添加stream操作工具类

hutool-core/src/main/java/cn/hutool/core/util/StreamUtil.java

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+package cn.hutool.core.util;
+
+import cn.hutool.core.collection.CollUtil;
+import cn.hutool.core.lang.Assert;
+import java.util.Collections;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Map;
+import java.util.Objects;
+import java.util.Set;
+import java.util.function.BinaryOperator;
+import java.util.function.Function;
+import java.util.function.Predicate;
+import java.util.stream.Collectors;
+
+/**
+ * java Stream工具类
+ * <pre>
+ *  在实际项目中，我们经常需要处理集合对象, 提取集合元素的某个字段集合。
+ *  此工具类可以简化一些代码, 如
+ *  原来需要提取User对象的id集合：
+ *      1 - 写一个for循环
+ *          {@code
+ *          List<Long> userIds = new ArrayList<>();
+ *          for(User user : userList) {
+ *              userIds.add(user.getId());
+ *          }
+ *          }
+ *      2 - 使用java8 引入的stream方式
+ *          {@code List<Long> userIds = userList.stream().map(User::getId).collect(Collectors.toList());}
+ *
+ *      3 - 此工具类提供了更简洁的stream写法(无需每次都写{@code collect(Collectors.toList())})
+ *          {@code List<Long> userId = StreamUtil.list(userList, User::getId);}
+ * </pre>
+ *
+ * @author yiming
+ */
+public class StreamUtil {
+
+    /**
+     * 将指定List元素的某个field提取成新的List(过滤null元素)
+     *
+     * @param sourceList 原list
+     * @param mapperFunction 映射方法
+     * @return 转化后的list
+     */
+    public static <T, R> List<R> list(List<T> sourceList, Function<T, R> mapperFunction) {
+        if (CollUtil.isEmpty(sourceList)) {
+            return Collections.emptyList();
+        }
+        Assert.notNull(mapperFunction);
+        return sourceList.stream().map(mapperFunction).filter(Objects::nonNull).distinct().collect(Collectors.toList());
+    }
+
+    /**
+     * 获取对象列表的某个字段list
+     *
+     * @param sourceList 源数据
+     * @param mapperFunction 映射方法
+     * @param filter 过滤器
+     * @param <T> 源数据类型
+     * @param <R> 结果数据类型
+     * @return 转化后的列表数据
+     */
+    public static <T, R> List<R> list(List<T> sourceList, Function<T, R> mapperFunction, Predicate<? super T> filter) {
+        if (CollUtil.isEmpty(sourceList)) {
+            return Collections.emptyList();
+        }
+        Assert.notNull(mapperFunction);
+        Assert.notNull(filter);
+        return sourceList.stream().filter(filter).map(mapperFunction).distinct().collect(Collectors.toList());
+    }
+
+    /**
+     * 将指定List元素的某个field提取成新的Set(过滤null元素)
+     *
+     * @param sourceList 原list
+     * @param mapperFunction 映射方法
+     * @return 转化后的set
+     */
+    public static <T, R> Set<R> toSet(List<T> sourceList, Function<T, R> mapperFunction) {
+        return new HashSet<>(list(sourceList, mapperFunction));
+    }
+
+    /**
+     * 将指定List元素的某个field提取成新的Set(过滤null元素)
+     *
+     * @param sourceList 原list
+     * @param mapperFunction 映射方法
+     * @param filter 过滤器
+     * @return 转化后的set
+     */
+    public static <T, R> Set<R> toSet(List<T> sourceList, Function<T, R> mapperFunction, Predicate<T> filter) {
+        return new HashSet<>(list(sourceList, mapperFunction, filter));
+    }
+
+    /**
+     * list 转 map
+     *
+     * @param sourceList 源数据
+     * @param keyMapper key映射
+     * @param valueMapper value映射
+     * @param mergeFunction value合并策略
+     * @param <T> 对象集合类型
+     * @param <K> map键类型
+     * @param <R> map值类型
+     * @return 由list转化而的map
+     */
+    public static <T, K, R> Map<K, R> toMap(List<T> sourceList, Function<T, K> keyMapper, Function<T, R> valueMapper,
+        BinaryOperator<R> mergeFunction) {
+        if (CollUtil.isEmpty(sourceList) || keyMapper == null || valueMapper == null) {
+            return Collections.emptyMap();
