Using IntelliJ IDEA StreamTrace to Debug Java Stream Operations

Java's Stream API can be difficult to debug, but IntelliJ IDEA provides a StreamTrace feature that visualizes each step of a stream pipeline, showing element counts, results, and object contents.

First Encounter with StreamTrace

The article demonstrates converting strings to their lengths and setting a breakpoint to start debugging.

The visual output displays the number of elements at each stage, the transformation results, and the correspondence between input and output, making the process clear.

Using StreamTrace

StreamTrace works only in debug mode; after placing a breakpoint on the stream code and launching the debugger, clicking the stream button opens the trace view, initially in Split mode.

Users can switch to FlatMode with the button at the lower left, and toggle back as needed.

Practical Demonstration

A sample stream filters strings whose length is greater than 5, using the following test method:

@Test
public void TestTrace() {
    Stream.of("beijing","tianjin","shanghai","wuhan")
        .map(String::length)
        .filter(e -> e > 5)
        .collect(Collectors.toList());
}