JarCastingLazy
If you are programming in Scala you can write
lazy val z = "Hello"
and the expression will only be evaluated when z is accessed the first time.
If you progam in Kotlin you can write something like
val z: String by lazy { "Hello" }
and the expression will only be evaluated when z is accessed the first time.
What can you write in Java?
Using
<groupId>com.javax0</groupId>
<artifactId>lazylet</artifactId>
<version>1.0.0</version>
you can write
var z = Lazy.let(()->"Hello");
and then you can write z.get() to access to the value. The first time get() is invoked the supplier will be evaluated and any later time the already calculated value will be returned.
This is not part of the Java language but it is an extremely simple class. The use is simple as demonstrated in the unit tests:
private static class TestSupport {
int count = 0;
boolean callMe() {
count++;
return true;
}
}
...
final var ts = new TestSupport();
var z = Lazy.let(ts::callMe);
if (false && z.get()) {
Assertions.fail();
}
Assertions.assertEquals(0, ts.count);
z.get();
Assertions.assertEquals(1, ts.count);
z.get();
Assertions.assertEquals(1, ts.count);
What is a bit more, even if it may not be useful that you can say
final var ts = new TestSupport();
var z = Lazy.sync(ts::callMe);
if (false && z.get()) {
Assertions.fail();
}
Assertions.assertEquals(0, ts.count);
z.get();
Assertions.assertEquals(1, ts.count);
z.get();
Assertions.assertEquals(1, ts.count);
to get a Lazy supplier that can be used by multiple threads and it is still guaranted that the supplier passed as argument is passed only once.