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 + "]");
}
//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！
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：
// 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();
}
