Java 8 Stream API – Exception I

In this part we will discuss a common exception that we may encounter when working with Stream in Java 8

IllegalStateException: stream has already been operated upon or closed.

In Java 8, each Stream represents a single-purpose data sequence and supports multiple input/output operations.

A Stream must only be operated (by invoking an intermediate or terminal stream operation) once.An implementation of Stream may throw the IllegalStateException if it detects that the Stream is being reused.

Each time a terminal operation is called on a Stream object, the instance is consumed and closed.

Therefore, we are only allowed to perform a single operation that consumes a Stream, otherwise we will get an exception indicating that the Stream has already been exploited or closed.

Example

import java.util.stream.Stream;
 
/**
* @author aroundducode
*/
public class Main {
public static void main(String[] args) {
Stream<String> fruit
= Stream.of("Apple", "Mango", "Strawberry", "Banana", "Avocado", "Raspberry");
fruits.filter(f -> f.contains("m")).map(String::toUpperCase).forEach(System.out::println);
boolean anyMatch = fruits.anyMatch(f -> f.length() > 6);
System.out.println("Any Match "+anyMatch);
}
}

After invoking the #forEach() method, the Stream is closed, therefore any further operation on the Stream will throw IllegalStateException, and this is what happened after invoking the #anyMatch() method.

Exception in thread "main" java.lang.IllegalStateException: stream has already been operated upon or closed
at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:229)
at java.util.stream.ReferencePipeline.anyMatch(ReferencePipeline.java:516)
at Main.main(Main.java:11)

Solution approach

Simply put, the solution is to create a new feed every time we need one.

We can, of course, do this manually

Stream<String> fruit
= Stream.of("Apple", "Mango", "Strawberry", "Banana", "Avocado", "Raspberry");
fruits.filter(f -> f.contains("m")).map(String::toUpperCase).forEach(System.out::println);
fruit
= Stream.of("Apple", "Mango", "Strawberry", "Banana", "Avocado", "Raspberry");
boolean anyMatch = fruits.anyMatch(f -> f.length() > 6);
System.out.println("Any Match "+anyMatch);

but this is where the functional interface Supplier  comes really handy

Supplier<Stream<String>> fruit
= () -> Stream.of("Apple", "Mango", "Strawberry", "Banana", "Avocado", "Raspberry");
fruits.get().filter(f -> f.contains("m")).map(String::toUpperCase).forEach(System.out::println);
boolean anyMatch = fruits.get().anyMatch(f -> f.length() > 6);
System.out.println("Any Match "+anyMatch);

We defined the stream Supplier object with type Stream<String>, which is exactly the same type that the #get() method returns.The Supplier is based on a lambda expression that takes no input and returns a new Stream.

Calling the get() functional method on the Supplier returns a freshly created Stream object, on which we can safely perform another Stream operation.

APPLE
RASPBERRY
Any Match true

We have seen how to perform terminal operations on a Stream several times, while avoiding the famous IllegalStateException which is thrown when the Stream is already closed or operated.

I hope this article was useful to you.Thanks for reading it.

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