Java-Collection、Map和Array之间的转换

1 List -> Map

---

设个User类：

public class User {
    private String userName;
    private String userId;
    private String userInfo;

    public User(){}

    public User(String userName, String userId, String userInfo) {
        this.userName = userName;
        this.userId = userId;
        this.userInfo = userInfo;
    }

    //getter setter
}

1）foreach()方法

    //foreach
    public Map<String,User> ltmForEach(List<User> list){
        Map<String,User> map = new HashMap<>();
        list.forEach(user -> {
            map.put(user.getUserId(), user);
        });
        return map;
    }

2）collect()方法

    //collect()
    public Map<String,User> ltmStream(List<User> list){
        /**
         * Collectors.toMap()方法参数:
         * keyMapper: User::getUserId,调用User的getter方法
         * valueMapper: user->user, User类作为value
         * mergeFunction: 当key冲突时，value合并的方法：(n1,n2)->n2为替换，(n1,n2)->n1+n2为累加
         * mapSupplier: Map构造器，需要返回特定Map的时候使用，如TreeMap::new返回以key排序的Map
         */
        return list.stream()
                   .collect(Collectors.toMap(User::getUserId,user->user,(n1,n2)->n2,TreeMap::new));

    }

3）Map常用遍历方法

// 1. 增强for循环
Map<String, User> ltmStreamMap = l.ltmStream(list);
for(Map.Entry entry : ltmStreamMap.entrySet()){
     System.out.println(entry.getKey()+" : "+entry.getValue());
}

// 2. Iterator迭代器
Map<String, User> ltmForEachMap = l.ltmForEach(list);
Iterator<Map.Entry<String, User>> iterator = ltmForEachMap.entrySet().iterator();
while(iterator.hasNext()){
    Map.Entry<String, User> next = iterator.next();
    System.out.println(next.getKey()+" : "+next.getValue());
}

// 3. 内循环
Map<String, User> ltmForEachMap = l.ltmForEach(list);
ltmForEachMap.forEach((k,v)->{
    System.out.println(k+" : "+v);
});

2 Array -> List

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Array指数组类型数据，如：String[]、Object[]、int[]等

一般使用工具类Arrays的asList方法：

public class arrayToList {
    public static void main(String[] args) {
        String[] strs = new String[]{"a","b","c"};
        List<String> list = Arrays.asList(strs);
        for(String str : list){
            System.out.println(str);
        }
    }
}

3 List -> Array

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集合List转为数组类型Array

通常使用Collection的toArray方法

public class listToArray {
    public static void main(String[] args) {
        List<String> list = new ArrayList<>();
        list.add("a");
        list.add("b");
        list.add("c");
        //List->Array，申请大小为list.size()的数组空间
        String[] strs = list.toArray(new String[list.size()]);
    }
}

4 Array <-> Set

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public class arrayToSet {
    public static void main(String[] args) {
        /**
         * Array数组类型转为Set类型集合
         * 需要Array->List->Set
         */
        String[] strs = new String[]{"a","b","c"};
        Set<String> set = new HashSet<>(Arrays.asList(strs));

        /**
         * Set转为Array和List转为Array原理相同
         */
        strs = set.toArray(new String[set.size()]);
    }
}

5 List <-> Set

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List和Set都实现了Collection借口，Collection.addAll()方法

相互转换可以通过：

　　1）Collection.addAll()

　　2）构造方法直接传入List/Set

/** * List -> Set
 */
Set set = new HashSet(list);//构造传参
set.addAll(list);//Collection.addAll()

/**
 * Set -> List
 */
List list = new ArrayList(set);
list.addAll(set);