没有比正直更富的遗产。——莎士比亚
我们在使用JDK自带的Collectors.toMap时，可能会遇到如下问题（什么？你不知道toMap干嘛的？戳我了解）：
1.key重复，出现java.lang.IllegalStateException: Duplicate key ***异常
例如：
List<User> users = new ArrayList<>(Arrays.asList(new User(null, "HiNo"), new User(null, "SuPa"))); Map<String, String> map = users.stream().collect(Collectors.toMap(User::getUsername, User::getPassword));
2.value为空，出现NPE： java.lang.NullPointerException
例如:
List<User> users = new ArrayList<>(Arrays.asList(new User("SuPa", null), new User("HiNo", null))); Map<String, String> map = users.stream().collect(Collectors.toMap(User::getUsername, User::getPassword));
这里我们可以不使用Collectors，而是直接在collect中传入参数
Map<String, String> map = users.stream().collect(HashMap::new, (m, v) -> m.put(v.getUsername(), v.getPassword()), HashMap::putAll);
或者是在后面追加参数，例如我这篇博客
这样确实可以解决，但每次都要写这一长串，有点难受——本人比较懒，所以写一个工具类
这里是直接put进去，所以不存在说key重复或者为NPE问题
import com.baomidou.mybatisplus.core.toolkit.ExceptionUtils; import com.baomidou.mybatisplus.core.toolkit.support.SerializedLambda; import com.ruben.pojo.User; import java.io.*; import java.lang.reflect.InvocationTargetException; import java.util.*; import java.util.function.BiConsumer; import java.util.function.BinaryOperator; import java.util.function.Function; import java.util.function.Supplier; import java.util.stream.Collector; /** * @ClassName: FunctionUtils * @Description: 我还没有写描述 * @Date: 2020/11/9 0009 23:43 * * * @author: <achao1441470436@gmail.com> * @version: 1.0 * @since: JDK 1.8 */ public class FunctionUtils { /** * @MethodName: toHashMap * @Description: list -> hashMap * @Date: 2021/1/31 0031 16:17 * * * @author: <achao1441470436@gmail.com> * @param: [list, keyFunction, valueFunction] * @returnValue: java.util.Map<K, U> */ public static <T, K, U> Map<K, U> toHashMap(List<T> list, Function<T, K> keyFunction, Function<T, U> valueFunction) { if (Objects.isNull(list) || list.isEmpty()) { return new HashMap<>(0); } return list.stream().collect(MyCollectors.toHashMap(keyFunction, valueFunction)); } public static <T, K> Map<K, T> toHashMap(List<T> list, Function<T, K> keyFunction) { return toHashMap(list, keyFunction, null); } public static <T> Map<?, T> toHashMap(List<T> list) { return toHashMap(list, null, null); } /** * @ClassName: MyCollectors * @Date: 2020/12/4 0004 11:16 * @Description: 我的自定义Collectors * @Author: <achao1441470436@gmail.com> */ public static class MyCollectors { /** * @MethodName: toHashMap * @Description: 转换成hashmap * @Date: 2020/12/4 0004 13:39 * * * @author: <achao1441470436@gmail.com> * @param: [methodName] * @returnValue: java.util.stream.Collector */ public static <T, K, U> Collector<T, HashMap<K, U>, HashMap<K, U>> toHashMap(Function<T, K> keyFunction, Function<T, U> valueFunction) { return new MyCollectorImpl<>(HashMap::new, (m, v) -> m.put(Optional.ofNullable(keyFunction).map(keyF -> keyF.apply(v)).orElseGet(() -> { try { return (K) v.getClass().getMethod("getId").invoke(v); } catch (IllegalAccessException | InvocationTargetException | NoSuchMethodException e) { e.printStackTrace(); } return null; }), Optional.ofNullable(valueFunction).map(valueF -> valueF.apply(v)).orElse((U) v)), (u, v) -> null, i -> i, Collections.unmodifiableSet(EnumSet.of(Collector.Characteristics.IDENTITY_FINISH))); } public static class MyCollectorImpl<T, A, R> implements Collector<T, A, R> { private final Supplier<A> supplier; private final BiConsumer<A, T> accumulator; private final BinaryOperator<A> combiner; private final Function<A, R> finisher; private final Set<Characteristics> characteristics; MyCollectorImpl(Supplier<A> supplier, BiConsumer<A, T> accumulator, BinaryOperator<A> combiner, Function<A, R> finisher, Set<Characteristics> characteristics) { this.supplier = supplier; this.accumulator = accumulator; this.combiner = combiner; this.finisher = finisher; this.characteristics = characteristics; } @Override public Supplier<A> supplier() { return supplier; } @Override public BiConsumer<A, T> accumulator() { return accumulator; } @Override public BinaryOperator<A> combiner() { return combiner; } @Override public Function<A, R> finisher() { return finisher; } @Override public Set<Characteristics> characteristics() { return characteristics; } } } }
使用方式（注意返回值）：
指定key，value使用它本身
Map<String, User> map = toHashMap(users, User::getUsername);
指定key、value
Map<String, String> map = toHashMap(users, User::getUsername, User::getPassword);
默认不指定，key为id、value为它本身的方式（需要有id属性并包含getter）
Map<?, User> map = toHashMap(users); // 可以强转 Map<Integer, User> map = (Map<Integer, User>) toHashMap(users);
