无涯教程-Java9 - Collection工厂方法

使用Java 9，新的工厂方法被添加到List，Set和Map接口以创建不可变的实例。用于以较少的冗长和简洁的方式创建集合。

Collections旧方法

import java.util.ArrayList;
import java.util.Collections;
import java.util.HashMap;
import java.util.HashSet;
import java.util.List;
import java.util.Map;
import java.util.Set;
public class Tester {
public static void main(String []args) {
Set<String> set=new HashSet<>();
set.add("A");
set.add("B");
set.add("C");
set=Collections.unmodifiableSet(set);
System.out.println(set);
List<String> list=new ArrayList<>();
  list</span><span class="pun">.</span><span class="kwd">add</span><span class="pun">(</span><span class="str">"A"</span><span class="pun">);</span><span class="pln">
  list</span><span class="pun">.</span><span class="kwd">add</span><span class="pun">(</span><span class="str">"B"</span><span class="pun">);</span><span class="pln">
  list</span><span class="pun">.</span><span class="kwd">add</span><span class="pun">(</span><span class="str">"C"</span><span class="pun">);</span><span class="pln">
  list</span><span class="pun">=</span><span class="typ">Collections</span><span class="pun">.</span><span class="pln">unmodifiableList</span><span class="pun">(</span><span class="pln">list</span><span class="pun">);</span><span class="pln">
  </span><span class="typ">System</span><span class="pun">.</span><span class="kwd">out</span><span class="pun">.</span><span class="pln">println</span><span class="pun">(</span><span class="pln">list</span><span class="pun">);</span><span class="pln">
  </span><span class="typ">Map</span><span class="pun">&lt;</span><span class="typ">String</span><span class="pun">,</span><span class="pln"> </span><span class="typ">String</span><span class="pun">&gt;</span><span class="pln"> map</span><span class="pun">=</span><span class="kwd">new</span><span class="pln"> </span><span class="typ">HashMap</span><span class="pun">&lt;&gt;();</span><span class="pln">

  map</span><span class="pun">.</span><span class="pln">put</span><span class="pun">(</span><span class="str">"A"</span><span class="pun">,</span><span class="str">"Apple"</span><span class="pun">);</span><span class="pln">
  map</span><span class="pun">.</span><span class="pln">put</span><span class="pun">(</span><span class="str">"B"</span><span class="pun">,</span><span class="str">"Boy"</span><span class="pun">);</span><span class="pln">
  map</span><span class="pun">.</span><span class="pln">put</span><span class="pun">(</span><span class="str">"C"</span><span class="pun">,</span><span class="str">"Cat"</span><span class="pun">);</span><span class="pln">
  map</span><span class="pun">=</span><span class="typ">Collections</span><span class="pun">.</span><span class="pln">unmodifiableMap</span><span class="pun">(</span><span class="pln">map</span><span class="pun">);</span><span class="pln">
  </span><span class="typ">System</span><span class="pun">.</span><span class="kwd">out</span><span class="pun">.</span><span class="pln">println</span><span class="pun">(</span><span class="pln">map</span><span class="pun">);</span><span class="pln">

}
}

它将打印以下输出。

[A, B, C]
[A, B, C]
{A=Apple, B=Boy, C=Cat}

Collections新方法

使用Java 9，会将以下方法及其重载的对应方法添加到List，Set和Map接口。

static <E> List<E> of(E e1, E e2, E e3);
static <E> Set<E>  of(E e1, E e2, E e3);
static <K,V> Map<K,V> of(K k1, V v1, K k2, V v2, K k3, V v3);
static <K,V> Map<K,V> ofEntries(Map.Entry<? extends K,? extends V>... entries)

注意事项

• 对于List和Set接口，of(...)方法重载了0到10个参数，其中一个带有var args参数。

• 对于Map接口，of(...)方法重载了0到10个参数。

• 如果Map接口的参数超过10个，则可以使用ofEntries(...)方法接受var args参数。

import java.util.ArrayList;
import java.util.Collections;
import java.util.HashMap;
import java.util.HashSet;
import java.util.List;
import java.util.AbstractMap;
import java.util.Map;
import java.util.Set;
public class Tester {
public static void main(String []args) {
Set<String> set=Set.of("A", "B", "C");

System.out.println(set);
List<String> list=List.of("A", "B", "C");
System.out.println(list);
Map<String, String> map=Map.of("A","Apple","B","Boy","C","Cat");
System.out.println(map);
  </span><span class="typ">Map</span><span class="pun">&lt;</span><span class="typ">String</span><span class="pun">,</span><span class="pln"> </span><span class="typ">String</span><span class="pun">&gt;</span><span class="pln"> map1</span><span class="pun">=</span><span class="typ">Map</span><span class="pun">.</span><span class="pln">ofEntries </span><span class="pun">(</span><span class="pln">
     </span><span class="kwd">new</span><span class="pln"> </span><span class="typ">AbstractMap</span><span class="pun">.</span><span class="typ">SimpleEntry</span><span class="pun">&lt;&gt;(</span><span class="str">"A"</span><span class="pun">,</span><span class="str">"Apple"</span><span class="pun">),</span><span class="pln">
     </span><span class="kwd">new</span><span class="pln"> </span><span class="typ">AbstractMap</span><span class="pun">.</span><span class="typ">SimpleEntry</span><span class="pun">&lt;&gt;(</span><span class="str">"B"</span><span class="pun">,</span><span class="str">"Boy"</span><span class="pun">),</span><span class="pln">
     </span><span class="kwd">new</span><span class="pln"> </span><span class="typ">AbstractMap</span><span class="pun">.</span><span class="typ">SimpleEntry</span><span class="pun">&lt;&gt;(</span><span class="str">"C"</span><span class="pun">,</span><span class="str">"Cat"</span><span class="pun">));</span><span class="pln">
  </span><span class="typ">System</span><span class="pun">.</span><span class="kwd">out</span><span class="pun">.</span><span class="pln">println</span><span class="pun">(</span><span class="pln">map1</span><span class="pun">);</span><span class="pln">

}
}

它将打印以下输出。

[A, B, C]
[A, B, C]
{A=Apple, B=Boy, C=Cat}
{A=Apple, B=Boy, C=Cat}

参考链接

www.learnfk.com/java/java9-…