删除集合转 Map 的工具方法，使用流即可。

src/main/java/xyz/zhouxy/plusone/commons/collection/CollectionTools.java

@@ -1,17 +1,10 @@
 package xyz.zhouxy.plusone.commons.collection;
 
-import java.util.Arrays;
 import java.util.Collection;
-import java.util.Comparator;
-import java.util.HashMap;
 import java.util.Map;
-import java.util.TreeMap;
-import java.util.function.Function;
 
 import javax.annotation.Nullable;
 
-import com.google.common.base.Preconditions;
-
 public class CollectionTools {
 
     // isEmpty
@@ -34,123 +27,6 @@ public class CollectionTools {
         return map != null && !map.isEmpty();
     }
 
-    // Collection -> Map
-
-    public static <K, V> HashMap<K, V> toHashMap( // NOSONAR return implementation
-            Iterable<V> c,
-            Function<? super V, K> keyGenerator,
-            int initialCapacity) {
-        HashMap<K, V> map = new HashMap<>(initialCapacity);
-        fillIntoEmptyMap(map, c, keyGenerator);
-        return map;
-    }
-
-    public static <K, V> HashMap<K, V> toHashMap( // NOSONAR return implementation
-            Collection<V> c,
-            Function<? super V, K> keyGenerator) {
-        return toHashMap(c, keyGenerator, c.size());
-    }
-
-    public static <K, V> SafeConcurrentHashMap<K, V> toConcurrentHashMap(
-            Iterable<V> c,
-            Function<? super V, K> keyGenerator,
-            int initialCapacity) {
-        SafeConcurrentHashMap<K, V> map = new SafeConcurrentHashMap<>(initialCapacity);
-        fillIntoEmptyMap(map, c, keyGenerator);
-        return map;
-    }
-
-    public static <K, V> SafeConcurrentHashMap<K, V> toConcurrentHashMap(
-            Collection<V> c,
-            Function<? super V, K> keyGenerator) {
-        return toConcurrentHashMap(c, keyGenerator, c.size());
-    }
-
-    public static <K extends Comparable<? super K>, V> TreeMap<K, V> toTreeMap( // NOSONAR return implementation
-            Iterable<V> c,
-            Function<? super V, K> keyGenerator) {
-        TreeMap<K, V> map = new TreeMap<>();
-        fillIntoEmptyMap(map, c, keyGenerator);
-        return map;
-    }
-
-    public static <K, V> TreeMap<K, V> toTreeMap( // NOSONAR return implementation
-            Iterable<V> c,
-            Function<? super V, K> keyGenerator,
-            Comparator<? super K> keycComparator) {
-        TreeMap<K, V> map = new TreeMap<>(keycComparator);
-        fillIntoEmptyMap(map, c, keyGenerator);
-        return map;
-    }
-
-    public static <K, V> void fillIntoEmptyMap(
-            Map<K, ? super V> map,
-            Iterable<V> c,
-            Function<? super V, K> keyGenerator) {
-        Preconditions.checkNotNull(map);
-        Preconditions.checkNotNull(c);
-        Preconditions.checkNotNull(keyGenerator);
-        Preconditions.checkArgument(map.isEmpty(), "The map should be empty.");
-        for (V v : c) {
-            map.put(keyGenerator.apply(v), v);
-        }
-    }
-
-    // array -> map
-
-    public static <K, V> HashMap<K, V> toHashMap( // NOSONAR return implementation
-            V[] c,
-            Function<? super V, K> keyGenerator,
-            int initialCapacity) {
-        HashMap<K, V> map = new HashMap<>(initialCapacity);
-        fillIntoEmptyMap(map, c, keyGenerator);
-        return map;
-    }
-
-    public static <K, V> HashMap<K, V> toHashMap( // NOSONAR return implementation
-            V[] c,
-            Function<? super V, K> keyGenerator) {
-        return toHashMap(c, keyGenerator, c.length);
-    }
-
-    public static <K, V> SafeConcurrentHashMap<K, V> toConcurrentHashMap(
-            V[] c,
-            Function<? super V, K> keyGenerator,
-            int initialCapacity) {
-        SafeConcurrentHashMap<K, V> map = new SafeConcurrentHashMap<>(initialCapacity);
-        fillIntoEmptyMap(map, c, keyGenerator);
-        return map;
-    }
-
-    public static <K, V> SafeConcurrentHashMap<K, V> toConcurrentHashMap(
-            V[] c,
-            Function<? super V, K> keyGenerator) {
-        return toConcurrentHashMap(c, keyGenerator, c.length);
-    }
-
-    public static <K extends Comparable<? super K>, V> TreeMap<K, V> toTreeMap( // NOSONAR return implementation
-            V[] c,
-            Function<? super V, K> keyGenerator) {
-        TreeMap<K, V> map = new TreeMap<>();
-        fillIntoEmptyMap(map, c, keyGenerator);
-        return map;
-    }
-
-    public static <K, V> TreeMap<K, V> toTreeMap( // NOSONAR return implementation
-            V[] c,
-            Function<? super V, K> keyGenerator,
-            Comparator<? super K> keyComparator) {
-        TreeMap<K, V> map = new TreeMap<>(keyComparator);
-        fillIntoEmptyMap(map, c, keyGenerator);
-        return map;
-    }
-
-    public static <K, V> void fillIntoEmptyMap(
-            Map<K, ? super V> map, V[] c,
-            Function<? super V, K> keyGenerator) {
-        fillIntoEmptyMap(map, Arrays.asList(c), keyGenerator);
-    }
-
     private CollectionTools() {
         throw new IllegalStateException("Utility class");
     }

src/main/java/xyz/zhouxy/plusone/commons/util/TreeBuilder.java

@@ -8,8 +8,6 @@ import java.util.function.BiConsumer;
 import java.util.function.Function;
 import java.util.stream.Collectors;
 
-import xyz.zhouxy.plusone.commons.collection.CollectionTools;
-
 public class TreeBuilder<T, TSubTree extends T, TIdentity> {
     private final Function<T, TIdentity> identityGetter;
     private final Function<T, Optional<TIdentity>> parentIdentityGetter;
@@ -23,7 +21,8 @@ public class TreeBuilder<T, TSubTree extends T, TIdentity> {
     }
 
     public List<T> buildTree(Collection<T> nodes) {
-        Map<TIdentity, T> identityNodeMap = CollectionTools.toHashMap(nodes, identityGetter);
+        Map<TIdentity, T> identityNodeMap = nodes.stream()
+                .collect(Collectors.toMap(identityGetter, Function.identity(), (n1, n2) -> n1));
         List<T> result = nodes.stream()
                 .filter(node -> !this.parentIdentityGetter.apply(node).isPresent())
                 .collect(Collectors.toList());