常用函数式接口
Function<T,R> 转换型：接受一个输入参数，返回一个结果
Consumer<T> 消费型：接受一个输入参数，无返回
Predicate<T> 判断型：接受一个输入参数，返回Boolean结果
Supplier<T> 供给型：无参数，返回结果
Pojo UserInfo
代码语言：javascript复制public class UserInfo {
private Integer id;
private String name;
private Integer isDelete;
public UserInfo(Integer id, String name, Integer isDelete) {
this.id = id;
this.name = name;
this.isDelete = isDelete;
}
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public Integer getIsDelete() {
return isDelete;
}
public void setIsDelete(Integer isDelete) {
this.isDelete = isDelete;
}
@Override
public String toString() {
return "UserInfo{"
"id=" id
", name='" name '''
", isDelete=" isDelete
'}';
}
}List -> Map
代码语言：javascript复制 public static void main(String[] args) {
ArrayList<UserInfo> list = new ArrayList();
list.add(new UserInfo(1, "张三", 1));
list.add(new UserInfo(2, "李四", 2));
list.add(new UserInfo(3, "王五", 3));
list.add(new UserInfo(3, "赵六", 4));
Map<Integer, UserInfo> userInfoMap = list.stream().collect(
// Collectors.toMap参数是3个lambda表达式：生成键的映射函数、生成值的映射函数、合并函数（因为Map存储可能会重复，导致元素丢失，我们需要指定规则保留哪个）
Collectors.toMap(
(UserInfo::getId), (userInfo -> userInfo), (k1, k2) -> {
// 合并函数，如果ID相同，保留isDelete的元素
if (k1.getIsDelete() > k2.getIsDelete()) {
return k1;
} else {
return k2;
}
})
);
userInfoMap.values().forEach(k -> {
System.out.println(String.format("name=%s,isDelete=%s",k.getName(),k.getIsDelete()));
});Filter 过滤
代码语言：javascript复制public static void main(String[] args) {
ArrayList<UserInfo> list = new ArrayList();
list.add(new UserInfo(1, "张三", 1));
list.add(new UserInfo(2, "李四", 2));
list.add(new UserInfo(3, "王五", 3));
list.add(new UserInfo(3, "赵六", 4));
List<UserInfo> collect = list.stream()
// 保留isDelete > 2 的元素
.filter(userInfo -> userInfo.getIsDelete() > 2)
.collect(Collectors.toList());
collect.forEach(k -> {
System.out.println(String.format("name=%s,isDelete=%s", k.getName(), k.getIsDelete()));
});
// 只获取保留元素ID列表
LinkedList<Integer> collect1 = collect.stream()
.map(UserInfo::getId)
.collect(Collectors.toCollection(LinkedList::new));
System.out.println("获取到的ID列表是" collect1);
}GroupingBy分组
我们日常SQL使用的时候，都是Group By实现分组。Stream也有类似功能，比如一个集合有3个组的员工，我们想获取每个组的员工，就可以通过.collect(Collectors.groupingBy(Obj::getXXX));实现。返回的集合是Map集合！
代码语言：javascript复制public static void main(String[] args) {
ArrayList<UserInfo> list = new ArrayList();
list.add(new UserInfo(1, "张三", 1));
list.add(new UserInfo(2, "李四", 2));
list.add(new UserInfo(3, "王五", 3));
list.add(new UserInfo(3, "赵六", 4));
Map<Integer, List<UserInfo>> collect = list.stream()
.collect(Collectors.groupingBy(UserInfo::getId));
collect.forEach((k,v) -> {
System.out.println(String.format("GroupID=%s,Element=%s",k.toString(),Arrays.toString(v.toArray())));
});
}排序 sorted Comparator
代码语言：javascript复制 public static void main(String[] args) {
ArrayList<UserInfo> list = new ArrayList();
list.add(new UserInfo(1, "张三", 1));
list.add(new UserInfo(2, "李四", 2));
list.add(new UserInfo(3, "王五", 3));
list.add(new UserInfo(3, "赵六", 4));
List<UserInfo> collect = list.stream()
.sorted(Comparator
.comparing(UserInfo::getId)
.thenComparing(UserInfo::getIsDelete, Comparator.reverseOrder()) // 默认升序,多一个参数实现此字段倒序