+        }
+        return sourceList.stream().collect(Collectors.toMap(keyMapper, valueMapper, mergeFunction));
+    }
+
+    /**
+     * 提取list指定字段转成map
+     *
+     * @param sourceList 原list
+     * @param keyMapper key映射方法
+     * @param valueMapper value映射方法
+     * @return 指定map
+     */
+    public static <T, K, R> Map<K, R> toMap(List<T> sourceList, Function<T, K> keyMapper, Function<T, R> valueMapper) {
+        if (CollUtil.isEmpty(sourceList) || keyMapper == null || valueMapper == null) {
+            return Collections.emptyMap();
+        }
+        return sourceList.stream().collect(Collectors.toMap(keyMapper, valueMapper, (v1, v2) -> v1));
+    }
+
+    /**
+     * 提取list指定字段转成map
+     *
+     * @param sourceList 原list
+     * @param keyMapper key映射方法
+     * @param valueMapper value映射方法
+     * @param filter 过滤器
+     * @return 指定map
+     */
+    public static <T, K, R> Map<K, R> toMap(List<T> sourceList, Function<T, K> keyMapper, Function<T, R> valueMapper,
+        Predicate<? super T> filter) {
+        if (CollUtil.isEmpty(sourceList) || keyMapper == null || valueMapper == null) {
+            return Collections.emptyMap();
+        }
+        return sourceList.stream().filter(filter).collect(Collectors.toMap(keyMapper, valueMapper, (v1, v2) -> v1));
+    }
+
+    /**
+     * list -> map (map值类型同list元素类型)
+     *
+     * @param sourceList 源数据
+     * @param keyMapper key映射
+     * @param <T> 对象集合类型
+     * @param <K> map键类型
+     * @return 由list转化而来的map
+     */
+    public static <T, K> Map<K, T> toMap(List<T> sourceList, Function<T, K> keyMapper) {
+        return toMap(sourceList, keyMapper, Function.identity(), (v1, v2) -> v1);
+    }
+
+    /**
+     * 按指定字段分组
+     *
+     * @param sourceList 原list
+     * @param keyMapper 字段映射
+     * @return 分组
+     */
+    public static <T, K> Map<K, List<T>> groupingBy(List<T> sourceList, Function<T, K> keyMapper) {
+        if (CollUtil.isEmpty(sourceList) || keyMapper == null) {
+            return Collections.emptyMap();
+        }
+        return sourceList.stream().collect(Collectors.groupingBy(keyMapper));
+    }
+}

hutool-core/src/test/java/cn/hutool/core/util/StreamUtilTest.java

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+package cn.hutool.core.util;
+
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.List;
+import java.util.Map;
+import java.util.Set;
+import java.util.stream.Collectors;
+import org.junit.Assert;
+import org.junit.Test;
+
+/**
+ * @author yiming
+ * @date 2020/9/4 13:40
+ */
+public class StreamUtilTest {
+
+    static List<User> userList = Arrays.asList(
+        new User(1L, "A", 3, true, true, true),
+        new User(2L, "B", 4, true, true, false),
+        new User(3L, "C", 5, true, false, true),
+        new User(4L, "E", 6, true, false, false),
+        new User(5L, "D", 7, false, true, true),
+        new User(6L, "F", 7, false, false, true),
+        new User(7L, "G", 8, false, true, false),
+        new User(8L, "H", 8, false, false, false)
+    );
+
+    @Test
+    public void testList() {
+        List<Long> userIds = StreamUtil.list(userList, User::getId);
+        Assert.assertEquals(userIds.size(), userList.size());
+    }
+
+    @Test
+    public void testListFilter() {
+        // 提取管理员用户名称
+        List<String> names = StreamUtil.list(userList, User::getName, x -> Boolean.TRUE.equals(x.isAdmin));
+        Assert.assertEquals(names.size(), 4);
+    }
+
+    @Test
+    public void testToSet() {
+        Set<Integer> ages = StreamUtil.toSet(userList, User::getAge);
+        Assert.assertEquals(ages.size(), 6);
+    }
+
+    @Test
+    public void testToSetFilter() {
