Java把 List中重複的元素移除的方法如下。

把 List物件做為 HashSet建構式的參數取得 HashSet物件即可得到不重複元素的集合，因為 SET的特性是所含的元素必須是唯一的。

List<Integer> integerList = Arrays.asList(1, 2, 2, 3, 3, 4, 4, 4, 5);
Set<Integer> integerSet = new HashSet<>(integerList);
System.out.println(integerSet); // [1, 2, 3, 4, 5]

或是轉為 Stream用 distinct()排除多的重覆。

List<Integer> integerList = Arrays.asList(1, 2, 2, 3, 3, 4, 4, 4, 5);
integerList = integerList.stream().distinct().collect(Collectors.toList());
System.out.println(integerList); // [1, 2, 3, 4, 5]

若 List的元素為自訂類別的物件則該類別必須覆寫 equals()與 hashCode()才有效果。

例如下面的 Order依照 id來區別是否相同，所以複寫 equals()及 hashCode()時僅利用 id比較及計算。

Order

package com.abc.demo;
import java.util.Objects;
public class Order {
    private long id; // key
    private long amount;
    Order(long id, long amount) {
        this.id = id;
        this.amount = amount;
    }
    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        Order order = (Order) o;
        return id == order.id;
    }
    @Override
    public int hashCode() {
        return Objects.hash(id);
    }
    @Override
    public String toString() {
        return String.format("id=%s, amount=%s", id, amount);
    }
}

下面建立有重覆 Order的 List<Order>，利用 HashSet排除重覆的物件。

List<Order> orderList = Arrays.asList(
        new Order(1, 10),
        new Order(2, 100),
        new Order(2, 100), // duplicate
        new Order(3, 1000),
        new Order(3, 1000) // duplicate
);
Set<Order> orderSet = new HashSet<>(orderList);
System.out.println(orderSet); // [id=1, amount=10, id=2, amount=100, id=3, amount=1000]

利用 Stream.distinct()排除重覆的物件。

List<Order> orderList = Arrays.asList(
        new Order(1, 10),
        new Order(2, 100),
        new Order(2, 100), // duplicate
        new Order(3, 1000),
        new Order(3, 1000) // duplicate
);
orderList = orderList.stream().distinct().collect(Collectors.toList());
System.out.println(orderList); // [id=1, amount=10, id=2, amount=100, id=3, amount=1000]

若要依物件的多個非 equals()及 hashCode()的屬性來排除重覆，可利用 TreeSet，其為有排序的 Set可定義排序的規則。

例如下面的 OrderPayment的 equals()及 hashCode()僅依 id來區別是否相同。

OrderPayment

package com.abc.demo;
import java.util.Objects;
public class OrderPayment {
    private long id;
    private String orderId;
    private String paymentId;
    public OrderPayment(long id, String orderId, String paymentId) {
        this.id = id;
        this.orderId = orderId;
        this.paymentId = paymentId;
    }
    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        OrderPayment that = (OrderPayment) o;
        return id == that.id;
    }
    @Override
    public int hashCode() {
        return Objects.hash(id);
    }
    @Override
    public String toString() {
        return "OrderPayment{" +
                "id=" + id +
                ", orderId='" + orderId + '\'' +
                ", paymentId='" + paymentId + '\'' +
                '}';
    }
    // getters and setters
}

但下面的 List<OrderPayment>要根據 orderId及 paymentId是否相同來排除重覆的元素，則在建構 TreeSet時傳入依 orderId結合 paymentId的 Comparator來比較。

List<OrderPayment> orderPaymentList = Arrays.asList(
        new OrderPayment(1, "H001", "C001"),
        new OrderPayment(1, "H001", "C001"), // duplicate
        new OrderPayment(2, "H001", "C002"),
        new OrderPayment(3, "H002", "C003"),
        new OrderPayment(3, "H002", "C003") // duplicate
);
Set<OrderPayment> set = new TreeSet<>(
        Comparator.comparing(orderPayment -> 
                orderPayment.getOrderId() + orderPayment.getPaymentId()));
set.addAll(orderPaymentList);
orderPaymentList = new ArrayList<>(set);
System.out.println(orderPaymentList); // [OrderPayment{id=1, orderId='H001', paymentId='C001'}, OrderPayment{id=2, orderId='H001', paymentId='C002'}, OrderPayment{id=3, orderId='H002', paymentId='C003'}]

或用 Stream.collect()搭配 TreeSet排除多餘的重覆。

List<OrderPayment> orderPaymentList = Arrays.asList(
        new OrderPayment(1, "H001", "C001"),
        new OrderPayment(1, "H001", "C001"), // duplicate
        new OrderPayment(2, "H001", "C002"),
        new OrderPayment(3, "H002", "C003"),
        new OrderPayment(3, "H002", "C003") // duplicate
);
orderPaymentList = orderPaymentList.stream().collect(Collectors.collectingAndThen(
        Collectors.toCollection(() -> new TreeSet<>(
                Comparator.comparing(orderPayment -> 
                        orderPayment.getOrderId() + orderPayment.getPaymentId()
        ))), ArrayList::new));
System.out.println(orderPaymentList); // [OrderPayment{id=1, orderId='H001', paymentId='C001'}, OrderPayment{id=2, orderId='H001', paymentId='C002'}, OrderPayment{id=3, orderId='H002', paymentId='C003'}]