如何从列表中删除重复项? [英] How to remove duplicates from a list?

问题描述

我想从列表中删除重复项，但我所做的不起作用:

I want to remove duplicates from a list but what I am doing is not working:

List<Customer> listCustomer = new ArrayList<Customer>();    
for (Customer customer: tmpListCustomer)
{
  if (!listCustomer.contains(customer)) 
  {
    listCustomer.add(customer);
  }
 }

推荐答案

如果您问题中的代码不起作用，您可能没有在 Customer 上实现 equals(Object) 适当地分类.

If the code in your question doesn't work, you probably have not implemented equals(Object) on the Customer class appropriately.

大概有一些唯一标识客户的密钥(让我们称之为customerId)；例如

Presumably there is some key (let us call it customerId) that uniquely identifies a customer; e.g.

class Customer {
    private String customerId;
    ...

equals(Object) 的适当定义如下所示:

An appropriate definition of equals(Object) would look like this:

    public boolean equals(Object obj) {
        if (obj == this) {
            return true;
        }
        if (!(obj instanceof Customer)) {
            return false;
        }
        Customer other = (Customer) obj;
        return this.customerId.equals(other.customerId);
    }

为了完整性，您应该还实现hashCode，以便两个相等的Customer 对象将返回相同的哈希值.与上述 equals 定义匹配的 hashCode 将是:

For completeness, you should also implement hashCode so that two Customer objects that are equal will return the same hash value. A matching hashCode for the above definition of equals would be:

    public int hashCode() {
        return customerId.hashCode();
    }

还值得注意的是，如果列表很大，这不是删除重复项的有效方法.(对于包含 N 个客户的列表，您将需要在最坏的情况下执行 N*(N-1)/2 比较；即，当没有重复项时.)对于更有效的解决方案，您应该使用类似 HashSet 的东西来做重复检查.

It is also worth noting that this is not an efficient way to remove duplicates if the list is large. (For a list with N customers, you will need to perform N*(N-1)/2 comparisons in the worst case; i.e. when there are no duplicates.) For a more efficient solution you should use something like a HashSet to do the duplicate checking.

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