+        // 提取大于6的元素
+        Set<Integer> ages = StreamUtil.toSet(userList, User::getAge, x -> x.getAge() > 6);
+        Assert.assertEquals(ages.size(), 2);
+    }
+
+    @Test
+    public void testToMap() {
+        Map<Long, User> userMap = StreamUtil.toMap(userList, User::getId);
+        Assert.assertEquals(userMap.size(), userList.size());
+    }
+
+    @Test
+    public void testToMapValue() {
+        Map<Long, String> userNameMap = StreamUtil.toMap(userList, User::getId, User::getName);
+        Assert.assertEquals(userNameMap.size(), userList.size());
+    }
+
+    @Test
+    public void testToMapFilter() {
+        // 提取年龄>5的用户姓名map
+        Map<Long, String> userNameMap = StreamUtil.toMap(userList, User::getId, User::getName, x -> x.getAge() > 5);
+        Assert.assertEquals(userNameMap.size(), 5);
+    }
+
+    @Test
+    public void testToMapMerge() {
+        // 提取用户姓名map - 自定义map的value覆盖策略
+        User tempUser1 = new User(10L, "X", 10, true, false, false);
+        User tempUser2 = new User(10L, "Y", 10, true, false, false);
+        List<User> users = new ArrayList<>();
+        users.add(tempUser1);
+        users.add(tempUser2);
+        // 若新的value同原value，取原value
+        Map<Long, String> userNameMap = StreamUtil.toMap(users, User::getId, User::getName, (v1, v2) -> v1);
+        Assert.assertEquals(userNameMap.get(10L), "X");
+        // 若新的value同原value，取新value
+        userNameMap = StreamUtil.toMap(users, User::getId, User::getName, (v1, v2) -> v2);
+        Assert.assertEquals(userNameMap.get(10L), "Y");
+    }
+
+    @Test
+    public void groupingBy() {
+        // 按年龄分组
+        Map<Integer, List<User>> ageGroupMap = StreamUtil.groupingBy(userList, User::getAge);
+        Assert.assertEquals(ageGroupMap.size(), 6);
+    }
+
+    /**
+     * 测试bean
+     */
+    public static class User {
+
+        private Long id;
+        private String name;
+        private int age;
+        private boolean isAdmin;
+        private boolean isSuper;
+        private boolean gender;
+
+        public User() {
+        }
+
+        public User(Long id, String name, int age, boolean isAdmin, boolean isSuper, boolean gender) {
+            this.id = id;
+            this.name = name;
+            this.age = age;
+            this.isAdmin = isAdmin;
+            this.isSuper = isSuper;
+            this.gender = gender;
+        }
+
+        public Long getId() {
+            return id;
+        }
+
+        public void setId(Long id) {
+            this.id = id;
+        }
+
+        public String getName() {
+            return name;
+        }
+
+        public void setName(String name) {
+            this.name = name;
+        }
+
+        public int getAge() {
+            return age;
+        }
+
+        public User setAge(int age) {
+            this.age = age;
+            return this;
+        }
+
+        public String testMethod() {
+            return "test for " + this.name;
+        }
+
+        public boolean isAdmin() {
+            return isAdmin;
+        }
+
+        public void setAdmin(boolean isAdmin) {
+            this.isAdmin = isAdmin;
+        }
+
+        public boolean isIsSuper() {
+            return isSuper;
+        }
+
+        public void setIsSuper(boolean isSuper) {
+            this.isSuper = isSuper;
+        }
+
+        public boolean isGender() {
+            return gender;
+        }
+
+        public void setGender(boolean gender) {
+            this.gender = gender;
+        }
+
+        @Override
+        public String toString() {
+            return "User [name=" + name + ", age=" + age + ", isAdmin=" + isAdmin + ", gender=" + gender + "]";
+        }
+    }
+}