to the
* specified value. More formally, returns {@code true} if and only if
* this map contains at least one mapping to a value {@code v} such that
* {@code Objects.equals(value, v)}. This operation
* will probably require time linear in the map size for most
* implementations of the {@code Map} interface.
*
* @param value value whose presence in this map is to be tested
* @return {@code true} if this map maps one or more keys to the
* specified value
* @throws ClassCastException if the value is of an inappropriate type for
* this map
* (<a href="Collection.html#optional-restrictions">optional</a>)
* @throws NullPointerException if the specified value is null and this
* map does not permit null values
* (<a href="Collection.html#optional-restrictions">optional</a>)
*/
boolean containsValue(Object value);
/**
* Returns the value to which the specified key is mapped,
* or {@code null} if this map contains no mapping for the key.
*
* <p>More formally, if this map contains a mapping from a key
* {@code k} to a value {@code v} such that
* {@code Objects.equals(key, k)},
* then this method returns {@code v}; otherwise
* it returns {@code null}. (There can be at most one such mapping.)
*
* <p>If this map permits null values, then a return value of
* {@code null} does not <i>necessarily</i> indicate that the map
* contains no mapping for the key; it's also possible that the map
* explicitly maps the key to {@code null}. The {@link #containsKey
* containsKey} operation may be used to distinguish these two cases.
*
* @param key the key whose associated value is to be returned
* @return the value to which the specified key is mapped, or
* {@code null} if this map contains no mapping for the key
* @throws ClassCastException if the key is of an inappropriate type for
* this map
* (<a href="Collection.html#optional-restrictions">optional</a>)
* @throws NullPointerException if the specified key is null and this map
* does not permit null keys
* (<a href="Collection.html#optional-restrictions">optional</a>)
*/
V get(Object key);
// Modification Operations
/**
* Associates the specified value with the specified key in this map
* (optional operation). If the map previously contained a mapping for
* the key, the old value is replaced by the specified value. (A map
* {@code m} is said to contain a mapping for a key {@code k} if and only
* if {@link #containsKey(Object) m.containsKey(k)} would return
* {@code true}.)
*
* @param key key with which the specified value is to be associated
* @param value value to be associated with the specified key
* @return the previous value associated with {@code key}, or
* {@code null} if there was no mapping for {@code key}.
* (A {@code null} return can also indicate that the map
* previously associated {@code null} with {@code key},
* if the implementation supports {@code null} values.)
* @throws UnsupportedOperationException if the {@code put} operation
* is not supported by this map
* @throws ClassCastException if the class of the specified key or value
* prevents it from being stored in this map
* @throws NullPointerException if the specified key or value is null
* and this map does not permit null keys or values
* @throws IllegalArgumentException if some property of the specified key
* or value prevents it from being stored in this map
*/
V put(K key, V value);
/**
* Removes the mapping for a key from this map if it is present
* (optional operation). More formally, if this map contains a mapping
* from key {@code k} to value {@code v} such that
* {@code Objects.equals(key, k)}, that mapping
* is removed. (The map can contain at most one such mapping.)
*
* <p>Returns the value to which this map previously associated the key,
* or {@code null} if the map contained no mapping for the key.
*
* <p>If this map permits null values, then a return value of
* {@code null} does not <i>necessarily</i> indicate that the map
* contained no mapping for the key; it's also possible that the map
* explicitly mapped the key to {@code null}.
*
* <p>The map will not contain a mapping for the specified key once the
* call returns.
*
* @param key key whose mapping is to be removed from the map
* @return the previous value associated with {@code key}, or
* {@code null} if there was no mapping for {@code key}.
* @throws UnsupportedOperationException if the {@code remove} operation
* is not supported by this map
* @throws ClassCastException if the key is of an inappropriate type for
* this map
* (<a href="Collection.html#optional-restrictions">optional</a>)
* @throws NullPointerException if the specified key is null and this
* map does not permit null keys
* (<a href="Collection.html#optional-restrictions">optional</a>)
*/
V remove(Object key);
// Bulk Operations
/**
* Copies all of the mappings from the specified map to this map
* (optional operation). The effect of this call is equivalent to that
* of calling {@link #put(Object,Object) put(k, v)} on this map once
* for each mapping from key {@code k} to value {@code v} in the
* specified map. The behavior of this operation is undefined if the
* specified map is modified while the operation is in progress.
*
* @param m mappings to be stored in this map
* @throws UnsupportedOperationException if the {@code putAll} operation
* is not supported by this map
* @throws ClassCastException if the class of a key or value in the
* specified map prevents it from being stored in this map
* @throws NullPointerException if the specified map is null, or if
* this map does not permit null keys or values, and the
* specified map contains null keys or values
* @throws IllegalArgumentException if some property of a key or value in
* the specified map prevents it from being stored in this map
*/
void putAll(Map<? extends K, ? extends V> m);
/**
* Removes all of the mappings from this map (optional operation).
