如何在Java 8 中打印 Stream 的元素

在Java 8 中引入的 Stream API 用于处理对象的集合。流是支持各种方法的对象序列，这些方法可以流水线化以产生所需的结果。
Java流的特点是——

流不是数据结构，而是从集合、数组或 I/O 通道获取输入。
流不会更改原始数据结构，它们仅根据流水线方法提供结果。
每个中间操作都被延迟执行并返回一个流作为结果，因此可以对各种中间操作进行流水线化。终端操作标记流的结束并返回结果。

有 3 种方法可以在Java中打印 Stream 的元素：

forEach()
println() 和 collect()
窥视（）

以下是详细打印 Stream 的三种方式：

Stream forEach(Consumer action) ：此方法对流的每个元素执行一个操作。 Stream forEach(Consumer action) 是一个终端操作，即它可以遍历流以产生结果或副作用。

句法：

void forEach(Consumer action)

Where, Consumer is a functional interface
and T is the type of stream elements.

下面是如何使用 forEach() 方法打印 Stream 的元素：

方案一：

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

输出：

Geeks
For
Geeks
A
Computer
Portal

程序 2：使用简写 lambda 表达式

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

输出：

Geeks
For
Geeks
A
Computer
Portal

程序 3：这种方法消耗流并使其不可用以供将来使用。因此，由于流已被消耗，以下代码将引发错误。

// Java code to print the elements of Stream
  
import java.util.stream.*;
  
class GFG {
    public static void main(String[] args)
    {
  
        // Get the stream
        Stream 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);
        }
    }
}

输出：

Geeks
For
Geeks
A
Computer
Portal

Exception: java.lang.IllegalStateException: 
stream has already been operated upon or closed

将 println() 与 collect() 一起使用：此方法将流的元素收集为收集器实例，例如 List。因此，使用 println() 方法可以轻松完成 List 的打印。

句法：

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

方案一：

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

输出：

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

程序 2：这种方法还消耗流并使其不可用以供将来使用。因此，由于流已被消耗，以下代码将引发错误。

// Java code to print the elements of Stream
  
import java.util.stream.*;
  
class GFG {
    public static void main(String[] args)
    {
  
        // Get the stream
        Stream 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：

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

Exception: java.lang.IllegalStateException: 
stream has already been operated upon or closed

Stream peek(Consumer action) ：此方法返回由该流的元素组成的流，在从结果流中消耗元素时，另外对每个元素执行提供的操作。这是一个中间操作，即它创建一个新流，当遍历该流时，它包含与给定谓词匹配的初始流的元素。

句法：

Stream 
    peek(Consumer 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.

方案一：

// Java code to print the elements of Stream
  
import java.util.stream.*;
  
class GFG {
    public static void main(String[] args)
    {
  
        // Get the stream
        Stream 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：

Geeks
For
Geeks
A
Computer
Portal

方案 2：这种方法不消耗流。因此下面的代码不会抛出任何错误。

// Java code to print the elements of Stream
  
import java.util.stream.*;
  
class GFG {
    public static void main(String[] args)
    {
  
        // Get the stream
        Stream stream = Stream.of("Geeks", "For",
                                          "GeeksForGeeks", "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：

Filtered value: Geeks
Uppercase value :GEEKS
Filtered value: GeeksForGeeks
Uppercase value :GEEKSFORGEEKS