如何在Java 8中打印Stream的元素 _JavaStream

在Java 8中引入Stream API，它用于处理对象的集合。Stream是支持各种方法的对象序列, 可以对这些方法进行流水线处理以产生所需的结果。
Java Stream的功能是–

Stream不是数据结构, 而是从集合, 数组或I/O通道获取输入。
Stream不会改变原始数据结构, 它们只会按照流水线方法提供结果。
每个中间操作都是延迟执行的, 并因此返回一个流, 因此可以对各种中间操作进行流水线处理。终端操作标记流的结尾并返回结果。

有3种方法可以用Java打印Stream元素：

forEach()
println()与collect()
peek()

以下是详细打印Stream的三种方法：
Stream forEach(消费者动作):这个方法对流的每个元素执行一个动作。流forEach(消费者操作)是一个终端操作，也就是说，它可以遍历流来产生结果或副作用。
语法：

void forEach(Consumer< ? super T> action)Where, Consumer is a functional interface and T is the type of stream elements.

以下是使用forEach()方法打印Stream元素的方法：
程序1：

// Java code to print the elements of Streamimport java.util.stream.*; class GFG { public static void main(String[] args) {// Get the stream Stream< String> stream = Stream.of( "Geeks" , "For" , "Geeks" , "A" , "Computer" , "Portal" ); // Print the stream stream.forEach(s -> System.out.println(s)); } }

输出如下：

Geeks For Geeks A Computer Portal

程序2：使用Short Hand Lambda表达式

// Java code to print the elements of Streamimport java.util.stream.*; class GFG { public static void main(String[] args) {// Get the stream Stream< String> stream = Stream.of( "Geeks" , "For" , "Geeks" , "A" , "Computer" , "Portal" ); // Print the stream stream.forEach(System.out::println); } }

输出如下：

Geeks For Geeks A Computer Portal

程序3：这种方法消耗了Stream, 并使其无法将来使用。因此, 以下代码将引发错误, 因为Stream已被使用。

// Java code to print the elements of Streamimport java.util.stream.*; class GFG { public static void main(String[] args) {// Get the stream Stream< String> stream = Stream.of( "Geeks" , "For" , "Geeks" , "A" , "Computer" , "Portal" ); // Print the stream stream.forEach(s -> System.out.println(s)); // Since the stream has been already consumed // this will throw exception try {// Print the stream stream.forEach(s -> System.out.println(s)); }catch (Exception e) {System.out.println( "\nException: " + e); } } }

【如何在Java 8中打印Stream的元素】输出如下：

Geeks For Geeks A Computer PortalException: java.lang.IllegalStateException: stream has already been operated upon or closed

将println()与collect()结合使用：
此方法收集Stream元素作为收集器实例, 例如List。因此, 使用println()方法可以轻松完成List的打印。
语法如下：

System.out.println(stream.collect(Collectors.toList()));

程序1：

// Java code to print the elements of Streamimport java.util.stream.*; class GFG { public static void main(String[] args) {// Get the stream Stream< String> stream = Stream.of( "Geeks" , "For" , "Geeks" , "A" , "Computer" , "Portal" ); // Print the stream System.out.println(stream.collect(Collectors.toList())); } }

输出如下：

[Geeks, For, Geeks, A, Computer, Portal]

程序2：这种方法还会消耗该Stream, 并使其无法将来使用。因此, 以下代码将引发错误, 因为Stream已被使用。

// Java code to print the elements of Streamimport java.util.stream.*; class GFG { public static void main(String[] args) {// Get the stream Stream< String> stream = Stream.of( "Geeks" , "For" , "Geeks" , "A" , "Computer" , "Portal" ); // Print the stream System.out.println(stream.collect(Collectors.toList())); // Since the stream has been already consumed // this will throw exception try {// Print the stream System.out.println(stream.collect(Collectors.toList())); }catch (Exception e) {System.out.println( "\nException: " + e); } } }

输出如下：

[Geeks, For, Geeks, A, Computer, Portal]Exception: java.lang.IllegalStateException: stream has already been operated upon or closed

Stream peek(消费者行为)：此方法返回由该流的元素组成的流, 并在从结果流中消耗元素时对每个元素另外执行提供的操作。这是个中间操作，即, 它创建了一个新的流, 该流在遍历时将包含与给定谓词匹配的初始流的元素。
语法：

Stream< T> peek(Consumer< ? super T> action)Where, Stream is an interface and T is the type of stream elements. action is a non-interfering action to perform on the elements as they are consumed from the stream and the function returns the new stream.

程序1：

// Java code to print the elements of Streamimport java.util.stream.*; class GFG { public static void main(String[] args) {// Get the stream Stream< String> stream = Stream.of( "Geeks" , "For" , "Geeks" , "A" , "Computer" , "Portal" ); // Print the stream using peek() // by providing a terminal operation count() stream.peek(s -> System.out.println(s)).count(); } }

输出如下：

Geeks For Geeks A Computer Portal

程序2：这种方法不消费流。因此, 以下代码不会引发任何错误。

// Java code to print the elements of Streamimport java.util.stream.*; class GFG { public static void main(String[] args) {// Get the stream Stream< String> stream = Stream.of( "Geeks" , "For" , "srcmini" , "A" , "Computer" , "Portal" ); // Since the stream is not being consumed // this will not throw any exception// Print the stream stream.filter(s -> s.startsWith( "G" )) .peek(s -> System.out.println( "Filtered value: " + s)) .map(String::toUpperCase) .peek(s -> System.out.println( "Uppercase value :" + s)) .count(); } }

输出如下：