java-core-examples/collections

collections

Companion code for the ankurm.com post "How to Sort a HashMap by Value (and Key) in Java." Module collections in java-core-examples.

All explanation lives in the post; this module holds the runnable evidence and the captured output.

Versions

Component	Version
JDK	25.0.4.1+1 (Temurin, LTS)
JMH	1.37
JUnit Jupiter	5.11.0
Hardware	2 vCPU x86-64 VM, shared with other jobs (timings are indicative, not a leaderboard)

Quickstart

export JDK25_HOME=/path/to/jdk-25
./scripts/run-all.sh        # rebuilds and regenerates everything in output/

What is in here

File	Shows	Output
SortByValueDemo	stream into LinkedHashMap, descending, by key, list of entries, the toMap() default-HashMap trap, tie-break	01
TieBugDemo	a TreeMap with a value-only comparator silently drops equal values; the tie-break fix	02
TopNDemo	top-N by sort+limit and by a size-N PriorityQueue	03
SequencedDemo	JDK 21+ reversed(), firstEntry(), putLast(), live-view behaviour	04
src/broken/*.java	HashMap has no firstEntry(); Collections.sort(map) does not compile; javac's real message is captured	05-06
SortingBenchmark	JMH: sort-all vs heap top-10 at 1,000 and 100,000 entries; full sort into LinkedHashMap	07
SortingTest	6 assertions behind the claims above	08

The JMH run is 2 forks, 3 warmup and 5 measurement iterations of 2 s per fork. The sandbox it ran on was shared with other jobs, so re-running moves the numbers by tens of percent but not the ordering.