* The map will be empty after this call returns.
*
* @throws UnsupportedOperationException if the {@code clear} operation
* is not supported by this map
*/
void clear();
// Views
/**
* Returns a {@link Set} view of the keys contained in this map.
* The set is backed by the map, so changes to the map are
* reflected in the set, and vice-versa. If the map is modified
* while an iteration over the set is in progress (except through
* the iterator's own {@code remove} operation), the results of
* the iteration are undefined. The set supports element removal,
* which removes the corresponding mapping from the map, via the
* {@code Iterator.remove}, {@code Set.remove},
* {@code removeAll}, {@code retainAll}, and {@code clear}
* operations. It does not support the {@code add} or {@code addAll}
* operations.
*
* @return a set view of the keys contained in this map
*/
Set<K> keySet();
/**
* Returns a {@link Collection} view of the values contained in this map.
* The collection is backed by the map, so changes to the map are
* reflected in the collection, and vice-versa. If the map is
* modified while an iteration over the collection is in progress
* (except through the iterator's own {@code remove} operation),
* the results of the iteration are undefined. The collection
* supports element removal, which removes the corresponding
* mapping from the map, via the {@code Iterator.remove},
* {@code Collection.remove}, {@code removeAll},
* {@code retainAll} and {@code clear} operations. It does not
* support the {@code add} or {@code addAll} operations.
*
* @return a collection view of the values contained in this map
*/
Collection<V> values();
/**
* Returns a {@link Set} view of the mappings contained in this map.
* The set is backed by the map, so changes to the map are
* reflected in the set, and vice-versa. If the map is modified
* while an iteration over the set is in progress (except through
* the iterator's own {@code remove} operation, or through the
* {@code setValue} operation on a map entry returned by the
* iterator) the results of the iteration are undefined. The set
* supports element removal, which removes the corresponding
* mapping from the map, via the {@code Iterator.remove},
* {@code Set.remove}, {@code removeAll}, {@code retainAll} and
* {@code clear} operations. It does not support the
* {@code add} or {@code addAll} operations.
*
* @return a set view of the mappings contained in this map
*/
Set<Map.Entry<K, V>> entrySet();
/**
* A map entry (key-value pair). The {@code Map.entrySet} method returns
* a collection-view of the map, whose elements are of this class. The
* <i>only</i> way to obtain a reference to a map entry is from the
* iterator of this collection-view. These {@code Map.Entry} objects are
* valid <i>only</i> for the duration of the iteration; more formally,
* the behavior of a map entry is undefined if the backing map has been
* modified after the entry was returned by the iterator, except through
* the {@code setValue} operation on the map entry.
*
* @see Map#entrySet()
* @since 1.2
*/
interface Entry<K, V> {
/**
* Returns the key corresponding to this entry.
*
* @return the key corresponding to this entry
* @throws IllegalStateException implementations may, but are not
* required to, throw this exception if the entry has been
* removed from the backing map.
*/
K getKey();
/**
* Returns the value corresponding to this entry. If the mapping
* has been removed from the backing map (by the iterator's
* {@code remove} operation), the results of this call are undefined.
*
* @return the value corresponding to this entry
* @throws IllegalStateException implementations may, but are not
* required to, throw this exception if the entry has been
* removed from the backing map.
*/
V getValue();
/**
* Replaces the value corresponding to this entry with the specified
* value (optional operation). (Writes through to the map.) The
* behavior of this call is undefined if the mapping has already been
* removed from the map (by the iterator's {@code remove} operation).
*
* @param value new value to be stored in this entry
* @return old value corresponding to the entry
* @throws UnsupportedOperationException if the {@code put} operation
* is not supported by the backing map
* @throws ClassCastException if the class of the specified value
* prevents it from being stored in the backing map
* @throws NullPointerException if the backing map does not permit
* null values, and the specified value is null
* @throws IllegalArgumentException if some property of this value
* prevents it from being stored in the backing map
* @throws IllegalStateException implementations may, but are not
* required to, throw this exception if the entry has been
* removed from the backing map.
*/
V setValue(V value);
/**
* Compares the specified object with this entry for equality.
* Returns {@code true} if the given object is also a map entry and
* the two entries represent the same mapping. More formally, two
* entries {@code e1} and {@code e2} represent the same mapping
* if<pre>
* (e1.getKey()==null ?
* e2.getKey()==null : e1.getKey().equals(e2.getKey())) &&
* (e1.getValue()==null ?
