Various Ways to Remove Duplicates from a List in Java Using Set Implementations and Streams

Java provides several built‑in collection types and APIs that can be used to remove duplicate elements from a List<Integer>. The following sections illustrate six common approaches with full source code and sample output.

1. HashSet Deduplication

Since HashSet inherently disallows duplicate entries, converting a list to a HashSet removes duplicates instantly.

public class ListDistinctExample {
    public static void main(String[] args) {
        List<Integer> list = new ArrayList<Integer>() {{
            add(1);add(3);add(5);add(2);add(1);
            add(3);add(7);add(2);
        }};
        System.out.println("原集合:" + list);
        method_2(list);
    }

    /**
     * 使用 HashSet 去重
     * @param list
     */
    public static void method_2(List<Integer> list) {
        HashSet<Integer> set = new HashSet<>(list);
        System.out.println("去重集合:" + set);
    }
}

2. TreeSet Deduplication

TreeSet

also eliminates duplicates while optionally providing sorted order.

public class ListDistinctExample {
    public static void main(String[] args) {
        List<Integer> list = new ArrayList<Integer>() {{
            add(1);add(3);add(5);add(2);add(1);
            add(3);add(7);add(2);
        }};
        System.out.println("原集合:" + list);
        method_4(list);
    }

    /**
     * 使用 TreeSet 去重(无序)
     * @param list
     */
    public static void method_4(List<Integer> list) {
        TreeSet<Integer> set = new TreeSet<>(list);
        System.out.println("去重集合:" + set);
    }
}

3. LinkedHashSet Deduplication

LinkedHashSet

removes duplicates while preserving the original insertion order.

public class ListDistinctExample {
    public static void main(String[] args) {
        List<Integer> list = new ArrayList<Integer>() {{
            add(1);add(3);add(5);add(2);add(1);
            add(3);add(7);add(2);
        }};
        System.out.println("原集合:" + list);
        method_3(list);
    }

    /**
     * 使用 LinkedHashSet 去重
     * @param list
     */
    public static void method_3(List<Integer> list) {
        LinkedHashSet<Integer> set = new LinkedHashSet<>(list);
        System.out.println("去重集合:" + set);
    }
}

4. Iterator‑Based Deduplication

This method iterates through the list, removing an element when a duplicate is detected using the iterator’s remove() method.

public class ListDistinctExample {
    public static void main(String[] args) {
        List<Integer> list = new ArrayList<Integer>() {{
            add(1);add(3);add(5);add(2);add(1);
            add(3);add(7);add(2);
        }};
        System.out.println("原集合:" + list);
        method_1(list);
    }

    /**
     * 使用迭代器去重
     * @param list
     */
    public static void method_1(List<Integer> list) {
        Iterator<Integer> iterator = list.iterator();
        while (iterator.hasNext()) {
            // 获取循环的值
            Integer item = iterator.next();
            // 如果存在两个相同的值
            if (list.indexOf(item) != list.lastIndexOf(item)) {
                // 移除最后那个相同的值
                iterator.remove();
            }
        }
        System.out.println("去重集合:" + list);
    }
}

5. Stream Distinct Deduplication

Java 8’s Stream API provides a concise distinct() operation to filter duplicates.

public class ListDistinctExample {
    public static void main(String[] args) {
        List<Integer> list = new ArrayList<Integer>() {{
            add(1);add(3);add(5);add(2);add(1);
            add(3);add(7);add(2);
        }};
        System.out.println("原集合:" + list);
        method_5(list);
    }

    /**
     * 使用 Stream 去重
     * @param list
     */
    public static void method_5(List<Integer> list) {
        list = list.stream().distinct().collect(Collectors.toList());
        System.out.println("去重集合:" + list);
    }
}

6. Contains‑Check Manual Deduplication

A new list is built by iterating over the original list and adding elements only if they are not already present in the new list.

public class ListDistinctExample {
    public static void main(String[] args) {
        List<Integer> list = new ArrayList<Integer>() {{
            add(1);add(3);add(5);add(2);add(1);
            add(3);add(7);add(2);
        }};
        System.out.println("原集合:" + list);
        method(list);
    }

    /**
     * 自定义去重
     * @param list
     */
    public static void method(List<Integer> list) {
        // 新集合
        List<Integer> newList = new ArrayList<>(list.size());
        list.forEach(i -> {
            if (!newList.contains(i)) { // 如果新集合中不存在则插入
                newList.add(i);
            }
        });
        System.out.println("去重集合:" + newList);
    }
}

Each snippet prints the original list followed by the deduplicated result, demonstrating how different Set implementations and Java APIs can be leveraged for duplicate removal.