將 List 轉為 Map<String, T>
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public class AnswerApp { public static void main(String[] args) throws Exception { List<String> names = Lists.newArrayList("Answer", "AnswerAIL", "AI"); Map<String, Integer> map = names.stream().collect(Collectors.toMap(v -> v, v -> 1)); System.out.println(map); }} |
程序運行輸出
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{Answer=1, AnswerAIL=1, AI=1} |
將 List 轉為 Map<K, V>
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public static void main(String[] args) throws Exception { List<User> users = new ArrayList<>(); for (int i = 0; i < 3; i++) { users.add(new User("answer" + i, new Random().nextInt(100))); } System.out.println(JSON.toJSONString(users)); System.out.println(); Map<String, Integer> map = users.stream().collect(Collectors.toMap(User::getName, User::getAge)); System.out.println(map);} |
程序運行輸出
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[{"age":78,"name":"answer0"},{"age":89,"name":"answer1"},{"age":72,"name":"answer2"}]{answer2=72, answer1=89, answer0=78} |
將 List 轉為 Map<String, T>
實現方式1
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public class AnswerApp { public static void main(String[] args) throws Exception { List<User> users = new ArrayList<>(); for (int i = 0; i < 3; i++) { // 改為此代碼, 轉map時會報錯 Duplicate key User // users.add(new User("answer", new Random().nextInt(100))); users.add(new User("answer" + i, new Random().nextInt(100))); } System.out.println(JSON.toJSONString(users)); System.out.println(); Map<String, User> map = users.stream().collect(Collectors.toMap(User::getName, Function.identity())); System.out.println(JSON.toJSONString(map)); }} |
該方式如果 map 的 key(如上述例子的 User::getName 的值) 重復， 會拋錯java.lang.IllegalStateException: Duplicate key User
程序運行輸出
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[{"age":22,"name":"answer0"},{"age":79,"name":"answer1"},{"age":81,"name":"answer2"}]{"answer2":{"age":81,"name":"answer2"},"answer1":{"age":79,"name":"answer1"},"answer0":{"age":22,"name":"answer0"}} |
實現方式2
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public class AnswerApp { public static void main(String[] args) throws Exception { List<User> users = new ArrayList<>(); for (int i = 0; i < 3; i++) { users.add(new User("answer", new Random().nextInt(100))); } System.out.println(JSON.toJSONString(users)); System.out.println(); // 如果 key 重復, 則根據 沖突方法 ·(key1, key2) -> key2· 判斷. 解釋: key1 key2 沖突時 取 key2 Map<String, User> map = users.stream().collect(Collectors.toMap(User::getName, Function.identity(), (key1, key2) -> key2)); System.out.println(JSON.toJSONString(map)); }} |
程序運行輸出
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[{"age":24,"name":"answer"},{"age":89,"name":"answer"},{"age":68,"name":"answer"}]{"answer":{"age":68,"name":"answer"}} |
如果改為 (key1, key2) -> key1 則輸出 {"answer":{"age":24,"name":"answer"}}
User 實體
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@Data@NoArgsConstructor@AllArgsConstructorpublic class User { private Long id; private String name; private Integer age; public User(String name) { this.name = name; } public User(String name, Integer age) { this.name = name; this.age = age; }} |
補充：java8中使用Lambda表達式將list中實體類的兩個字段轉Map
代碼：
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List<Entity> list = new ArrayList<>();Map<Integer, String> map = list.stream().collect(Collectors.toMap(Entity::getId, Entity::getType)); |
常用的lambda表達式：
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** * List -> Map * 需要注意的是： * toMap 如果集合對象有重復的key，會報錯Duplicate key .... * apple1,apple12的id都為1。 * 可以用 (k1,k2)->k1 來設置，如果有重復的key,則保留key1,舍棄key2 */Map<Integer, Apple> appleMap = appleList.stream().collect(Collectors.toMap(Apple::getId, a -> a,(k1,k2)->k1)); 安照某一字段去重list = list.stream().filter(distinctByKey(p -> ((ModCreditColumn) p).getFieldCode())).collect(Collectors.toList()); List<Double> unitNetValue = listIncreaseDto.stream().map(IncreaseDto :: getUnitNetValue).collect(Collectors.toList()); //求和 對象ListBigDecimal allFullMarketPrice = entityList.stream().filter(value -> value.getFullMarketPrice()!= null).map(SceneAnalysisRespVo::getFullMarketPrice).reduce(BigDecimal.ZERO, BigDecimal::add); List<BigDecimal> naturalDayList; BigDecimal total = naturalDayList.stream().reduce(BigDecimal.ZERO, BigDecimal::add); 分組函數Map<String, List<SceneAnalysisRespVo>> groupMap = total.getGroupList().stream().collect(Collectors.groupingBy(SceneAnalysisRespVo::getVmName)); //DV01之和BigDecimal allDV01 = values.stream().filter(sceneAnalysisRespVo -> sceneAnalysisRespVo.getDv() != null).map(SceneAnalysisRespVo::getDv).reduce(BigDecimal.ZERO, BigDecimal::add); |
以上為個人經驗，希望能給大家一個參考，也希望大家多多支持服務器之家。如有錯誤或未考慮完全的地方，望不吝賜教。
原文鏈接：https://jaemon.blog.csdn.net/article/details/92685846