* e2.getValue()==null : e1.getValue().equals(e2.getValue()))
* </pre>
* This ensures that the {@code equals} method works properly across
* different implementations of the {@code Map.Entry} interface.
*
* @param o object to be compared for equality with this map entry
* @return {@code true} if the specified object is equal to this map
* entry
*/
boolean equals(Object o);
/**
* Returns the hash code value for this map entry. The hash code
* of a map entry {@code e} is defined to be: <pre>
* (e.getKey()==null ? 0 : e.getKey().hashCode()) ^
* (e.getValue()==null ? 0 : e.getValue().hashCode())
* </pre>
* This ensures that {@code e1.equals(e2)} implies that
* {@code e1.hashCode()==e2.hashCode()} for any two Entries
* {@code e1} and {@code e2}, as required by the general
* contract of {@code Object.hashCode}.
*
* @return the hash code value for this map entry
* @see Object#hashCode()
* @see Object#equals(Object)
* @see #equals(Object)
*/
int hashCode();
/**
* Returns a comparator that compares {@link Map.Entry} in natural order on key.
*
* <p>The returned comparator is serializable and throws {@link
* NullPointerException} when comparing an entry with a null key.
*
* @param <K> the {@link Comparable} type of then map keys
* @param <V> the type of the map values
* @return a comparator that compares {@link Map.Entry} in natural order on key.
* @see Comparable
* @since 1.8
*/
public static <K extends Comparable<? super K>, V> Comparator<Map.Entry<K, V>> comparingByKey() {
return (Comparator<Map.Entry<K, V>> & Serializable)
(c1, c2) -> c1.getKey().compareTo(c2.getKey());
}
/**
* Returns a comparator that compares {@link Map.Entry} in natural order on value.
*
* <p>The returned comparator is serializable and throws {@link
* NullPointerException} when comparing an entry with null values.
*
* @param <K> the type of the map keys
* @param <V> the {@link Comparable} type of the map values
* @return a comparator that compares {@link Map.Entry} in natural order on value.
* @see Comparable
* @since 1.8
*/
public static <K, V extends Comparable<? super V>> Comparator<Map.Entry<K, V>> comparingByValue() {
return (Comparator<Map.Entry<K, V>> & Serializable)
(c1, c2) -> c1.getValue().compareTo(c2.getValue());
}
/**
* Returns a comparator that compares {@link Map.Entry} by key using the given
* {@link Comparator}.
*
* <p>The returned comparator is serializable if the specified comparator
* is also serializable.
*
* @param <K> the type of the map keys
* @param <V> the type of the map values
* @param cmp the key {@link Comparator}
* @return a comparator that compares {@link Map.Entry} by the key.
* @since 1.8
*/
public static <K, V> Comparator<Map.Entry<K, V>> comparingByKey(Comparator<? super K> cmp) {
Objects.requireNonNull(cmp);
return (Comparator<Map.Entry<K, V>> & Serializable)
(c1, c2) -> cmp.compare(c1.getKey(), c2.getKey());
}
/**
* Returns a comparator that compares {@link Map.Entry} by value using the given
* {@link Comparator}.
*
* <p>The returned comparator is serializable if the specified comparator
* is also serializable.
*
* @param <K> the type of the map keys
* @param <V> the type of the map values
* @param cmp the value {@link Comparator}
* @return a comparator that compares {@link Map.Entry} by the value.
* @since 1.8
*/
public static <K, V> Comparator<Map.Entry<K, V>> comparingByValue(Comparator<? super V> cmp) {
Objects.requireNonNull(cmp);
return (Comparator<Map.Entry<K, V>> & Serializable)
(c1, c2) -> cmp.compare(c1.getValue(), c2.getValue());
}
}
// Comparison and hashing
/**
* Compares the specified object with this map for equality. Returns
* {@code true} if the given object is also a map and the two maps
* represent the same mappings. More formally, two maps {@code m1} and
* {@code m2} represent the same mappings if
* {@code m1.entrySet().equals(m2.entrySet())}. This ensures that the
* {@code equals} method works properly across different implementations
* of the {@code Map} interface.
*
* @param o object to be compared for equality with this map
* @return {@code true} if the specified object is equal to this map
*/
boolean equals(Object o);
/**
* Returns the hash code value for this map. The hash code of a map is
* defined to be the sum of the hash codes of each entry in the map's
* {@code entrySet()} view. This ensures that {@code m1.equals(m2)}
* implies that {@code m1.hashCode()==m2.hashCode()} for any two maps
* {@code m1} and {@code m2}, as required by the general contract of
* {@link Object#hashCode}.
