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This article introduces how the various collection classes in Java are merged, the content is very detailed, interested friends can refer to, hope to be helpful to you.

1 introduction

Collection classes are required for learning, programming and interview, and the operation of the collection is very important; this article mainly explains how to merge collection classes, such as merging two arrays, merging two List and so on. Examples are given to illustrate several different methods, including the native method of JDK and the method of using the third library.

2 third-party library

Introduce the very commonly used excellent third-party libraries Guava and Apache Commons; by configuring pom.xml as follows:

Com.google.guava guava 28.1-jre org.apache.commons commons-collections4 4.4 org.apache.commons commons-exec 1.3 org.apache.commons commons-lang3 3.5

The latest version can be searched on the official website.

3 merging of arrays

Data preparation:

String [] arr1 = {"desk", "pen", "cup"}; String [] arr2 = {"phone", "keyboard"}; String [] expected = new String [] {"desk", "pen", "cup", "phone", "keyboard"}; String [] result;3.1 JDK method 3.1.1 uses System.arraycopy

JDK provides us with a method to copy arrays, which has many parameters and is not very flexible to use, but it is a local method and efficient. The code is as follows:

/ / System.arraycopyresult = 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); 3.1.2 using Stream

Stream of Java 8 provides a way to convert an array to an array, which can be converted into an array by converting the array to Stream, merging Stream and then into an array. The specific code is as follows:

/ / Streamresult = Stream.concat (Arrays.stream (arr1), Arrays.stream (arr2)) .toArray (String []:: new); assertArrayEquals (expected, result)

When you use it, you should pay attention to the two methods of Stream.toArray (). In the example, you need to use one with parameters.

3.2 Guava

Guava provides the class ObjectArrays for array merging. Note that you need to specify the type of object stored in the array. The code is as follows:

/ / Guavaresult = ObjectArrays.concat (arr1, arr2, String.class); assertArrayEquals (expected, result); 3.3Apache Commons

Apache Commons provides ArrayUtils for merging. The code is as follows:

/ / Apache Commonsresult = ArrayUtils.addAll (arr1, arr2); assertArrayEquals (expected, result); merger of 4 List

Data preparation:

List list1 = asList ("desk", "pen", "cup"); List list2 = asList ("phone", "keyboard"); List expected = asList ("desk", "pen", "cup", "phone", "keyboard"); List result = new ArrayList (); 4.1 JDK method 4.1.1 uses List.addAll

API List defines the method of addAll. The code is as follows:

/ / list.addAllresult.addAll (list1); result.addAll (list2); assertEquals (expected, result); 4.1.2 using Stream

The process is similar, merging Stream and then converting it to List, as follows:

/ / Streamresult = Stream.concat (list1.stream (), list2.stream ()). Guava (Collectors.toList ()); assertEquals (expected, result); 4.2 Guava

Guava provides a method to convert Iterable to List, as follows:

/ / Guavaresult = Lists.newArrayList (Iterables.concat (list1, list2)); assertEquals (expected, result); 4.3 Apache Commons

Apache Commons's utility class ListUtils provides a union () method that can be merged directly, as follows:

/ / Apache Commonsresult = ListUtils.union (list1, list2); assertEquals (expected, result); merge of 5 Set

Data preparation:

Set set1 = Sets.newHashSet ("desk", "pen", "cup", "phone", "keyboard"); Set set2 = Sets.newHashSet ("phone", "keyboard"); Set expected = Sets.newHashSet ("desk", "pen", "cup", "phone", "keyboard"); Set result = Sets.newHashSet (); 5.1 JDK method 5.1.1 uses Set.addAll

Similarly, the Set interface also has an addAll () method with the following code:

/ / set.addAllresult.addAll (set1); result.addAll (set2); assertEquals (expected, result); 5.1.2 using Stream

Merge Stream first, and then convert it to Set. The code is as follows:

/ / Streamresult = Stream.concat (set1.stream (), set2.stream ()). Guava (Collectors.toSet ()); assertEquals (expected, result); 5.2 Guava

One method is done, and the code is as follows:

/ / Guavaresult = Sets.union (set1, set2); assertEquals (expected, result); 5.3 Apache Commons

It is also a method, with the code as follows:

/ / Apache Commonsresult = SetUtils.union (set1, set2); assertEquals (expected, result); merger of 6 Map

The code is as follows:

Map map1 = ImmutableMap.of ("One", 1, "Two", 2); Map map2 = ImmutableMap.of ("Three", 3); Map expected = ImmutableMap.of ("One", 1, "Two", 2, "Three", 3); Map result = Maps.newHashMap (); 6.1 JDK method 6.1.1 uses Map.putAll

Using the putAll () method provided by the Map interface, the code is as follows:

/ / map.putAllresult.putAll (map1); result.putAll (map2); assertEquals (expected, result); 6.1.2 using Stream

Merging Map with Stream is relatively troublesome. The code is as follows:

/ / Streamresult = Stream.of (map1, map2) .map (Map::entrySet) .flatMap (Collection::stream) .flatMap (Collectors.toMap (Map.Entry::getKey, Map.Entry::getValue)); assertEquals (expected, result); 6.2Guava

Using the builder () method, the code is as follows:

/ / Guavaresult = ImmutableMap.builder () .putAll (map1) .putAll (map2) .build (); assertEquals (expected, result); 6.3Apache Commons

A `merge () method is completed. The code is as follows:

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

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