stream流相当于一个流水线，一共可以分成三大类的方法。
- 获取stream流
- 中间方法
- 终结方法（可以得到一个返回结果的）
获取stream流
单列集合获取Stream流
public class Demo01 {
// 单列集合获取stream流
public static void main(String[] args) {
ArrayList<String> list = new ArrayList<>();
Collections.addAll(list, "a", "b", "c", "d", "e", "f");
/*Stream<String> stream = list.stream(); // 获取到一条流水线，并把集合中的数据放到流水线上
stream.forEach((x)-> System.out.println(x));*/
list.stream().forEach(x -> System.out.println(x));
}
}
双列集合获取stream流
public class Demo02 {
public static void main(String[] args) {
HashMap<String, Integer> map = new HashMap<>();
map.put("a", 1);
map.put("b", 2);
map.put("c", 3);
map.put("d", 4);
// 双列集合获取stream流
map.keySet().stream().forEach(x -> System.out.println(x));
map.entrySet().stream().forEach(x -> System.out.println(x));
}
}
数组获取stream流
public class Demo03 {
public static void main(String[] args) {
int[] arr = {1, 2, 3, 4, 5};
Arrays.stream(arr).forEach(x -> System.out.println(x));
}
}
一堆零散的数据
public class Demo04 {
public static void main(String[] args) {
Stream.of("a", 1, 2, 3).forEach(x -> System.out.println(x));
}
}
流只能用一次，就算保存了流对象，后边也无法使用了。
中间方法
filter过滤
public class Demo01 {
public static void main(String[] args) {
ArrayList<String> list = new ArrayList<>();
Collections.addAll(list, "张无忌", "周芷若", "赵敏", "张三丰", "张翠山", "张良", "王二麻子", "谢广坤");
list.stream()
.filter(x -> x.startsWith("张")) // 过滤姓名张开头的
.filter(x -> x.length() == 3) // 过滤长度为3的
.forEach(x -> System.out.println(x));
}
}
skip跳过前n个
public class Demo02 {
public static void main(String[] args) {
ArrayList<String> list = new ArrayList<>();
Collections.addAll(list, "张无忌", "周芷若", "赵敏", "张三丰", "张翠山", "张良", "王二麻子", "谢广坤");
list.stream()
.limit(2) // 获取前两[个]
.forEach(x -> System.out.println(x));
System.out.println("=================");
list.stream()
.skip(2) // 跳过前两个
.forEach(x -> System.out.println(x));
System.out.println("=================");
list.stream()
.skip(3)
.limit(3)
.forEach(x -> System.out.println(x));
}
}
distinct去除集合中重复的元素
public class Demo03 {
public static void main(String[] args) {
ArrayList<String> list = new ArrayList<>();
Collections.addAll(list, "张无忌","张无忌","张无忌","张无忌","张无忌", "周芷若","周芷若","周芷若","周芷若", "赵敏", "张三丰", "张翠山", "张良", "王二麻子", "谢广坤");
list.stream()
.distinct() // 去重
.forEach(x -> System.out.println(x));
}
}
concat连接两个集合
public class Demo04 {
public static void main(String[] args) {
ArrayList<String> list1 = new ArrayList<>();
Collections.addAll(list1, "张无忌", "周芷若", "赵敏", "张三丰", "张翠山", "张良");
ArrayList<String> list2 = new ArrayList<>();
Collections.addAll(list2, "张无忌", "周芷若" , "王二麻子", "谢广坤");
Stream.concat(list1.stream(), list2.stream()) // 连接两个集合
.forEach(x -> System.out.println(x));
}
}
map类型转换
public class Demo05 {
public static void main(String[] args) {
ArrayList<String> list = new ArrayList<String>();
Collections.addAll(list, "张无忌-1", "周芷若-2", "赵敏-3", "张三丰-4", "张翠山-5", "张良-6", "王二麻子-7", "谢广坤-8");
list.stream()
.map(x -> { // map方法执行完后，流上的数据就变成了整数
String[] arr = x.split("-");
return Integer.parseInt(arr[1]);
})
.forEach(x -> System.out.println(x)); // 打印的是整数
}
}
flatMap将流中每个元素转成另一个流
public class Demo06 {
public static void main(String[] args) {
List<String> familyNameList = Arrays.asList("赵", "钱", "孙", "李");
List<String> boyNameList = Arrays.asList("男", "子", "孩");
List<String> cartesianProductList = familyNameList.stream()
.flatMap(familyName -> boyNameList.stream() // 将流中每个元素转换成另一个流
.map(boyName -> familyName + boyName))
.forEach(x -> System.out.println(x));
}
}
终结方法
forEach遍历集合
public class Demo01 {
public static void main(String[] args) {
ArrayList<String> list = new ArrayList<>();
Collections.addAll(list, "张无忌", "周芷若", "赵敏", "张三丰", "张翠山", "张良", "王二麻子", "谢广坤");
list.stream().forEach(x->System.out.println(x));
}
}
count计算集合中的元素
public class Demo02 {
public static void main(String[] args) {
ArrayList<String> list = new ArrayList<>();
Collections.addAll(list, "张无忌", "周芷若", "赵敏", "张三丰", "张翠山", "张良", "王二麻子", "谢广坤");
long count = list.stream().count();
System.out.println(count);
}
}
toArray收集流中的数据放到数组中
public class Demo03 {
// 收集流中的数据，放到数组中
public static void main(String[] args) {
ArrayList<String> list = new ArrayList<String>();
Collections.addAll(list, "张无忌", "周芷若", "赵敏", "张三丰", "张翠山", "张良", "王二麻子", "谢广坤");
Object[] arr1 = list.stream().toArray();
System.out.println(Arrays.toString(arr1));
System.out.println("==========");
String[] arr2 = list.stream().toArray(n -> new String[n]);
System.out.println(Arrays.toString(arr2));
}
}
collect收集流中的数据，放到集合中（List、Set、Map）
public class Demo04 {
// 收集流中的数据，放到集合中（List、Set、Map）
public static void main(String[] args) {
ArrayList<String> list = new ArrayList<String>();
Collections.addAll(list, "张无忌-男-15", "周芷若-女-14", "赵敏-女-14", "张三丰-男-100", "张翠山-男-40", "张良-男-35", "王二麻子-男-37", "谢广坤-男-41");
// 收集所有的男性到List集合中
List<String> list1 = list.stream()
.filter(x -> "男".equals(x.split("-")[1])) // 过滤所有男性
.collect(Collectors.toList());// 创建一个ArrayList集合
// 收集所有的男性到Set集合中
Set<String> set1 = list.stream()
.filter(x -> "男".equals(x.split("-")[1])) // 过滤所有男性
.collect(Collectors.toSet());
// 收集Map集合(键不能重复，否则代码报错)
// Key：姓名、Value：年龄
/**
* toMap(键的规则，值的规则)
*/
HashMap<String, String> mp1 = list.stream()
.filter(x -> "男".equals(x.split("-")[1])) // 过滤所有男性
.collect(Collectors.toMap(x -> x.split("-")[0], // 键的规则
x -> x.split("-")[1]));// 值的规则
}
}