*
* @return the hash code value for this map
* @see Map.Entry#hashCode()
* @see Object#equals(Object)
* @see #equals(Object)
*/
int hashCode();
// Defaultable methods
/**
* Returns the value to which the specified key is mapped, or
* {@code defaultValue} if this map contains no mapping for the key.
*
* @implSpec
* The default implementation makes no guarantees about synchronization
* or atomicity properties of this method. Any implementation providing
* atomicity guarantees must override this method and document its
* concurrency properties.
*
* @param key the key whose associated value is to be returned
* @param defaultValue the default mapping of the key
* @return the value to which the specified key is mapped, or
* {@code defaultValue} if this map contains no mapping for the key
* @throws ClassCastException if the key is of an inappropriate type for
* this map
* (<a href="Collection.html#optional-restrictions">optional</a>)
* @throws NullPointerException if the specified key is null and this map
* does not permit null keys
* (<a href="Collection.html#optional-restrictions">optional</a>)
* @since 1.8
*/
default V getOrDefault(Object key, V defaultValue) {
V v;
return (((v = get(key)) != null) || containsKey(key))
? v
: defaultValue;
}
/**
* Performs the given action for each entry in this map until all entries
* have been processed or the action throws an exception. Unless
* otherwise specified by the implementing class, actions are performed in
* the order of entry set iteration (if an iteration order is specified.)
* Exceptions thrown by the action are relayed to the caller.
*
* @implSpec
* The default implementation is equivalent to, for this {@code map}:
* <pre> {@code
* for (Map.Entry<K, V> entry : map.entrySet())
* action.accept(entry.getKey(), entry.getValue());
* }</pre>
*
* The default implementation makes no guarantees about synchronization
* or atomicity properties of this method. Any implementation providing
* atomicity guarantees must override this method and document its
* concurrency properties.
*
* @param action The action to be performed for each entry
* @throws NullPointerException if the specified action is null
* @throws ConcurrentModificationException if an entry is found to be
* removed during iteration
* @since 1.8
*/
default void forEach(BiConsumer<? super K, ? super V> action) {
Objects.requireNonNull(action);
for (Map.Entry<K, V> entry : entrySet()) {
K k;
V v;
try {
k = entry.getKey();
v = entry.getValue();
} catch (IllegalStateException ise) {
// this usually means the entry is no longer in the map.
throw new ConcurrentModificationException(ise);
}
action.accept(k, v);
}
}
/**
* Replaces each entry's value with the result of invoking the given
* function on that entry until all entries have been processed or the
* function throws an exception. Exceptions thrown by the function are
* relayed to the caller.
*
* @implSpec
* <p>The default implementation is equivalent to, for this {@code map}:
* <pre> {@code
* for (Map.Entry<K, V> entry : map.entrySet())
* entry.setValue(function.apply(entry.getKey(), entry.getValue()));
* }</pre>
*
* <p>The default implementation makes no guarantees about synchronization
* or atomicity properties of this method. Any implementation providing
* atomicity guarantees must override this method and document its
* concurrency properties.
*
* @param function the function to apply to each entry
* @throws UnsupportedOperationException if the {@code set} operation
* is not supported by this map's entry set iterator.
* @throws ClassCastException if the class of a replacement value
* prevents it from being stored in this map
* @throws NullPointerException if the specified function is null, or the
* specified replacement value is null, and this map does not permit null
* values
* @throws ClassCastException if a replacement value is of an inappropriate
* type for this map
* (<a href="Collection.html#optional-restrictions">optional</a>)
* @throws NullPointerException if function or a replacement value is null,
* and this map does not permit null keys or values
* (<a href="Collection.html#optional-restrictions">optional</a>)
* @throws IllegalArgumentException if some property of a replacement value
* prevents it from being stored in this map
* (<a href="Collection.html#optional-restrictions">optional</a>)
* @throws ConcurrentModificationException if an entry is found to be
* removed during iteration
* @since 1.8
*/
default void replaceAll(BiFunction<? super K, ? super V, ? extends V> function) {
Objects.requireNonNull(function);
for (Map.Entry<K, V> entry : entrySet()) {
K k;
V v;
try {
k = entry.getKey();
v = entry.getValue();
} catch (IllegalStateException ise) {
// this usually means the entry is no longer in the map.
throw new ConcurrentModificationException(ise);
}
// ise thrown from function is not a cme.
v = function.apply(k, v);
try {
entry.setValue(v);
} catch (IllegalStateException ise) {
// this usually means the entry is no longer in the map.
throw new ConcurrentModificationException(ise);
}
}
}
/**
* If the specified key is not already associated with a value (or is mapped
* to {@code null}) associates it with the given value and returns
* {@code null}, else returns the current value.
