java中 List去重的五种方式
https://www.jianshu.com/p/066c9445bbc5
日常开发中经常会使用到list，那么如何更高效的对list进行去重，让代码看起来更优雅则需要在实际开发中不断的优化。推荐还是使用第二种方式，代码更简洁一些。
本文的五种去重方式分别是：
1.使用LinkedHashSet删除arraylist中的重复数据
2.使用java8新特性stream的distinct 进行List去重
3.利用HashSet不能添加重复数据的特性 由于HashSet不能保证添加顺序，所以只能作为判断条件保证顺序：
4.利用List的contains方法循环遍历,重新排序,只添加一次数据,避免重复：
5.双重for循环去重
1.使用LinkedHashSet删除arraylist中的重复数据
LinkedHashSet是在一个ArrayList删除重复数据的最佳方法。LinkedHashSet在内部完成两件事：
- 删除重复数据
- 保持添加到其中的数据的顺序
Java示例使用LinkedHashSet删除arraylist中的重复项。在给定的示例中，numbersList是包含整数的arraylist，其中一些是重复的数字。
例如1,3和5.我们将列表添加到LinkedHashSet，然后将内容返回到列表中。结果arraylist没有重复的整数。
import java.util.ArrayList; import java.util.Arrays; import java.util.LinkedHashSet;public class ArrayListExample
{
public static void main(String[] args)<span class="token punctuation">{</span> ArrayList<span class="token operator"><</span>Integer<span class="token operator">></span> numbersList <span class="token operator">=</span> <span class="token keyword">new</span> ArrayList<span class="token operator"><</span><span class="token operator">></span><span class="token punctuation">(</span>Arrays<span class="token punctuation">.</span><span class="token function">asList</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">,</span> <span class="token number">4</span><span class="token punctuation">,</span> <span class="token number">5</span><span class="token punctuation">,</span> <span class="token number">6</span><span class="token punctuation">,</span> <span class="token number">6</span><span class="token punctuation">,</span> <span class="token number">6</span><span class="token punctuation">,</span> <span class="token number">7</span><span class="token punctuation">,</span> <span class="token number">8</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span> System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>numbersList<span class="token punctuation">)</span><span class="token punctuation">;</span> LinkedHashSet<span class="token operator"><</span>Integer<span class="token operator">></span> hashSet <span class="token operator">=</span> <span class="token keyword">new</span> LinkedHashSet<span class="token operator"><</span><span class="token operator">></span><span class="token punctuation">(</span>numbersList<span class="token punctuation">)</span><span class="token punctuation">;</span> ArrayList<span class="token operator"><</span>Integer<span class="token operator">></span> listWithoutDuplicates <span class="token operator">=</span> <span class="token keyword">new</span> ArrayList<span class="token operator"><</span><span class="token operator">></span><span class="token punctuation">(</span>hashSet<span class="token punctuation">)</span><span class="token punctuation">;</span> System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>listWithoutDuplicates<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span>
}
输出结果
[1, 1, 2, 3, 3, 3, 4, 5, 6, 6, 6, 7, 8]
[1, 2, 3, 4, 5, 6, 7, 8]
2.使用java8新特性stream进行List去重
要从arraylist中删除重复项，我们也可以使用java 8 stream api。使用steam的distinct()方法返回一个由不同数据组成的流，通过对象的equals（）方法进行比较。
收集所有区域数据List使用Collectors.toList()。
Java程序，用于在不使用Set的情况下从java中的arraylist中删除重复项。
import java.util.ArrayList; import java.util.Arrays; import java.util.List; import java.util.stream.Collectors;public class ArrayListExample
{
public static void main(String[] args)<span class="token punctuation">{</span> <span class="token builtin">ArrayList</span><span class="token operator"><</span><span class="token builtin">Integer</span><span class="token operator">></span> numbersList <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">ArrayList</span><span class="token operator"><</span><span class="token operator">></span><span class="token punctuation">(</span><span class="token builtin">Arrays</span><span class="token punctuation">.</span><span class="token function">asList</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">,</span> <span class="token number">4</span><span class="token punctuation">,</span> <span class="token number">5</span><span class="token punctuation">,</span> <span class="token number">6</span><span class="token punctuation">,</span> <span class="token number">6</span><span class="token punctuation">,</span> <span class="token number">6</span><span class="token punctuation">,</span> <span class="token number">7</span><span class="token punctuation">,</span> <span class="token number">8</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token builtin">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>numbersList<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token builtin">List</span><span class="token operator"><</span><span class="token builtin">Integer</span><span class="token operator">></span> listWithoutDuplicates <span class="token operator">=</span> numbersList<span class="token punctuation">.</span><span class="token function">stream</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">distinct</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">collect</span><span class="token punctuation">(</span><span class="token builtin">Collectors</span><span class="token punctuation">.</span><span class="token function">toList</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token builtin">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>listWithoutDuplicates<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span>
}
输出结果
[1, 1, 2, 3, 3, 3, 4, 5, 6, 6, 6, 7, 8][1, 2, 3, 4, 5, 6, 7, 8]
3.利用HashSet不能添加重复数据的特性 由于HashSet不能保证添加顺序，所以只能作为判断条件保证顺序：
private static void removeDuplicate(List<String> list) {
HashSet<String> set = new HashSet<String>(list.size());
List<String> result = new ArrayList<String>(list.size());
for (String str : list) {
if (set.add(str)) {
result.add(str);
}
}
list.clear();
list.addAll(result);
}
4.利用List的contains方法循环遍历,重新排序,只添加一次数据,避免重复：
private static void removeDuplicate(List<String> list) {
List<String> result = new ArrayList<String>(list.size());
for (String str : list) {
if (!result.contains(str)) {
result.add(str);
}
}
list.clear();
list.addAll(result);
}
5.双重for循环去重
for (int i = 0; i < list.size(); i++) {
for (int j = 0; j < list.size(); j++) {
if(i!=j&&list.get(i)==list.get(j)) {
list.remove(list.get(j));
}
}
