1 常规去重
碰到List去重的问题，除了遍历去重，我们常常想到利用Set集合不允许重复元素的特点，通过List和Set互转，来去掉重复元素。
// 遍历后判断赋给另一个List集合，保持原来顺序 public static void ridRepeat1(List<String> list) { System.out.println("list = [" + list + "]"); List<String> listNew = new ArrayList<String>(); for (String str : list) { if (!listNew.contains(str)) { listNew.add(str); } } System.out.println("listNew = [" + listNew + "]"); } // Set集合去重，保持原来顺序 public static void ridRepeat2(List<String> list) { System.out.println("list = [" + list + "]"); List<String> listNew = new ArrayList<String>(); Set set = new HashSet(); for (String str : list) { if (set.add(str)) { listNew.add(str); } } System.out.println("listNew = [" + listNew + "]"); } // Set去重 由于Set(HashSet)的无序性，不会保持原来顺序 public static void ridRepeat3(List<String> list) { System.out.println("list = [" + list + "]"); Set set = new HashSet(); List<String> listNew = new ArrayList<String>(); set.addAll(list); listNew.addAll(set); System.out.println("listNew = [" + listNew + "]"); } // Set通过HashSet去重（将ridRepeat3方法缩减为一行） 无序 public static void ridRepeat4(List<String> list) { System.out.println("list = [" + list + "]"); List<String> listNew = new ArrayList<String>(new HashSet(list)); System.out.println("listNew = [" + listNew + "]"); } // Set通过TreeSet去重 会按字典顺序重排序 public static void ridRepeat5(List<String> list) { System.out.println("list = [" + list + "]"); List<String> listNew = new ArrayList<String>(new TreeSet<String>(list)); System.out.println("listNew = [" + listNew + "]"); } // Set通过LinkedHashSet去重 保持原来顺序 public static void ridRepeat6(List<String> list) { System.out.println("list = [" + list + "]"); List<String> listNew = new ArrayList<String>(new LinkedHashSet<String>(list)); System.out.println("listNew = [" + listNew + "]"); }
stream去重
//利用java8的stream去重 List uniqueList = list.stream().distinct().collect(Collectors.toList()); System.out.println(uniqueList.toString());
上面的方法在List元素为基本数据类型及String类型时是可以的，但是如果List集合元素为对象，却失效了！
2 对象去重
可利用for循环遍历的方式进行判断去重，很简单不赘述，看如下方法：
2.1 stream去重
// 根据name属性去重 List<User> uniqueNameList = userList .stream() .collect(collectingAndThen( toCollection(() -> new TreeSet<>(comparing(User::getName))), ArrayList::new) ); System.out.println("根据name去重后集合元素：" + uniqueNameList.toString()); // 根据name,age属性去重 List<User> uniqueNameAndAgeList = userList .stream() .collect(collectingAndThen( toCollection(() -> new TreeSet<>(comparing(o -> o.getName() + ";" + o.getAge()))), ArrayList::new) ); System.out.println("根据name,age去重后集合元素：" + uniqueNameAndAgeList.toString());
输出结果：
重写equals()、hashCode()
//重写equals方法 @Override public boolean equals(Object obj) { User user = (User) obj; return name.equals(user.getName()) && (age==user.getAge()); } //重写hashCode方法 @Override public int hashCode() { String str = name + age; return str.hashCode(); }