*
* @implSpec
* The default implementation is equivalent to, for this {@code
* map}:
*
* <pre> {@code
* V v = map.get(key);
* if (v == null)
* v = map.put(key, value);
*
* return v;
* }</pre>
*
* <p>The default implementation makes no guarantees about synchronization
* or atomicity properties of this method. Any implementation providing
* atomicity guarantees must override this method and document its
* concurrency properties.
*
* @param key key with which the specified value is to be associated
* @param value value to be associated with the specified key
* @return the previous value associated with the specified key, or
* {@code null} if there was no mapping for the key.
* (A {@code null} return can also indicate that the map
* previously associated {@code null} with the key,
* if the implementation supports null values.)
* @throws UnsupportedOperationException if the {@code put} operation
* is not supported by this map
* (<a href="Collection.html#optional-restrictions">optional</a>)
* @throws ClassCastException if the key or value is of an inappropriate
* type for this map
* (<a href="Collection.html#optional-restrictions">optional</a>)
* @throws NullPointerException if the specified key or value is null,
* and this map does not permit null keys or values
* (<a href="Collection.html#optional-restrictions">optional</a>)
* @throws IllegalArgumentException if some property of the specified key
* or value prevents it from being stored in this map
* (<a href="Collection.html#optional-restrictions">optional</a>)
* @since 1.8
*/
default V putIfAbsent(K key, V value) {
V v = get(key);
if (v == null) {
v = put(key, value);
}
return v;
}
/**
* Removes the entry for the specified key only if it is currently
* mapped to the specified value.
*
* @implSpec
* The default implementation is equivalent to, for this {@code map}:
*
* <pre> {@code
* if (map.containsKey(key) && Objects.equals(map.get(key), value)) {
* map.remove(key);
* return true;
* } else
* return false;
* }</pre>
*
* <p>The default implementation makes no guarantees about synchronization
* or atomicity properties of this method. Any implementation providing
* atomicity guarantees must override this method and document its
* concurrency properties.
*
* @param key key with which the specified value is associated
* @param value value expected to be associated with the specified key
* @return {@code true} if the value was removed
* @throws UnsupportedOperationException if the {@code remove} operation
* is not supported by this map
* (<a href="Collection.html#optional-restrictions">optional</a>)
* @throws ClassCastException if the key or value is of an inappropriate
* type for this map
* (<a href="Collection.html#optional-restrictions">optional</a>)
* @throws NullPointerException if the specified key or value is null,
* and this map does not permit null keys or values
* (<a href="Collection.html#optional-restrictions">optional</a>)
* @since 1.8
*/
default boolean remove(Object key, Object value) {
Object curValue = get(key);
if (!Objects.equals(curValue, value) ||
(curValue == null && !containsKey(key))) {
return false;
}
remove(key);
return true;
}
/**
* Replaces the entry for the specified key only if currently
* mapped to the specified value.
*
* @implSpec
* The default implementation is equivalent to, for this {@code map}:
*
* <pre> {@code
* if (map.containsKey(key) && Objects.equals(map.get(key), value)) {
* map.put(key, newValue);
* return true;
* } else
* return false;
* }</pre>
*
* The default implementation does not throw NullPointerException
* for maps that do not support null values if oldValue is null unless
* newValue is also null.
*
* <p>The default implementation makes no guarantees about synchronization
* or atomicity properties of this method. Any implementation providing
* atomicity guarantees must override this method and document its
* concurrency properties.
*
* @param key key with which the specified value is associated
* @param oldValue value expected to be associated with the specified key
* @param newValue value to be associated with the specified key
* @return {@code true} if the value was replaced
* @throws UnsupportedOperationException if the {@code put} operation
* is not supported by this map
* (<a href="Collection.html#optional-restrictions">optional</a>)
* @throws ClassCastException if the class of a specified key or value
* prevents it from being stored in this map
* @throws NullPointerException if a specified key or newValue is null,
* and this map does not permit null keys or values
* @throws NullPointerException if oldValue is null and this map does not
* permit null values
* (<a href="Collection.html#optional-restrictions">optional</a>)
* @throws IllegalArgumentException if some property of a specified key
* or value prevents it from being stored in this map
* @since 1.8
*/
default boolean replace(K key, V oldValue, V newValue) {
Object curValue = get(key);
if (!Objects.equals(curValue, oldValue) ||
(curValue == null && !containsKey(key))) {
return false;
}
put(key, newValue);
return true;
}
/**
* Replaces the entry for the specified key only if it is
* currently mapped to some value.
*
* @implSpec
* The default implementation is equivalent to, for this {@code map}:
*
* <pre> {@code
* if (map.containsKey(key)) {
* return map.put(key, value);
* } else
* return null;
* }</pre>
*
* <p>The default implementation makes no guarantees about synchronization
* or atomicity properties of this method. Any implementation providing
* atomicity guarantees must override this method and document its
* concurrency properties.
