Stream结果收集
结果收集到集合中
// Stream<String> stream = Stream.of("aa", "bb", "cc");
List<String> list = Stream.of("aa", "bb", "cc","aa")
.collect(Collectors.toList());
System.out.println(list);
// 收集到 Set集合中
Set<String> set = Stream.of("aa", "bb", "cc", "aa")
.collect(Collectors.toSet());
System.out.println(set);
// 如果需要获取的类型为具体的实现，比如：ArrayList HashSet
ArrayList<String> arrayList = Stream.of("aa", "bb", "cc", "aa")
//.collect(Collectors.toCollection(() -> new ArrayList<>()));
.collect(Collectors.toCollection(ArrayList::new));
System.out.println(arrayList);
HashSet<String> hashSet = Stream.of("aa", "bb", "cc", "aa")
.collect(Collectors.toCollection(HashSet::new));
System.out.println(hashSet);
Stream结果收集到数组中
/**
* Stream结果收集到数组中
*/
@Test
public void test02(){
Object[] objects = Stream.of("aa", "bb", "cc", "aa")
.toArray(); // 返回的数组中的元素是 Object类型
System.out.println(Arrays.toString(objects));
// 如果我们需要指定返回的数组中的元素类型
String[] strings = Stream.of("aa", "bb", "cc", "aa")
.toArray(String[]::new);
System.out.println(Arrays.toString(strings));
}
Stream流中数据的聚合计算
/**
* Stream流中数据的聚合计算
*/
@Test
public void test03(){
// 获取年龄的最大值
Optional<Person> maxAge = Stream.of(
new Person("张三", 18)
, new Person("李四", 22)
, new Person("张三", 13)
, new Person("王五", 15)
, new Person("张三", 19)
).collect(Collectors.maxBy((p1, p2) -> p1.getAge() - p2.getAge()));
System.out.println("最大年龄：" + maxAge.get());
// 获取年龄的最小值
Optional<Person> minAge = Stream.of(
new Person("张三", 18)
, new Person("李四", 22)
, new Person("张三", 13)
, new Person("王五", 15)
, new Person("张三", 19)
).collect(Collectors.minBy((p1, p2) -> p1.getAge() - p2.getAge()));
System.out.println("最新年龄:" + minAge.get());
// 求所有人的年龄之和
Integer sumAge = Stream.of(
new Person("张三", 18)
, new Person("李四", 22)
, new Person("张三", 13)
, new Person("王五", 15)
, new Person("张三", 19)
)
//.collect(Collectors.summingInt(s -> s.getAge()))
.collect(Collectors.summingInt(Person::getAge))
;
System.out.println("年龄总和：" + sumAge);
// 年龄的平均值
Double avgAge = Stream.of(
new Person("张三", 18)
, new Person("李四", 22)
, new Person("张三", 13)
, new Person("王五", 15)
, new Person("张三", 19)
).collect(Collectors.averagingInt(Person::getAge));
System.out.println("年龄的平均值：" + avgAge);
// 统计数量
Long count = Stream.of(
new Person("张三", 18)
, new Person("李四", 22)
, new Person("张三", 13)
, new Person("王五", 15)
, new Person("张三", 19)
).filter(p->p.getAge() > 18)
.collect(Collectors.counting());
System.out.println("满足条件的记录数:" + count);
}
分组计算
/**
* 分组计算
*/
@Test
public void test04() {
// 根据账号对数据进行分组
Map<String, List<Person>> collect = Stream.of(
new Person("张三", 18, 175)
, new Person("李四", 22, 177)
, new Person("张三", 14, 165)
, new Person("李四", 15, 166)
, new Person("张三", 19, 182)
).collect(Collectors.groupingBy(Person::getName));
collect.forEach((k, v) -> System.out.println("k=" + k + "\t" + "v=" + v));
System.out.println("-----------");
// 根据年龄分组 如果大于等于18 成年否则未成年
Map<String, List<Person>> collect1 = Stream.of(
new Person("张三", 18, 175)
, new Person("李四", 22, 177)
, new Person("张三", 14, 165)
, new Person("李四", 15, 166)
, new Person("张三", 19, 182)
).collect(Collectors.groupingBy(p -> p.getAge() >= 18 ? "成年" : "未成年"));
collect1.forEach((k, v) -> System.out.println("k=" + k + "\t" + "v=" + v));
}
分组计算--多级分组
/**
* 分组计算--多级分组
*/
@Test
public void test05() {
// 先根据name分组，然后根据age(成年和未成年)分组
Map<String, Map<String, Long>> collect = Stream.of(
new Person("张三", 18, 175)
, new Person("李四", 22, 177)
, new Person("张三", 14, 165)
, new Person("李四", 15, 166)
, new Person("张三", 19, 182)
).collect(Collectors.groupingBy(
Person::getName, Collectors.groupingBy((p) -> p.getAge() >= 18 ? "成年" : "未成年", Collectors.counting())
)
);
collect.forEach((k, v) -> {
System.out.println(k);
System.out.println(v);
});
}
分区操作
/**
* 分区操作
*/
@Test
public void test06() {
Map<Boolean, List<Person>> map = Stream.of(
new Person("张三", 18, 175)
, new Person("李四", 22, 177)
, new Person("张三", 14, 165)
, new Person("李四", 15, 166)
, new Person("张三", 19, 182)
).collect(Collectors.partitioningBy(p -> p.getAge() > 18));
map.forEach((k, v) -> System.out.println(k + "\t" + v));
}
对流中的数据做拼接操作
/**
* 对流中的数据做拼接操作
*/
@Test
public void test07() {
String s1 = Stream.of(
new Person("张三", 18, 175)
, new Person("李四", 22, 177)
, new Person("张三", 14, 165)
, new Person("李四", 15, 166)
, new Person("张三", 19, 182)
).map(Person::getName)
.collect(Collectors.joining());
// 张三李四张三李四张三
System.out.println(s1);
String s2 = Stream.of(
new Person("张三", 18, 175)
, new Person("李四", 22, 177)
, new Person("张三", 14, 165)
, new Person("李四", 15, 166)
, new Person("张三", 19, 182)
).map(Person::getName)
.collect(Collectors.joining("_"));
// 张三_李四_张三_李四_张三
System.out.println(s2);
String s3 = Stream.of(
new Person("张三", 18, 175)
, new Person("李四", 22, 177)
, new Person("张三", 14, 165)
, new Person("李四", 15, 166)
, new Person("张三", 19, 182)
).map(Person::getName)
.collect(Collectors.joining("_", "###", "$$$"));
// ###张三_李四_张三_李四_张三$$$
System.out.println(s3);
}