如何对(数组、List、Set、Map)执行合并操作呢?

下文笔者讲述对数组、list、Set、Map进行合并操作的方法分享,如下所示

集合合并的实现思路

可使用 guava或Apache commons中的方法对集合进行合并操作
当然，我们也可以使用Stream流方法及原生JDK方法也可以对list,set,map进行合并

集合合并的实现思路

//引入 guava或Apache commons方法  
<dependency>
  <groupId>com.google.guava</groupId>
  <artifactId>guava</artifactId>
  <version>28.1-jre</version>
</dependency>
<dependency>
  <groupId>org.apache.commons</groupId>
  <artifactId>commons-collections4</artifactId>
  <version>4.4</version>
</dependency>
<dependency>
  <groupId>org.apache.commons</groupId>
  <artifactId>commons-exec</artifactId>
  <version>1.3</version>
</dependency>
<dependency>
  <groupId>org.apache.commons</groupId>
  <artifactId>commons-lang3</artifactId>
  <version>3.5</version>
</dependency> 


//数组的合并的示例
String[] arr1 = {"java爱好者", "java265.com", "最高Java的编程者"};
String[] arr2 = {"a", "b"};
String[] expected = new String[]{"java爱好者", "java265.com", "最高Java的编程者", "a", "b"};
String[] result;

使用System.arraycopy

JDK源码实现数组合并操作

//System.arraycopy
result = new String[arr1.length + arr2.length];
System.arraycopy(arr1, 0, result, 0, arr1.length);
System.arraycopy(arr2, 0, result, arr1.length, arr2.length);
assertArrayEquals(expected, result);

使用Stream进行数组合并

Java 8的Stream提供了转化成数组的方法，可以通过将数组转化成Stream，合并Stream后再转化为数组，具体代码如下：

//Stream
result = Stream.concat(Arrays.stream(arr1), Arrays.stream(arr2))
  .toArray(String[]::new);
assertArrayEquals(expected, result);
使用的时候要注意Stream.toArray()的两个方法，例子中需要使用带参数的。

Guava进行数组合并

Guava提供类ObjectArrays进行数组合并
注意需要指定数组存储的对象的类型

例

//Guava
result = ObjectArrays.concat(arr1, arr2, String.class);
assertArrayEquals(expected, result);

Apache Commons进行数组合并

Apache Commons中可使用ArrayUtils进行数组合并

例

//Apache Commons
result = ArrayUtils.addAll(arr1, arr2);
assertArrayEquals(expected, result);

---

List的合并

List<String> list1 = asList("java爱好者", "java265.com", "最高Java的编程者");
List<String> list2 = asList("a", "b");
List<String> expected = asList("java爱好者", "java265.com", "最高Java的编程者", "a", "b");
List<String> result = new ArrayList<>();

原生JDK之List合并方法

使用List.addAll进行数组合并

//list.addAll
result.addAll(list1);
result.addAll(list2);
assertEquals(expected, result);

使用Stream进行List合并

//Stream
result = Stream.concat(list1.stream(), list2.stream())
  .collect(Collectors.toList());
assertEquals(expected, result);

Guava进行list合并

Guava提供将Iterable转化为List的方法

例

//Guava
result = Lists.newArrayList(Iterables.concat(list1, list2));
assertEquals(expected, result);

Apache Commons进行List合并

Apache Commons的工具类ListUtils中union()方法可对List进行直接合并

//Apache Commons
result = ListUtils.union(list1, list2);
assertEquals(expected, result);

Set合并

 Set<String> set1 = Sets.newHashSet("java爱好者", "java265.com", "最高Java的编程者", "a", "b");
Set<String> set2 = Sets.newHashSet("a", "b");
Set<String> expected = Sets.newHashSet("java爱好者", "java265.com", "最高Java的编程者", "a", "b");
Set<String> result = Sets.newHashSet();

原生JDK方法对set合并

使用Set.addAll对set合并

//set.addAll
result.addAll(set1);
result.addAll(set2);
assertEquals(expected, result);

使用Stream对set合并

先合并Stream，再转化成Set
达到set集合合并的操作

//Stream
result = Stream.concat(set1.stream(), set2.stream())
  .collect(Collectors.toSet());
assertEquals(expected, result);

Guava对set合并

//Guava
result = Sets.union(set1, set2);
assertEquals(expected, result);

Apache Commons对set合并

//Apache Commons
result = SetUtils.union(set1, set2);
assertEquals(expected, result);

Map合并

Map<String, Integer> map1 = ImmutableMap.of("One", 1, "Two", 2);
Map<String, Integer> map2 = ImmutableMap.of("Three", 3);
Map<String, Integer> expected = ImmutableMap.of("One", 1, "Two", 2, "Three", 3);
Map<String, Integer> result = Maps.newHashMap();

原生map合并方法之Map.putAll

//map.putAll
result.putAll(map1);
result.putAll(map2);
assertEquals(expected, result);

使用Stream对Map进行合并操作

//Stream
result = Stream.of(map1, map2)
  .map(Map::entrySet)
  .flatMap(Collection::stream)
  .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));
assertEquals(expected, result);

Guava对map合并

使用builder()方法对map合并

//Guava
result = ImmutableMap.<String, Integer>builder()
  .putAll(map1)
  .putAll(map2)
  .build();
assertEquals(expected, result);

Apache Commons对map合并

使用merge()方法即可对map进行合并操作

//Apache Commons
result = MapUtils.merge(map1, map2);
assertEquals(expected, result);