*
* @param key key with which the specified value is associated
* @param value value to be associated with the specified key
* @return the previous value associated with the specified key, or
* {@code null} if there was no mapping for the key.
* (A {@code null} return can also indicate that the map
* previously associated {@code null} with the key,
* if the implementation supports null values.)
* @throws UnsupportedOperationException if the {@code put} operation
* is not supported by this map
* (<a href="Collection.html#optional-restrictions">optional</a>)
* @throws ClassCastException if the class of the specified key or value
* prevents it from being stored in this map
* (<a href="Collection.html#optional-restrictions">optional</a>)
* @throws NullPointerException if the specified key or value is null,
* and this map does not permit null keys or values
* @throws IllegalArgumentException if some property of the specified key
* or value prevents it from being stored in this map
* @since 1.8
*/
default V replace(K key, V value) {
V curValue;
if (((curValue = get(key)) != null) || containsKey(key)) {
curValue = put(key, value);
}
return curValue;
}
/**
* If the specified key is not already associated with a value (or is mapped
* to {@code null}), attempts to compute its value using the given mapping
* function and enters it into this map unless {@code null}.
*
* <p>If the mapping function returns {@code null}, no mapping is recorded.
* If the mapping function itself throws an (unchecked) exception, the
* exception is rethrown, and no mapping is recorded. The most
* common usage is to construct a new object serving as an initial
* mapped value or memoized result, as in:
*
* <pre> {@code
* map.computeIfAbsent(key, k -> new Value(f(k)));
* }</pre>
*
* <p>Or to implement a multi-value map, {@code Map<K,Collection<V>>},
* supporting multiple values per key:
*
* <pre> {@code
* map.computeIfAbsent(key, k -> new HashSet<V>()).add(v);
* }</pre>
*
* <p>The mapping function should not modify this map during computation.
*
* @implSpec
* The default implementation is equivalent to the following steps for this
* {@code map}, then returning the current value or {@code null} if now
* absent:
*
* <pre> {@code
* if (map.get(key) == null) {
* V newValue = mappingFunction.apply(key);
* if (newValue != null)
* map.put(key, newValue);
* }
* }</pre>
*
* <p>The default implementation makes no guarantees about detecting if the
* mapping function modifies this map during computation and, if
* appropriate, reporting an error. Non-concurrent implementations should
* override this method and, on a best-effort basis, throw a
* {@code ConcurrentModificationException} if it is detected that the
* mapping function modifies this map during computation. Concurrent
* implementations should override this method and, on a best-effort basis,
* throw an {@code IllegalStateException} if it is detected that the
* mapping function modifies this map during computation and as a result
* computation would never complete.
*
* <p>The default implementation makes no guarantees about synchronization
* or atomicity properties of this method. Any implementation providing
* atomicity guarantees must override this method and document its
* concurrency properties. In particular, all implementations of
* subinterface {@link java.util.concurrent.ConcurrentMap} must document
* whether the mapping function is applied once atomically only if the value
* is not present.
*
* @param key key with which the specified value is to be associated
* @param mappingFunction the mapping function to compute a value
* @return the current (existing or computed) value associated with
* the specified key, or null if the computed value is null
* @throws NullPointerException if the specified key is null and
* this map does not support null keys, or the mappingFunction
* is null
* @throws UnsupportedOperationException if the {@code put} operation
* is not supported by this map
* (<a href="Collection.html#optional-restrictions">optional</a>)
* @throws ClassCastException if the class of the specified key or value
* prevents it from being stored in this map
* (<a href="Collection.html#optional-restrictions">optional</a>)
* @throws IllegalArgumentException if some property of the specified key
* or value prevents it from being stored in this map
* (<a href="Collection.html#optional-restrictions">optional</a>)
* @since 1.8
*/
default V computeIfAbsent(K key,
Function<? super K, ? extends V> mappingFunction) {
Objects.requireNonNull(mappingFunction);
V v;
if ((v = get(key)) == null) {
V newValue;
if ((newValue = mappingFunction.apply(key)) != null) {
put(key, newValue);
return newValue;
}
}
return v;
}
/**
* If the value for the specified key is present and non-null, attempts to
* compute a new mapping given the key and its current mapped value.
*
* <p>If the remapping function returns {@code null}, the mapping is removed.
* If the remapping function itself throws an (unchecked) exception, the
* exception is rethrown, and the current mapping is left unchanged.
*
* <p>The remapping function should not modify this map during computation.
