Java關於List集合去重方案詳細介紹
1 常規去重
碰到List去重的問題,除瞭遍歷去重,我們常常想到利用Set集合不允許重復元素的特點,通過List和Set互轉,來去掉重復元素。
// 遍歷後判斷賦給另一個List集合,保持原來順序 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,保持原來順序 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 由於Set(HashSet)的無序性,不會保持原來順序 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通過HashSet去重(將ridRepeat3方法縮減為一行) 無序 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通過TreeSet去重 會按字典順序重排序 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通過LinkedHashSet去重 保持原來順序 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元素為基本數據類型及String類型時是可以的,但是如果List集合元素為對象,卻失效瞭!
2 對象去重
可利用for循環遍歷的方式進行判斷去重,很簡單不贅述,看如下方法:
2.1 stream去重
// 根據name屬性去重 List<User> uniqueNameList = userList .stream() .collect(collectingAndThen( toCollection(() -> new TreeSet<>(comparing(User::getName))), ArrayList::new) ); System.out.println("根據name去重後集合元素:" + uniqueNameList.toString()); // 根據name,age屬性去重 List<User> uniqueNameAndAgeList = userList .stream() .collect(collectingAndThen( toCollection(() -> new TreeSet<>(comparing(o -> o.getName() + ";" + o.getAge()))), ArrayList::new) ); System.out.println("根據name,age去重後集合元素:" + uniqueNameAndAgeList.toString());
輸出結果:
重寫equals()、hashCode()
//重寫equals方法 @Override public boolean equals(Object obj) { User user = (User) obj; return name.equals(user.getName()) && (age==user.getAge()); } //重寫hashCode方法 @Override public int hashCode() { String str = name + age; return str.hashCode(); }
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