collection tomap
时间: 2023-11-12 18:48:49 浏览: 37
Collectors.toMap是Java 8中提供的一个收集器,它可以将集合中的元素转换为一个Map。它有三个重载的方法,分别是:
1. toMap(Function<? super T,? extends K> keyMapper, Function<? super T,? extends U> valueMapper):根据元素的某个属性作为键,将元素本身作为值,生成一个Map。
2. toMap(Function<? super T,? extends K> keyMapper, Function<? super T,? extends U> valueMapper, BinaryOperator<U> mergeFunction):在遇到键冲突时,通过提供的函数来处理值的合并操作。
3. toMap(Function<? super T, extends K> keyMapper, Function<? super T,? extends U> valueMapper, BinaryOperator<U> mergeFunction, Supplier<M> mapSupplier):在遇到键冲突时,通过提供的函数来处理值的合并操作,并提供一个指定类型的Map作为结果。
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mybatis collection
MyBatis provides a way to map relationships between objects in Java and tables in a database using collections. A collection is a group of objects that are related to each other in some way. In MyBatis, collections are used to map one-to-many relationships between objects and tables.
There are three types of collections in MyBatis:
1. List: A list is an ordered collection of objects. In MyBatis, a list is used to map a one-to-many relationship between an object and a table.
2. Set: A set is an unordered collection of objects. In MyBatis, a set is used to map a one-to-many relationship between an object and a table, but the order of the objects is not important.
3. Map: A map is a collection of key-value pairs. In MyBatis, a map is used to map a many-to-one relationship between objects and tables.
To map a collection in MyBatis, you can use the collection element in your mapping file. The collection element contains a property attribute that specifies the name of the collection property in your Java object, and a select attribute that specifies the SQL statement to retrieve the related objects from the database.
For example, if you have a User object that has a list of Orders, you can map the Orders collection in MyBatis like this:
```
<resultMap id="userResultMap" type="User">
<id property="id" column="user_id" />
<result property="username" column="username" />
<collection property="orders" ofType="Order" select="selectOrdersByUserId"/>
</resultMap>
<select id="selectOrdersByUserId" resultType="Order">
SELECT * FROM orders WHERE user_id = #{id}
</select>
```
In this example, the collection element maps the Orders property in the User object to the selectOrdersByUserId SQL statement. MyBatis will execute the SQL statement and populate the Orders list in the User object with the results.
Collections are a powerful feature in MyBatis that allow you to map complex relationships between objects and tables in your database. By using collections, you can simplify your code and make it easier to work with your data in Java.
mybatis collection 子查询
MyBatis allows you to perform collection subqueries, which can be useful when you need to retrieve related data for a collection of records. Collection subqueries are typically used in scenarios where you have a one-to-many or many-to-many relationship between tables.
To implement a collection subquery in MyBatis, you can use the `<collection>` element within your result mapping. Here's an example:
```xml
<resultMap id="userMap" type="User">
<id property="id" column="user_id" />
<result property="name" column="user_name" />
<collection property="orders" ofType="Order">
<id property="id" column="order_id" />
<result property="product" column="product_name" />
</collection>
</resultMap>
```
In this example, we have a `User` object that has a collection of `Order` objects. The `User` object has an `id` and a `name`, while the `Order` object has an `id` and a `product`. The `collection` element is used to define the mapping for the collection property (`orders`) of the `User` object.
To perform the collection subquery, you can use the `<select>` element with a nested `<collection>` element in your SQL statement. Here's an example:
```xml
<select id="getUserWithOrders" resultMap="userMap">
SELECT u.id AS user_id, u.name AS user_name,
o.id AS order_id, o.product AS product_name
FROM users u
INNER JOIN orders o ON u.id = o.user_id
WHERE u.id = #{userId}
</select>
```
In this example, we join the `users` and `orders` tables on the `user_id` column and retrieve the relevant columns for both the `User` and `Order` objects. The `<collection>` element is used to map the query results to the collection property (`orders`) of the `User` object.
You can then use this mapping and SQL statement in your MyBatis mapper interface to retrieve a `User` object with its associated `Order` objects.
I hope this explanation helps! Let me know if you have any further questions.