Java 8 API添加了一个新的抽象称为流Stream,可以让你以一种声明的方式处理数据。 Stream 使用一种类似用 SQL 语句从数据库查询数据的直观方式来提供一种对 Java 集合运算和表达的高阶抽象。 Stream API可以极大提供Java程序员的生产力,让程序员写出高效率、干净、简洁的代码。 这种风格将要处理的元素集合看作一种流, 流在管道中传输, 并且可以在管道的上进行操作处理, 比如筛选, 排序,聚合等。 元素流在管道中经过中间的一系列操作(intermediate operation)和处理,最后由最终操作(terminal operation)得到前面处理的结果。
Stream(流)是一个来自数据源的元素队列并支持聚合操作
和以前的Collection操作不同, Stream操作还有两个基础的特征:
JDK8之前 对集合进行过滤
public class Test1 {
public static void main(String[] args) {
List<String> strList = Arrays.asList("A", "B", "c", "D", "E");
System.out.println("初始化数据: " + strList);
/**
* JDK8之前 对集合进行过滤
*/
//过滤掉小写字母c
strList = getFilterOutput(strList, "c");
System.out.println("过滤小写字母c后数据:"+strList);
}
/**
* 过滤掉小写字母c
* @param strList 集合
* @param strFilter 需要过滤的字符
* @return
*/
public static List<String> getFilterOutput(List<String> strList, String strFilter) {
List<String> filterList = new ArrayList<>();
for (String s : strList) {
if (!s.equals("c")) {
filterList.add(s);
}
}
return filterList;
}
}
JDK8 后 使用Streams中 filter() and collect() 方法进行过滤数据和收集数据
public class Test2 {
public static void main(String[] args) {
List<String> strList = Arrays.asList("A", "B", "c", "D", "E");
System.out.println("初始化数据: " + strList);
strList = strList.stream().filter(s ->
!s.equals("c")
).collect(Collectors.toList());
System.out.println("过滤小写字母c后数据:" + strList);
}
}
strList.stream() : 将List转换为stram
filter() : 过滤并返回
collect() : 将返回的结果转换为List
JDK8 之前 通过username获取对象
public class Test3 {
public static void main(String[] args) {
List<User> userList = Arrays.asList(new User("user1", "password1"), new User("username2", "password2"), new User("username3", "password3"));
User user = getFilterUser(userList, "username2");
System.out.println(user);
}
/**
* 通过username获取对象
* @param userList
* @param username
* @return
*/
public static User getFilterUser(List<User> userList,String username){
for (User user : userList) {
if (username.equals(user.getUsername())){
return user;
}
}
return null;
}
}
class User{
private String username;
private String password;
public User() {
}
public User(String username, String password) {
this.username = username;
this.password = password;
}
public String getUsername() {
return username;
}
public void setUsername(String username) {
this.username = username;
}
public String getPassword() {
return password;
}
public void setPassword(String password) {
this.password = password;
}
@Override
public String toString() {
return "User{" +
"username='" + username + '\'' +
", password='" + password + '\'' +
'}';
}
}
JDK8 使用Streams中 filter(), findAny() and orElse() 方法进行过滤数据和收集数据
public class Test4 {
public static void main(String[] args) {
List<User> userList = Arrays.asList(new User("user1", "password1"), new User("username2", "password2"), new User("username3", "password3"));
User user = userList.stream().filter(username -> "username2".equals(username)).findAny().orElse(null);
System.out.println(user);
}
}
class User{
private String username;
private String password;
public User() {
}
public User(String username, String password) {
this.username = username;
this.password = password;
}
public String getUsername() {
return username;
}
public void setUsername(String username) {
this.username = username;
}
public String getPassword() {
return password;
}
public void setPassword(String password) {
this.password = password;
}
@Override
public String toString() {
return "User{" +
"username='" + username + '\'' +
", password='" + password + '\'' +
'}';
}
}
filter() : 过滤并返回
findAny() : 若有返回参数,直接返回
orElse() : 若没有返回参数,返回null
JDK 8 之前从对象集合中取出某个字段的集合
public class Test5 {
public static void main(String[] args) {
//定义list集合
List<admin> list = Arrays.asList(new admin(1, "周杰"), new admin(2, "周捷"), new admin(3, "周洁"));
List<String> names = new ArrayList<>();
for (admin admin : list) {
names.add(admin.getName());
}
System.out.println(names);
}
}
class admin {
int id;
String name;
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public admin() {
}
public admin(int id, String name) {
this.id = id;
this.name = name;
}
}
JDK8 使用Streams中 map(), collect()方法进行过滤数据和收集数据
public class Test6 {
public static void main(String[] args) {
//定义list集合
List<admin> list = Arrays.asList(new admin(1, "周杰"), new admin(2, "周捷"), new admin(3, "周洁"));
//从list集合中,取出字段name的列表
List<String> names = list.stream().map(p -> p.getName()).collect(Collectors.toList());
System.out.println(names);
}
}
class admin {
int id;
String name;
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public admin() {
}
public admin(int id, String name) {
this.id = id;
this.name = name;
}
}