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使用stream的Collectors.toMap()方法常见问题
java8开始的流式编程很大程度上简化了我们的代码,提高了开发效率。
我们经常会使用到stream的Collectors.toMap()来将List转换Map
在使用过程中有两个小坑需要注意
java.lang.IllegalStateException: Duplicate key
java.lang.NullPointerException
第一个是由于在List转Map过程中Map集合的key重复导致的;
第二个是由于在List转Map过程中Map集合的value有null导致的(当存在value值为空时,使用Collectors.toMap()会报NPE,因为底层调用了Map的merge方法,而map方法规定了此处的vlue不能为null,从而抛出空指针异常);
解决方案
Collectors.toMap(dto ->key值 , dto -> dto,(v1,v2) -> v1)
在后面添加(v1,v2)->v1 指定选取第一个值 当key值重复的时候,根据情况而定选取第一个还是第二个)
自定义一个Map来接收,不使用Collectors.toMap()
第一种情况示例:
import com.google.common.collect.Lists;import java.util.List;import java.util.Map;import java.util.stream.Collectors;import lombok.Data; public class Test { private static List<User> userList = Lists.newArrayList(); @Data public static class User { private String userCode; private String userName; } public static void initData() { User user1 = new User(); user1.setUserCode("10001"); user1.setUserName("张三"); User user2 = new User(); user2.setUserCode("10002"); user2.setUserName("李四"); User user3 = new User(); user3.setUserCode("10002"); user3.setUserName("王五"); userList.add(user1); userList.add(user2); userList.add(user3); } public static void main(String[] args) { initData(); //反例 // Map<String, String> userMap = userList.stream().collect(Collectors.toMap(User::getUserCode, User::getUserName)); //正例,在后面添加(u1,u2)->u1 指定选取第一个值 当key值重复的时候,根据情况而定选取第一个还是第二个 Map<String, String> userMap = userList.stream().collect(Collectors.toMap(User::getUserCode, User::getUserName, (u1, u2) -> u1)); System.out.println(userMap); }}
第二种情况示例:
import com.google.common.collect.Lists;import java.util.HashMap;import java.util.List;import java.util.Map;import java.util.stream.Collectors;import lombok.Data; public class Test { private static List<User> userList = Lists.newArrayList(); @Data public static class User { private String userCode; private String userName; } public static void initData() { User user1 = new User(); user1.setUserCode("10001"); user1.setUserName("张三"); User user2 = new User(); user2.setUserCode("10002"); user2.setUserName("李四"); User user3 = new User(); user3.setUserCode("10003"); user3.setUserName(null); userList.add(user1); userList.add(user2); userList.add(user3); } public static void main(String[] args) { initData(); //反例 // Map<String, String> userMap = userList.stream().collect(Collectors.toMap(User::getUserCode, User::getUserName)); //正例 (如果对转换后的顺序有要求,这里还可以使用LinkedHashMap) Map<String, String> userMap = userList.stream().collect(HashMap::new, (map, user) -> map.put(user.getUserCode(), user.getUserName()), HashMap::putAll); System.out.println(userMap); } }
Stream ToMap(Collectors.toMap) 实践
Requirements
List TO Map
List Stream 转换 Map时向collect()方法中传递Collector对象,对象由Collectors.toMap()方法返回。
如下实现List转换为Map
List<GroupBrandCateBO> list = new ArrayList<>( Arrays.asList( new GroupBrandCateBO("v1", "g1", "b1"), new GroupBrandCateBO("v1", "g1", "b1"), new GroupBrandCateBO("v3", "g3", "b3") ));Map<String, String> map = list.stream().collect(Collectors.toMap(item -> item.getVersion(), item -> item.getGroupCode(), (oldVal, currVal) -> oldVal, LinkedHashMap::new)); System.out.println(map.getClass());Map<String, String> map0 = list.stream().collect(Collectors.toMap(item -> item.getVersion(), item -> item.getGroupCode(), (oldVal, currVal) -> oldVal));System.out.println(map0.getClass());System.out.println(map0.toString());Map<String, String> map1 = list.stream().collect(Collectors.toMap(GroupBrandCateBO::getVersion, GroupBrandCateBO::getGroupCode));System.out.println(map1.toString());
Console
class java.util.LinkedHashMap
class java.util.HashMap
{v1=g1, v3=g3}
Exception in thread “main” java.lang.IllegalStateException: Duplicate key g1
at java.util.stream.Collectors.lambda$throwingMerger$0(Collectors.java:133)
…
问题分析
toMap()函数重载:
未指定合并函数mergeFunction情况下,传入throwingMerger()返回BinaryOperator对象,当出现key重复时,调用合并函数!
未指定Supplier实例情况下,默认生成HashMap实例。
public static <T, K, U>Collector<T, ?, Map<K,U>> toMap(Function<? super T, ? extends K> keyMapper, Function<? super T, ? extends U> valueMapper) { return toMap(keyMapper, valueMapper, throwingMerger(), HashMap::new);}public static <T, K, U>Collector<T, ?, Map<K,U>> toMap(Function<? super T, ? extends K> keyMapper, Function<? super T, ? extends U> valueMapper, BinaryOperator<U> mergeFunction) { return toMap(keyMapper, valueMapper, mergeFunction, HashMap::new);}public static <T, K, U, M extends Map<K, U>>Collector<T, ?, M> toMap(Function<? super T, ? extends K> keyMapper, Function<? super T, ? extends U> valueMapper, BinaryOperator<U> mergeFunction, Supplier<M> mapSupplier) { BiConsumer<M, T> accumulator = (map, element) -> map.merge(keyMapper.apply(element), valueMapper.apply(element), mergeFunction); return new CollectorImpl<>(mapSupplier, accumulator, mapMerger(mergeFunction), CH_ID);}private static <T> BinaryOperator<T> throwingMerger() { return (u,v) -> { throw new IllegalStateException(String.format("Duplicate key %s", u)); };}
补充
关于合并函数
List<GroupBrandCateBO> list = new ArrayList<>( Arrays.asList( new GroupBrandCateBO("v1", "g1", "b1"), new GroupBrandCateBO("v1", "g2", "b2"), new GroupBrandCateBO("v1", "g2", "b2"), new GroupBrandCateBO("v3", "g3", "b3") ));Map<String, String> map00 = list.stream().collect(Collectors.toMap(item -> item.getVersion(), item -> item.getGroupCode(), (oldVal, currVal) -> currVal));Map<String, String> map01 = list.stream().collect(Collectors.toMap(item -> item.getVersion(), item -> item.getGroupCode(), (oldVal, currVal) -> oldVal + currVal));System.out.println(map00.toString());System.out.println(map01.toString());
Console
{v1=g2, v3=g3}
{v1=g1g2g2, v3=g3}
传入Lambda表达式将转化为BinaryOperator<U> mergeFunction对象,合并处理value,非Key!!!
比如:
(oldVal, currVal) -> currVal) // key相同时当前值替换原始值(oldVal, currVal) -> oldVal + currVal //key相同时保留原始值和当前值
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