*
* @implSpec
* The default implementation is equivalent to performing the following
* steps for this {@code map}, then returning the current value or
* {@code null} if now absent:
*
* <pre> {@code
* if (map.get(key) != null) {
* V oldValue = map.get(key);
* V newValue = remappingFunction.apply(key, oldValue);
* if (newValue != null)
* map.put(key, newValue);
* else
* map.remove(key);
* }
* }</pre>
*
* <p>The default implementation makes no guarantees about detecting if the
* remapping function modifies this map during computation and, if
* appropriate, reporting an error. Non-concurrent implementations should
* override this method and, on a best-effort basis, throw a
* {@code ConcurrentModificationException} if it is detected that the
* remapping function modifies this map during computation. Concurrent
* implementations should override this method and, on a best-effort basis,
* throw an {@code IllegalStateException} if it is detected that the
* remapping function modifies this map during computation and as a result
* computation would never complete.
*
* <p>The default implementation makes no guarantees about synchronization
* or atomicity properties of this method. Any implementation providing
* atomicity guarantees must override this method and document its
* concurrency properties. In particular, all implementations of
* subinterface {@link java.util.concurrent.ConcurrentMap} must document
* whether the remapping function is applied once atomically only if the
* value is not present.
*
* @param key key with which the specified value is to be associated
* @param remappingFunction the remapping function to compute a value
* @return the new value associated with the specified key, or null if none
* @throws NullPointerException if the specified key is null and
* this map does not support null keys, or the
* remappingFunction is null
* @throws UnsupportedOperationException if the {@code put} operation
* is not supported by this map
* (<a href="Collection.html#optional-restrictions">optional</a>)
* @throws ClassCastException if the class of the specified key or value
* prevents it from being stored in this map
* (<a href="Collection.html#optional-restrictions">optional</a>)
* @throws IllegalArgumentException if some property of the specified key
* or value prevents it from being stored in this map
* (<a href="Collection.html#optional-restrictions">optional</a>)
* @since 1.8
*/
default V computeIfPresent(K key,
BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
Objects.requireNonNull(remappingFunction);
V oldValue;
if ((oldValue = get(key)) != null) {
V newValue = remappingFunction.apply(key, oldValue);
if (newValue != null) {
put(key, newValue);
return newValue;
} else {
remove(key);
return null;
}
} else {
return null;
}
}
/**
* Attempts to compute a mapping for the specified key and its current
* mapped value (or {@code null} if there is no current mapping). For
* example, to either create or append a {@code String} msg to a value
* mapping:
*
* <pre> {@code
* map.compute(key, (k, v) -> (v == null) ? msg : v.concat(msg))}</pre>
* (Method {@link #merge merge()} is often simpler to use for such purposes.)
*
* <p>If the remapping function returns {@code null}, the mapping is removed
* (or remains absent if initially absent). If the remapping function
* itself throws an (unchecked) exception, the exception is rethrown, and
* the current mapping is left unchanged.
*
* <p>The remapping function should not modify this map during computation.
*
* @implSpec
* The default implementation is equivalent to performing the following
* steps for this {@code map}, then returning the current value or
* {@code null} if absent:
*
* <pre> {@code
* V oldValue = map.get(key);
* V newValue = remappingFunction.apply(key, oldValue);
* if (oldValue != null) {
* if (newValue != null)
* map.put(key, newValue);
* else
* map.remove(key);
* } else {
* if (newValue != null)
* map.put(key, newValue);
* else
* return null;
* }
* }</pre>
*
* <p>The default implementation makes no guarantees about detecting if the
* remapping function modifies this map during computation and, if
* appropriate, reporting an error. Non-concurrent implementations should
* override this method and, on a best-effort basis, throw a
* {@code ConcurrentModificationException} if it is detected that the
* remapping function modifies this map during computation. Concurrent
* implementations should override this method and, on a best-effort basis,
* throw an {@code IllegalStateException} if it is detected that the
* remapping function modifies this map during computation and as a result
* computation would never complete.
*
* <p>The default implementation makes no guarantees about synchronization
* or atomicity properties of this method. Any implementation providing
* atomicity guarantees must override this method and document its
* concurrency properties. In particular, all implementations of
* subinterface {@link java.util.concurrent.ConcurrentMap} must document
* whether the remapping function is applied once atomically only if the
* value is not present.
