Java集合学习

Posted 2020-11-08 艾白羊

一、集合的分类：

 

二、常用的集合：

 

1、Java collection：Jdk中的集合

1、List

//List
List<String>  list = new ArrayList<>();
list.add("a");
list.add("b");
list.add("c");
System.out.println(list); // [a, b, c]

 

2、Map

//Map
Map<String,String>  map = new HashMap<>();
map.put("name","by");
map.put("age","18");
System.out.println(map); // {name=by, age=18}

 

3、Set

//Set
Set<String> set = new HashSet<>();
set.add("a");
set.add("b");
System.out.println(set); // [a, b]

 

4、Iterator遍历集合

//Iterator遍历List集合
Iterator iterator = list.iterator();
while (iterator.hasNext()){
    String parm = (String) iterator.next();
    System.out.println(parm);
    if(parm.equals("a")){
        iterator.remove();
    }
}
System.out.println(list); // [b, c]

 

5、遍历MAP

//Map遍历比较，迭代器遍历比增强for循环遍历效率高，使用（方法四）遍历效率最高

Map<String, String> mapdemo = new HashMap<>();
mapdemo.put("name", "by");
mapdemo.put("age", "18");
mapdemo.put("love", "reading");
mapdemo.put("girlfriend", "marry");

//增强for循环遍历
//法一：使用keySet()遍历
for (String key : mapdemo.keySet()) {
    System.out.println(key + ":" + mapdemo.get(key));
}

//法二：使用entrySet()遍历
for (Map.Entry<String, String> entry : mapdemo.entrySet()) {
    System.out.println(entry.getKey() + ":" + entry.getValue());
}

//迭代器遍历
//法三：使用keySet()遍历
Iterator<String> ite = mapdemo.keySet().iterator();
while (ite.hasNext()) {
    String key = ite.next();
    System.out.println(key + "　：" + mapdemo.get(key));
}

//法四：使用entrySet()遍历
Iterator<Map.Entry<String, String>> iter = mapdemo.entrySet().iterator();
while (iter.hasNext()) {
    Map.Entry<String, String> entry = iter.next();
    System.out.println(entry.getKey() + "　：" + entry.getValue());
}

 

2、Guava Collections（google开源工具 ）

1、引入依赖：

<!-- https://mvnrepository.com/artifact/com.google.guava/guava -->
<dependency>
    <groupId>com.google.guava</groupId>
    <artifactId>guava</artifactId>
    <version>21.0</version>
</dependency>

 

2、示例：

 

2.1 List

//创建List
List<String> list = Lists.newArrayList("a","b","c");
list.add("d");
//反转List
System.out.println(Lists.reverse(list)); // [d, c, b, a]

//将List集合转换为特定规则的字符串
String listResult = Joiner.on("-").join(list);
System.out.println(listResult); // a-b-c-d

 

2.2 Map

//定义Map
Map<String,String > map = Maps.newHashMap();
map.put("name","by");
map.put("age","23");
System.out.println(map); //{name=by, age=23}
//将Map集合转换为特定规则的字符串
String mapResult = Joiner.on(",").withKeyValueSeparator("=").join(map);
System.out.println(mapResult); // name=by,age=23

//定义Map中放List的形式（Map<String,List<Integer>>）
Multimap<String,Integer> maps = ArrayListMultimap.create();
maps.put("map",1);
maps.put("map",2);
System.out.println(maps); //{map=[1, 2]}

 

2.3 Set

//定义Set
Set<String> set = Sets.newHashSet();
set.add("value");

 

3、Trove

1、引入依赖：

<!-- https://mvnrepository.com/artifact/net.sf.trove4j/trove4j -->
<dependency>
    <groupId>net.sf.trove4j</groupId>
    <artifactId>trove4j</artifactId>
    <version>3.0.3</version>
</dependency>

 

2、示例：

 

2.1 构造基本类型的集合

//直接构造int类型的集合
TIntArrayList intList = new TIntArrayList();
intList.add(1);
intList.add(2);
intList.add(3);
intList.reverse();
System.out.println(intList);

 

4、Apache commons collections

1、引入依赖：

<!-- https://mvnrepository.com/artifact/commons-collections/commons-collections -->
<dependency>
    <groupId>commons-collections</groupId>
    <artifactId>commons-collections</artifactId>
    <version>3.2.1</version>
</dependency>

 

2、示例：

 

2.1. 集合的并集、交集、差集

User user = new User();
user.setId(1l);
user.setName("by");

User user1 = new User();
user1.setId(2l);
user1.setName("ly");

User user2 = new User();
user2.setId(3l);
user2.setName("hy");

List<User> list = new ArrayList<>();
list.add(user);
list.add(user1);

List<User> list1 = new ArrayList<>();
list1.add(user);

// 并集
Collection<User> unionList = CollectionUtils.union(list, list1);
System.out.println(unionList); // [User(id=1, name=by, alimony=null), User(id=2, name=ly, alimony=null)]
// 交集
Collection<User> intersectionList = CollectionUtils.intersection(list, list1);
System.out.println(intersectionList); // [User(id=1, name=by, alimony=null)]
// 集合相减
Collection<User> subtractList = CollectionUtils.subtract(list, list1);
System.out.println(subtractList); // [User(id=2, name=ly, alimony=null)]