// .reversed() // 实现所有条件排序后，再反转排序
)
.collect(Collectors.toList());
collect.forEach(k -> {
System.out.println(String.format("id=%s,name=%s,isDelete=%s", k.getId(), k.getName(), k.getIsDelete()));
});
}Distinct 对元素去重
代码语言：javascript复制public static void main(String[] args) {
ArrayList<String> list = new ArrayList();
list.add("A");
list.add("A");
list.add("B");
list.add("A");
List<String> collect = list.stream().distinct().collect(Collectors.toList());
collect.forEach(k -> {
System.out.println(String.format("Element=%s",k));
});
}FindFirst 返回第一个元素
代码语言：javascript复制 public static void main(String[] args) {
ArrayList<UserInfo> list = new ArrayList();
list.add(new UserInfo(1, "张三", 1));
list.add(new UserInfo(2, "李四", 2));
list.add(new UserInfo(3, "王五", 3));
list.add(new UserInfo(3, "赵六", 4));
list.stream().findFirst().ifPresent(System.out::println);
UserInfo userInfo = list.stream().findFirst().get();
System.out.println(userInfo);
}AnyMatch 与 AllMatch
AnyMatch匹配整个集合是否包含某元素
AllMatch匹配整个集合是否都是某元素
代码语言：javascript复制public static void main(String[] args) {
ArrayList<UserInfo> list = new ArrayList();
UserInfo user = new UserInfo(1, "张三", 1);
list.add(user);
list.add(new UserInfo(2, "李四", 2));
list.add(new UserInfo(3, "王五", 3));
list.add(new UserInfo(3, "赵六", 4));
// 注意比较的是 地址值
boolean anyMatch = list.stream().anyMatch(userInfo -> user == userInfo);
System.out.println("AnyMatch结果：" anyMatch);
boolean allMatch = list.stream().allMatch(userInfo -> user == userInfo);
System.out.println("AllMatch结果：" allMatch);
}Map用法
代码语言：javascript复制public static void main(String[] args) {
ArrayList<UserInfo> list = new ArrayList();
list.add(new UserInfo(1, "张三", 1));
list.add(new UserInfo(2, "李四", 2));
list.add(new UserInfo(3, "王五", 3));
list.add(new UserInfo(3, "赵六", 4));
List<String> collect = list.stream()
.distinct()
.map(UserInfo::getName)
.collect(Collectors.toList());
System.out.println(collect);
}Peek Stream打印日志
代码语言：javascript复制public static void main(String[] args) {
ArrayList<String> list = new ArrayList();
list.add("A");
list.add("A");
list.add("B");
list.add("A");
List<String> collect = list.stream().distinct()
.peek(e -> {
System.out.println(e);
})
.collect(Collectors.toList());
}Max、Min取值
Max min用法一致！
代码语言：javascript复制 public static void main(String[] args) {
ArrayList<UserInfo> list = new ArrayList();
list.add(new UserInfo(1, "张三", 1));
list.add(new UserInfo(2, "李四", 2));
list.add(new UserInfo(3, "王五", 3));
list.add(new UserInfo(3, "赵六", 4));
Integer integer = list.stream()
.distinct()
.map(UserInfo::getId)
// 如果List集合直接是int可直接用 .max(Integer::compare)
.max(Integer::compare)
.get();
System.out.println(integer);
}Count统计元素个数
代码语言：javascript复制public static void main(String[] args) {
ArrayList<UserInfo> list = new ArrayList();
list.add(new UserInfo(1, "张三", 1));
list.add(new UserInfo(2, "李四", 2));
list.add(new UserInfo(3, "王五", 3));
list.add(new UserInfo(3, "赵六", 4));
long count = list.stream().count();
System.out.println(count);
}特殊说明： 以上文章，均是我实际操作，写出来的笔记资料，不会盗用别人文章！烦请各位，请勿直接盗用！转载记得标注来源！
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