stream方法获取指向当前Collection对象的流对象，filter将对流中元素进行过滤，结合lambda表达式，需要在filter参数中实现一个类似于比较器的Predicate对象，返回一个boolean类型返回值，只有返回为true的Collection中的元素才会进入到forEach的循环中。
List strArr = Arrays.asList("21", "22", "3", "4");
strArr.stream().filter(str ->{
return str.startsWith("2");
}).filter(str ->{
return str.equals("22");
}).forEach(str ->{
System.out.println(str);
});
使用collect将stream转化为list
List result1 = lines.stream() // convert list to stream
.filter(line -> !"mkyong".equals(line)) // filter the line which equals to "mkyong"
.collect(Collectors.toList()); // collect the output and convert streams to a list
result1.forEach(System.out::println); // o
Stream.distinct() :字符串去重
List list = Arrays.asList("AA", "BB", "CC", "BB", "CC", "AA", "AA");
long l = list.stream().distinct().count();
System.out.println("No. of distinct elements:"+l);
String output = list.stream().distinct().collect(Collectors.joining(","));
System.out.println(output);
2. Stream.distinct() with List of Objects
在此示例中，我们有一个Book对象列表。 为了对列表进行去重，该类将重写hashCode()和equals()。
public class Book {
private String name;
private int price;
public Book(String name, int price) {
this.name = name;
this.price = price;
}
public String getName() {
return name;
}
public int getPrice() {
return price;
}
@Override
public boolean equals(final Object obj) {
if (obj == null) {
return false;
}
final Book book = (Book) obj;
if (this == book) {
return true;
} else {
return (this.name.equals(book.name) && this.price == book.price);
}
}
@Override
public int hashCode() {
int hashno = 7;
hashno = 13 * hashno + (name == null ? 0 : name.hashCode());
return hashno;
}
}
List list = new ArrayList<>();
{
list.add(new Book("Core Java", 200));
list.add(new Book("Core Java", 200));
list.add(new Book("Learning Freemarker", 150));
list.add(new Book("Spring MVC", 300));
list.add(new Book("Spring MVC", 300));
}
long l = list.stream().distinct().count();
System.out.println("No. of distinct books:"+l);
list.stream().distinct().forEach(b -> System.out.println(b.getName()+ "," + b.getPrice()));
3. Distinct by Property
distinct()不提供按照属性对对象列表进行去重的直接实现。它是基于hashCode()和equals()工作的。如果我们想要按照对象的属性，对对象列表进行去重，我们可以通过其它方法来实现。如下代码段所示：
static Predicate distinctByKey(Function super T, ?> keyExtractor) {
Map seen = new ConcurrentHashMap<>();
return t -> seen.putIfAbsent(keyExtractor.apply(t), Boolean.TRUE) == null;
}
上面的方法可以被Stream接口的 filter()接收为参数，如下所示：
list.stream().filter(distinctByKey(b -> b.getName()));
distinctByKey()方法返回一个使用ConcurrentHashMap来维护先前所见状态的 Predicate实例，如下是一个完整的使用对象属性来进行去重的示例。
public class DistinctByProperty {
public static void main(String[] args) {
List list = new ArrayList<>();
{
list.add(new Book("Core Java", 200));
list.add(new Book("Core Java", 300));
list.add(new Book("Learning Freemarker", 150));
list.add(new Book("Spring MVC", 200));
list.add(new Book("Hibernate", 300));
}
list.stream().filter(distinctByKey(b -> b.getName()))
.forEach(b -> System.out.println(b.getName()+ "," + b.getPrice()));
}
private static Predicate distinctByKey(Function super T, ?> keyExtractor) {
Map seen = new ConcurrentHashMap<>();
return t -> seen.putIfAbsent(keyExtractor.apply(t), Boolean.TRUE) == null;
}
}