Java HashSet
HashSet 的去重原理、常用方法与遍历顺序。
Key PointsHashSet 无序、不重复，基于 HashMap 实现；判断重复靠 hashCode + equals，自定义类必须同时重写。
创建与添加
add(e) inserts and returns false for duplicates. HashSet guarantees no order: iteration does not follow insertion. Use LinkedHashSet or TreeSet when order matters.
Java
import java.util.HashSet;
import java.util.Set;
public class HashSetAdd {
public static void main(String[] args) {
Set<String> set = new HashSet<>();
System.out.println(set.add("a"));
System.out.println(set.add("a"));
System.out.println(set.add("b"));
System.out.println(set.size());
System.out.println(set.contains("a"));
System.out.println(set.contains("z"));
}
}Output
true
false
true
2
true
false删除与遍历
remove(Object) deletes by value and clear() empties it. Iterate with for-each or forEach. For ordered output wrap in a TreeSet or sort afterwards.
Java
import java.util.HashSet;
import java.util.Set;
import java.util.TreeSet;
public class HashSetIter {
public static void main(String[] args) {
Set<String> set = new HashSet<>(Set.of("b", "a", "c"));
set.remove("b");
System.out.println(set.size());
System.out.println(new TreeSet<>(set));
set.clear();
System.out.println(set.isEmpty());
}
}Output
2
[a, c]
trueHashSet 常用方法
Beyond add, remove, contains, size, isEmpty and clear, set algebra is available: addAll for union, retainAll for intersection, removeAll for difference.
Java
import java.util.HashSet;
import java.util.Set;
public class HashSetOps {
public static void main(String[] args) {
Set<Integer> a = new HashSet<>(Set.of(1, 2, 3));
Set<Integer> b = new HashSet<>(Set.of(2, 3, 4));
Set<Integer> union = new HashSet<>(a);
union.addAll(b);
System.out.println(union.size());
Set<Integer> inter = new HashSet<>(a);
inter.retainAll(b);
System.out.println(inter);
Set<Integer> diff = new HashSet<>(a);
diff.removeAll(b);
System.out.println(diff);
}
}Output
4
[2, 3]
[1]Custom objects in a HashSet must override both
equals and hashCode on the same fields. Overriding only one breaks deduplication — the most common collection bug.创建与添加
add(e) 添加，重复元素会被忽略并返回 false。HashSet 不保证顺序 —— 遍历顺序与插入顺序无关，需要有序请用 LinkedHashSet 或 TreeSet。
Java
import java.util.HashSet;
import java.util.Set;
public class HashSetAdd {
public static void main(String[] args) {
Set<String> set = new HashSet<>();
System.out.println(set.add("a"));
System.out.println(set.add("a"));
System.out.println(set.add("b"));
System.out.println(set.size());
System.out.println(set.contains("a"));
System.out.println(set.contains("z"));
}
}Output
true
false
true
2
true
false删除与遍历
remove(Object) 按值删除，clear() 清空。遍历用 for-each 或 forEach。要按顺序输出可以配合 TreeSet 或先收集再排序。
Java
import java.util.HashSet;
import java.util.Set;
import java.util.TreeSet;
public class HashSetIter {
public static void main(String[] args) {
Set<String> set = new HashSet<>(Set.of("b", "a", "c"));
set.remove("b");
System.out.println(set.size());
System.out.println(new TreeSet<>(set));
set.clear();
System.out.println(set.isEmpty());
}
}Output
2
[a, c]
trueHashSet 常用方法
add、remove、contains、size、isEmpty、clear 之外，还有集合运算：addAll 并集、retainAll 交集、removeAll 差集。
Java
import java.util.HashSet;
import java.util.Set;
public class HashSetOps {
public static void main(String[] args) {
Set<Integer> a = new HashSet<>(Set.of(1, 2, 3));
Set<Integer> b = new HashSet<>(Set.of(2, 3, 4));
Set<Integer> union = new HashSet<>(a);
union.addAll(b);
System.out.println(union.size());
Set<Integer> inter = new HashSet<>(a);
inter.retainAll(b);
System.out.println(inter);
Set<Integer> diff = new HashSet<>(a);
diff.removeAll(b);
System.out.println(diff);
}
}Output
4
[2, 3]
[1]自定义对象放进 HashSet 时必须同时重写
equals 和 hashCode，且两者基于相同字段。只重写一个会导致去重失效 —— 这是最常见的集合 bug。