*
* @param key key with which the specified value is to be associated
* @param remappingFunction the remapping function to compute a value
* @return the new value associated with the specified key, or null if none
* @throws NullPointerException if the specified key is null and
* this map does not support null keys, or the
* remappingFunction is null
* @throws UnsupportedOperationException if the {@code put} operation
* is not supported by this map
* (<a href="Collection.html#optional-restrictions">optional</a>)
* @throws ClassCastException if the class of the specified key or value
* prevents it from being stored in this map
* (<a href="Collection.html#optional-restrictions">optional</a>)
* @throws IllegalArgumentException if some property of the specified key
* or value prevents it from being stored in this map
* (<a href="Collection.html#optional-restrictions">optional</a>)
* @since 1.8
*/
default V compute(K key,
BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
Objects.requireNonNull(remappingFunction);
V oldValue = get(key);
V newValue = remappingFunction.apply(key, oldValue);
if (newValue == null) {
// delete mapping
if (oldValue != null || containsKey(key)) {
// something to remove
remove(key);
return null;
} else {
// nothing to do. Leave things as they were.
return null;
}
} else {
// add or replace old mapping
put(key, newValue);
return newValue;
}
}
/**
* If the specified key is not already associated with a value or is
* associated with null, associates it with the given non-null value.
* Otherwise, replaces the associated value with the results of the given
* remapping function, or removes if the result is {@code null}. This
* method may be of use when combining multiple mapped values for a key.
* For example, to either create or append a {@code String msg} to a
* value mapping:
*
* <pre> {@code
* map.merge(key, msg, String::concat)
* }</pre>
*
* <p>If the remapping function returns {@code null}, the mapping is removed.
* If the remapping function itself throws an (unchecked) exception, the
* exception is rethrown, and the current mapping is left unchanged.
*
* <p>The remapping function should not modify this map during computation.
*
* @implSpec
* The default implementation is equivalent to performing the following
* steps for this {@code map}, then returning the current value or
* {@code null} if absent:
*
* <pre> {@code
* V oldValue = map.get(key);
* V newValue = (oldValue == null) ? value :
* remappingFunction.apply(oldValue, value);
* if (newValue == null)
* map.remove(key);
* else
* map.put(key, newValue);
* }</pre>
*
* <p>The default implementation makes no guarantees about detecting if the
* remapping function modifies this map during computation and, if
* appropriate, reporting an error. Non-concurrent implementations should
* override this method and, on a best-effort basis, throw a
* {@code ConcurrentModificationException} if it is detected that the
* remapping function modifies this map during computation. Concurrent
* implementations should override this method and, on a best-effort basis,
* throw an {@code IllegalStateException} if it is detected that the
* remapping function modifies this map during computation and as a result
* computation would never complete.
*
* <p>The default implementation makes no guarantees about synchronization
* or atomicity properties of this method. Any implementation providing
* atomicity guarantees must override this method and document its
* concurrency properties. In particular, all implementations of
* subinterface {@link java.util.concurrent.ConcurrentMap} must document
* whether the remapping function is applied once atomically only if the
* value is not present.
*
* @param key key with which the resulting value is to be associated
* @param value the non-null value to be merged with the existing value
* associated with the key or, if no existing value or a null value
* is associated with the key, to be associated with the key
* @param remappingFunction the remapping function to recompute a value if
* present
* @return the new value associated with the specified key, or null if no
* value is associated with the key
* @throws UnsupportedOperationException if the {@code put} operation
* is not supported by this map
* (<a href="Collection.html#optional-restrictions">optional</a>)
* @throws ClassCastException if the class of the specified key or value
* prevents it from being stored in this map
* (<a href="Collection.html#optional-restrictions">optional</a>)
* @throws IllegalArgumentException if some property of the specified key
* or value prevents it from being stored in this map
* (<a href="Collection.html#optional-restrictions">optional</a>)
* @throws NullPointerException if the specified key is null and this map
* does not support null keys or the value or remappingFunction is
* null
* @since 1.8
*/
default V merge(K key, V value,
BiFunction<? super V, ? super V, ? extends V> remappingFunction) {
Objects.requireNonNull(remappingFunction);
Objects.requireNonNull(value);
V oldValue = get(key);
V newValue = (oldValue == null) ? value :
remappingFunction.apply(oldValue, value);
if (newValue == null) {
remove(key);
} else {
put(key, newValue);
}
return newValue;
}
// Android-removed: OpenJDK 9 Immutable Map static methods
}
实现类HashMap，它可以继续泛型的使用。
public class HashMap<K,V> extends AbstractMap<K,V>
implements Map<K,V>, Cloneable, Serializable {
}
-
泛型方法
Pair类包括一个泛型的方法
public static <A, B> Pair <A, B> create(A a, B b) {
return new Pair<A, B>(a, b);
}
需要在方法修饰的地方放泛型的声明<A,B>，这样才能正确的使用泛型。
泛型解决了类似问题的解决方案，不用在定义的时候具体指定那些参数，具有动态变化的特性，简化了代码量。 以后在工作中要多多考虑使用泛型，接口和抽象。
