SoFunction
Updated on 2025-04-27

7 common errors and solutions when using Streams in Java

Here are some common mistakes when using Java Streams:

1. Do not use the termination operation

mistake: Forgot to call the termination operation (such ascollect()、forEach()orreduce()), which causes the stream to not be executed.

public static void main(String[] args) {
    List<String> names = ("Alice", "Bob", "Charlie", "David");

    // Create a stream but no termination operation is called    ()
    .filter(name -> ("A")); // No termination operation is called here
    // Since the stream is not executed, nothing will be printed    ("Stream operations have not been executed.");
}

Solution: Always end with a termination operation to trigger the processing of the stream.

public static void main(String[] args) {
    List<String> names = ("Alice", "Bob", "Charlie", "David");

    // Create a stream and call a termination operation    ()
    .filter(name -> ("A")) // Intermediate operation    .forEach(::println); // Terminate the operation
    // This will print "Alice" because the stream is executed}

2. Modify source data

mistake: Modify the source data structure when processing the stream (e.g.List) may lead to unknown results.

public static void main(String[] args) {
    List<String> names = ("Alice", "Bob", "Charlie", "David");
    // Try to modify the source list during stream processing    ()
    .filter(name -> {
        if (("B")) {
            (name); // Modify the source list        }
        return true;
    })
    .forEach(::println);
    // Due to concurrent modification, the output may not be as expected    ("Remaining names: " + names);
}

Solution: Do not modify the source data during a stream operation, but use the stream to create a new collection.

public static void main(String[] args) {
    List<String> names = ("Alice", "Bob", "Charlie", "David");
    // Create a new list based on filtering results    List<String> filteredNames = ()
    .filter(name -> ("B")) // Filter out names starting with 'B'    .collect(());
    // Show filtered list    ("Filtered names: " + filteredNames);
    ("Original names remain unchanged: " + names);
}

3. Ignore the overhead of parallel streams

mistake: Think parallel streams always improve performance regardless of context, such as small datasets or lightweight operations.

public static void main(String[] args) {
    List<Integer> numbers = (1, 2, 3, 4, 5); // Small dataset    // Use parallel streams on small datasets    ()
    .map(n -> {
        // Simulate lightweight operations        (().getName() + " processing: " + n);
        return n * n;
    })
    .forEach(::println);
    // The output may show that unnecessary threads are created for simple tasks}

Solution: Use parallel streams with caution, especially for CPU-intensive tasks with large data sets.

public static void main(String[] args) {
    List<Integer> numbers = (1, 1_000_000) // Big data set    .boxed()
    .collect(());
    // Use parallel streams for CPU-intensive operations on large data sets    List<Integer> squareNumbers = ()
    .map(n -> {
        // Simulate CPU intensive operation        return n * n;
    })
    .collect(());
    // Print the first 10 results    ("First 10 squared numbers: " + (0, 10));
}

4. Overuse intermediate operations

mistake: Too many intermediate operations in chain calls (such asfilter()andmap()) may introduce performance overhead.

public static void main(String[] args) {
    List<String> names = ("Alice", "Bob", "Charlie", "David", "Eve");

    // Overuse intermediate operation    List<String> result = ()
    .filter(name -> ("A")) // The first intermediate operation    .filter(name -> () > 3) // The second intermediate operation    .map(String::toUpperCase) // The third intermediate operation    .map(name -> name + " is a name") // The fourth intermediate operation    .toList(); // Terminal operation
    // Output result    (result);
}

Solution: Minimize intermediate operations in the flow pipeline and use flow fusion where possible.

public static void main(String[] args) {
    List<String> names = ("Alice", "Bob", "Charlie", "David", "Eve");

    // Optimize flow pipeline    List<String> result = ()
    .filter(name -> ("A") && () > 3) // Merge filters into one    .map(name -> () + " is a name") // Merge map operation    .toList(); // Terminal operation
    // Output result    (result);
}

5. Not processing Optional values

mistake: In usefindFirst()orreduce()Waiting for operation, it was not handled correctlyOptionalresult.

public static void main(String[] args) {
    List<String> names = ("Alice", "Bob", "Charlie");
    // Try to find names starting with "Z" (does not exist)    String firstNameStartingWithZ = ()
    .filter(name -> ("Z")) 
    .findFirst() // Return an Optional    .get(); // If Optional is empty, this will throw a NoSuchElementException    // Output result    (firstNameStartingWithZ);
}

Solution: On the visitOptionalAlways check whether it exists before the value ofNoSuchElementException。

public static void main(String[] args) {
    List<String> names = ("Alice", "Bob", "Charlie");
    // Correctly handle Optional    Optional<String> firstNameStartingWithZ = ()
    .filter(name -> ("Z")) 
    .findFirst(); // Return an Optional    // Check whether Optional exists    if (()) {
        (());
    } else {
        ("No name starts with 'Z'");
    }
}

6. Ignore thread safety

mistake: Using shared mutable states in parallel streams can lead to race conditions and inconsistent results.

public static void main(String[] args) {
    List<Integer> numbers = (1, 2, 3, 4, 5);
    List<Integer> results = new ArrayList<>(); // Shared variable state    // Use shared variable states in parallel streams    ().forEach(number -> {
        (number * 2); // This may lead to race conditions    });
    // Output result    ("Results: " + results);
}

Solution: Avoid sharing mutable state; use thread-safe collections or local variables.

public static void main(String[] args) {
    List<Integer> numbers = (1, 2, 3, 4, 5);
    List<Integer> results = new CopyOnWriteArrayList<>(); // Thread-safe collection    // Use thread-safe collections in parallel streams    ().forEach(number -> {
        (number * 2); // Avoid race conditions    });
    // Output result    ("Results: " + results);
}

7. Confusing intermediate operations and terminating operations

mistake: It is unclear about the difference between an intermediate operation (returning a new stream) and a terminating operation (producing a result).

public static void main(String[] args) {
    List<String> names = ("Alice", "Bob", "Charlie", "David");
    // Error: Try to use the intermediate operation as a termination operation    // This will not compile because 'filter' returns a Stream, not a List    ().filter(name -> ("A")).forEach(::println); // The termination operation is correctly used here}

Solution: Familiar with the characteristics of each operation type to avoid logical errors in the code.

public static void main(String[] args) {
    List<String> names = ("Alice", "Bob", "Charlie", "David");
    // Use intermediate and terminate operations correctly    List<String> filteredNames = ()
        .filter(name -> ("A")) // Intermediate operation        .collect(()); // Terminate the operation
    // Output filtered name    ("Filtered Names: " + filteredNames);
}

By mastering these tips and implementing these solutions, you can better use Java Streams and write cleaner, more efficient code.

This is the end of this article about 7 common errors and solutions when using Streams in Java. For more common errors in Java Streams, please search for my previous articles or continue browsing the related articles below. I hope you will support me in the future!