dtonhofer

dtonhofer

Functional Programming in Java, Second Edition: p 125: Going all the way with createCacheAndHeavy()

The example on page 125 comes closest to the mythical “self-modifying code” (a completely useless concept as modifying the data structure that the code works on is much easier, as seen here, but I digress!)

We can go one step further it seems and replace the anonymous class in createAndCacheHeavy() with another lambda (truly a closure that wraps around the Heavy instance) that matches Supplier<Lambda>. As we have no immediate way of testing the actual type of of the synthesized class that underpins a lambda (I think), we need to add a boolean to decide on whether we already did the replacement or not. This boolean does not need to be volatile or an AtomicBoolean as it is accessed from with synchronized code by the few earliest threads only. I hope I’m right about this.

Here we go:

class Holder {

    private Supplier<Heavy> heavy = () -> createAndCacheHeavy();
    private boolean firstCall = true;

    public Holder() {
        System.out.println("Holder created");
    }

    public Heavy getHeavy() {
        return heavy.get();
    }

    // We replace the supplier in the form of a lambda creating the Heavy instance
    // with another lambda that just returns the firstly created instance

    private synchronized Heavy createAndCacheHeavy() {
        if (firstCall) {
            System.out.println("First call to createAndCacheHeavy()");
            Heavy heavyInstance = new Heavy();
            heavy = () -> heavyInstance;
            firstCall = false;
        }
        else {
            System.out.println("Just another call to createAndCacheHeavy()");
            // Only happens if there was another thread that was queued on the
            // initial "Supplier" while the first thread performed the action
            // in the "firstCall" branch above. That initial "Supplier" has
            // already been replaced by the "Supplier" just returning
            // "heavyInstance". As this is a synchronized call, said "Supplier"
            // should be visible to this thread, so heavy.get() will do
            // what we expect.
        }
        return heavy.get();
    }
}

Where Next?

Popular Pragmatic Bookshelf topics Top

iPaul
page 37 ANTLRInputStream input = new ANTLRInputStream(is); as of ANTLR 4 .8 should be: CharStream stream = CharStreams.fromStream(i...
New
johnp
Running the examples in chapter 5 c under pytest 5.4.1 causes an AttributeError: ‘module’ object has no attribute ‘config’. In particula...
New
brianokken
Many tasks_proj/tests directories exist in chapters 2, 3, 5 that have tests that use the custom markers smoke and get, which are not decl...
New
GilWright
Working through the steps (checking that the Info,plist matches exactly), run the demo game and what appears is grey but does not fill th...
New
jesse050717
Title: Web Development with Clojure, Third Edition, pg 116 Hi - I just started chapter 5 and I am stuck on page 116 while trying to star...
New
brian-m-ops
#book-python-testing-with-pytest-second-edition Hi. Thanks for writing the book. I am just learning so this might just of been an issue ...
New
brunogirin
When installing Cards as an editable package, I get the following error: ERROR: File “setup.py” not found. Directory cannot be installe...
New
jonmac
The allprojects block listed on page 245 produces the following error when syncing gradle: “org.gradle.api.GradleScriptException: A prob...
New
jwandekoken
Book: Programming Phoenix LiveView, page 142 (157/378), file lib/pento_web/live/product_live/form_component.ex, in the function below: d...
New
mcpierce
@mfazio23 I’ve applied the changes from Chapter 5 of the book and everything builds correctly and runs. But, when I try to start a game,...
New

Other popular topics Top

AstonJ
If it’s a mechanical keyboard, which switches do you have? Would you recommend it? Why? What will your next keyboard be? Pics always w...
New
dimitarvp
Small essay with thoughts on macOS vs. Linux: I know @Exadra37 is just waiting around the corner to scream at me “I TOLD YOU SO!!!” but I...
New
PragmaticBookshelf
Build highly interactive applications without ever leaving Elixir, the way the experts do. Let LiveView take care of performance, scalabi...
New
AstonJ
Saw this on TikTok of all places! :lol: Anyone heard of them before? Lite:
New
PragmaticBookshelf
Build efficient applications that exploit the unique benefits of a pure functional language, learning from an engineer who uses Haskell t...
New
PragmaticBookshelf
Author Spotlight Mike Riley @mriley This month, we turn the spotlight on Mike Riley, author of Portable Python Projects. Mike’s book ...
New
PragmaticBookshelf
Author Spotlight Rebecca Skinner @RebeccaSkinner Welcome to our latest author spotlight, where we sit down with Rebecca Skinner, auth...
New
New
hilfordjames
There appears to have been an update that has changed the terminology for what has previously been known as the Taskbar Overflow - this h...
New
PragmaticBookshelf
Develop, deploy, and debug BEAM applications using BEAMOps: a new paradigm that focuses on scalability, fault tolerance, and owning each ...
New

Latest in Functional Programming in Java, Second Edition

Functional Programming in Java, Second Edition Portal

Sub Categories: