strings: Java Strings deep dive companion code (pool/intern, indy concat vs StringBuilder JMH, text blocks, string templates)

Co-Authored-By: Claude Sonnet 5.5 <[email protected]> Claude-Session: https://claude.ai/code/session_01KqJyCidz3ZgRyHABv2GVJh
2026-09-30 19:06:34 +00:00
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+# strings
+
+Companion code for the ankurm.com post *"Java Strings Deep Dive: Interning, StringBuilder, Text
+Blocks, and the Concatenation Benchmark."* Module `strings` 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 while measuring (timings are indicative, not a leaderboard) |
+
+## Quickstart
+
+```bash
+export JDK25_HOME=/path/to/jdk-25
+./scripts/run-all.sh        # rebuilds and regenerates everything in output/
+```
+
+`run-all.sh` ends with the JMH run, which takes a few minutes. JMH refuses to start if another JMH
+process holds `/tmp/jmh.lock`; wait for it rather than forcing it, or the numbers are worthless.
+
+## What is in here
+
+| File | Shows | Output |
+|---|---|---|
+| `StringPoolDemo` | `==` vs `equals`, constant folding, runtime concatenation, `intern()` (including the already-pooled `"java"` surprise) | `01` |
+| `TextBlockDemo` | closing-delimiter position, indentation stripping, trailing spaces, `\s`, `\<newline>`, escape timing, `formatted()` | `02` |
+| `ConcatLoopDemo` | `+=` in a loop allocates a new String each time; characters copied grows quadratically | `03` |
+| `ConcatShapes` | the same `+` expression compiled two ways; `javap` shows `invokedynamic` vs `StringBuilder` | `04`, `05` |
+| `src/broken/StringTemplate.java` | `STR."..."` is a compile error on JDK 25, with and without `--enable-preview` | `06` |
+| `run-all.sh` flags step | `StringTableSize`, `CompactStrings`, `OptimizeStringConcat`, `UseStringDeduplication` defaults | `07` |
+| `ConcatBenchmark`, `LoopConcatBenchmark` | JMH: `+` vs `StringBuilder` vs `String.format`; Latin-1 vs UTF-16; `+=` loop vs builder at 100/1000/10000 | `08` |
+| `StringBehaviourTest` | 8 assertions behind the claims above | `09` |
+
+`ConcatBenchmark` uses 2 forks; `LoopConcatBenchmark` uses 1 fork; both 3 warmup and 5 measurement
+iterations of 1 s. Re-running moves the numbers by tens of percent but not the ordering.