欢迎参观我的博客，一个Vue 与 SpringBoot结合的产物：https://poetize.cn
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原文链接：https://poetize.cn/article?id=50
HashMap的compute()方法
如果 key 对应的 value 不存在，则返回该 null，如果存在，则返回通过 remappingFunction 重新计算后的值。
Map<String, List<JSONObject>> map = new HashMap<>();
map.compute(jsonObject.getString("name"), (k, v) -> {
if (v == null) {
v = new ArrayList<>();
}
v.add(jsonObject);
return v;
});
Collectors.toMap
//List转Map
List<Map<String, String>> resultList = new ArrayList<>();
resultList.stream().collect(Collectors.toMap(data -> data.get("key"), Function.identity()));
//Map排序
Map<String, Long> map = new HashMap<>();
map.entrySet().stream().sorted(Map.Entry.comparingByValue(Comparator.reverseOrder()))
.collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue, (x, y) -> {
throw new AssertionError();
}, LinkedHashMap::new));
//Map计算
Map<String, Long> map = new HashMap<>();
map.entrySet().stream().collect(Collectors.toMap(data -> data.getKey(), Map.Entry::getValue, Long::sum));
//List转Map重复Key处理
Collectors.toMap(data -> data.getString("id"), data -> {
Set<String> s = new HashSet<>();
s.add(checkData(data));
return s;
}, (k1, k2) -> {
k1.addAll(k2);
return k1;
})
Collectors.joining
stream().collect(Collectors.joining("`,`", "`", "`"));
Stream分页
List<Person> persons = ...;
List<Person> page = persons.stream().skip((pageQuery.getPageNo() - 1) * pageQuery.getPageSize()).limit(pageQuery.getPageSize()).collect(Collectors.toList());
Stream.peek()
对流中的每个元素执行一个操作，而不改变流的内容。
List<String> result = new ArrayList<>();
data.stream().peek(d -> result.add(d)).filter();
Stream排序
//倒叙
List<Teacher> value = ...;
List<Teacher> sortList = value.stream().sorted(Comparator.comparing(Teacher::getName, (a, b) -> {
if (a == null) a = "";
if (b == null) b = "";
return b.compareTo(a);
}).thenComparing(Teacher::getUpdateTime, (a, b) -> {
if (a == null) a = new Date(0L);
if (b == null) b = new Date(0L);
return b.compareTo(a);
})).collect(Collectors.toList());
Collectors.groupingBy
Map<String, List<Teacher>> groupMap = list.stream().collect(Collectors.groupingBy(Teacher::getName, LinkedHashMap::new, Collectors.toList()));
Map<String, Map<String, List<Teacher>>> groupMap = list.stream().collect(Collectors.groupingBy(Teacher::getName, LinkedHashMap::new, Collectors.groupingBy(Teacher::getAge, Collectors.toList())));
Map<String, Set<String>> groupMap = list.stream().collect(Collectors.groupingBy(Teacher::getName, Collectors.mapping(Teacher::getAge, Collectors.toSet())));