# 6. Lazy constants (JEP 531, third preview)

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Compile and run with `--enable-preview --release 27`. Source: [`LazyDemo`](../jep-tour/src/LazyDemo.java), transcript [22](output/22-lazy.txt).

A **lazy constant** holds a value that is computed the first time somebody asks for it, exactly once, even when many threads ask at the same instant,
after which the JVM may treat it like a `final` field. It replaces the hand-written holder-class idiom and the double-checked-locking field.

```java
static final LazyConstant<Settings> SETTINGS = LazyConstant.of(LazyDemo::loadSettings);
Settings s = SETTINGS.get();   // computes on first call, returns the same instance afterwards
```

What the transcript shows, each backed by an assertion: the supplier ran once; 64 virtual threads racing on `get()` caused one initialisation; `List.ofLazy(5, i -> ...)`
computed only element 3 after `get(3)`; `Map.ofLazy(keys, f)` computed only the requested key; `Set.ofLazy(Set.of(2..7), isPrime)` answers `contains` on demand.

## What changed since 26 (the compile errors)

* `LazyConstant.orElse(...)` and `isInitialized()` are **gone** in 27 ([20-broken-lazy26.txt](output/20-broken-lazy26.txt); the class is `LazyConstant` in both, the old `StableValue` was already removed in 26 - [52](output/52-api-diff-25-to-26.txt)).
* `Set.ofLazy(Set, Predicate)` is new in 27 ([62](output/62-new-api.txt), [51](output/51-api-diff-26-to-27.txt)).

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