java-memory-playground/platforms/web/public/documentation.html at main · openpatch/java-memory-playground · GitHub

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<!doctype html>
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  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <link rel="icon" type="image/svg+xml" href="./logo.svg" />
    <title>Documentation — Java Memory Playground</title>
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      content="How to read and build stack and heap diagrams with the Java Memory Playground: steps, the garbage collector, the teacher's playground, sharing, and embedding."
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  <body>
    <header>
      <a class="back" href="./">← Back to the playground</a>
      <h1>Java Memory Playground</h1>
      <p>
        Draw what a Java program has in memory: the call stack on one side, the
        objects on the heap on the other, and the references between them.
        Nothing here runs your code — you build the picture, which is what makes
        it a thinking tool rather than a debugger.
      </p>
      <ul class="toc">
        <li><a href="#reading">Reading a diagram</a></li>
        <li><a href="#building">Building one</a></li>
        <li><a href="#values">Values and references</a></li>
        <li><a href="#steps">Steps and traces</a></li>
        <li><a href="#gc">The garbage collector</a></li>
        <li><a href="#edit">The teacher's playground</a></li>
        <li><a href="#classes">Classes from Java source</a></li>
        <li><a href="#options">Options</a></li>
        <li><a href="#shortcuts">Keyboard shortcuts</a></li>
        <li><a href="#sharing">Sharing a diagram</a></li>
        <li><a href="#embedding">Embedding</a></li>
        <li><a href="#files">.jmp files and VS Code</a></li>
      </ul>
    </header>

    <main>
      <h2 id="reading">Reading a diagram</h2>
      <p>
        A diagram has two kinds of box. A <strong>stack frame</strong> is one
        method call, labelled with its depth and name, holding that call's local
        variables. An <strong>object</strong> is one instance on the heap,
        labelled <code>:ClassName</code> and holding its fields.
      </p>
      <p>
        An arrow is a <strong>reference</strong>. It leaves the exact variable
        or field that holds it, so you can see which field of which object
        points where. A variable with no arrow holds <code>null</code>.
      </p>
      <p>
        Frames stack in call order, and only the frame on top can return — the
        <code>return</code> button on the others is disabled, because a call in
        the middle of the stack cannot finish before the ones above it. That
        restriction is the lesson, so the button stays visible rather than
        disappearing.
      </p>

      <h2 id="building">Building one</h2>
      <p>
        The palette in the top left corner is the source of new things. Drag one
        onto the canvas:
      </p>
      <ul>
        <li>
          <strong>new <em>ClassName</em></strong> — an instance of one of your
          configured classes, with a field for each attribute.
        </li>
        <li>
          <strong>new Array</strong> — pick a length and an element type. The
          <em>length</em> field is then fixed, as it is in Java: the elements
          can be edited, the number of them cannot.
        </li>
        <li>
          <strong>Call Method</strong> — a new frame on top of the stack.
        </li>
        <li>
          <strong>Declare Global Variable</strong> — a variable belonging to no
          frame. Java has no globals; this is scaffolding for the lessons that
          come before methods do.
        </li>
      </ul>
      <p>
        To make a reference, drag from the handle on the right of a variable or
        field to the object it should point at. Drag the arrow away again to set
        it back to <code>null</code>. Inside a frame,
        <strong>Declare Local Variable</strong> adds a variable, and
        <code>return</code> pops the frame — along with the references it held,
        which is often what makes something garbage.
      </p>

      <h2 id="values">Values and references</h2>
      <p>
        Primitive fields are edited in place: a number field for the numeric
        types, a checkbox for <code>boolean</code>. Reference fields get a
        handle to drag an arrow from instead.
      </p>
      <p>
        A <code>String</code> is shown inside the thing that references it, in
        quotes, rather than as its own box on the heap. A diagram about a linked
        list should not sprout a box for every name. When the point of the
        lesson <em>is</em> that a String is an object like any other —
        <code>==</code> against <code>.equals()</code>, the string pool — turn
        <code>inlineStrings</code> off and it gets a box and an arrow like
        everything else. The object is there either way; only the drawing
        changes.
      </p>

      <h2 id="steps">Steps and traces</h2>
      <p>
        A diagram can be a single picture or a sequence of them. The bar along
        the bottom moves between steps, and each step can carry a label saying
        what happens there — <em>head = new Node(1)</em>. It is one strip: the
        dots for the trace, then the step&rsquo;s label, then what you can do
        about it.
      </p>
      <p>
        <strong>Add step</strong> duplicates the current step and continues from
        it, so a trace is built by changing one thing at a time. What changed
        since the previous step is marked, which is usually the whole point of a
        trace; <code>hideStepChanges</code> turns that off.
      </p>
      <p>
        A step can be marked as an <strong>exercise</strong>. The student is
        handed the previous step and builds this one themselves, then checks
        their answer against what the step actually contains.
      </p>
      <p>
        Beside the label, a step can carry a <strong>hint</strong>. It is not on
        screen to begin with — a hint that is always visible is part of the
        task, and the exercise is then a different one. The student presses
        <strong>Hint</strong> when they want it, which puts a rung between
        trying again and <strong>Show solution</strong>. Write the hint as the
        nudge you would give over a shoulder, not as the answer in prose.
      </p>

      <p>
        A step can also carry <strong>statements</strong> — lines such as
        <code>neu.next = current.next</code> that the reader runs by pressing
        them, in whatever order they choose. Each one is evaluated against the
        diagram as it stands, so the right-hand side is read from the picture
        rather than from the text: run the two lines of an insertion the wrong
        way round and the new node ends up pointing at itself, with the rest of
        the list greyed out behind it. A numbered log keeps the order that was
        chosen, and <strong>Start over</strong> puts the diagram back to before
        the first line ran. Dragging edges still works — the lines are another
        way in, not a replacement. Authors write them beside the label,
        separated by semicolons.
      </p>

      <h2 id="gc">The garbage collector</h2>
      <p>
        <strong>Run Garbage Collector</strong>, under the palette, removes every
        object that cannot be reached from a variable or a stack frame.
        Unreachable objects are already tinted before you press it, so you can
        see what is about to go. It sits with the palette because the two are
        the same kind of thing: one puts objects on the canvas, the other takes
        them away.
      </p>
      <p>
        With the <code>gcPrediction</code> option on, you commit first: click
        the objects you believe are unreachable, then
        <strong>Check and collect</strong> scores what you found, what you
        missed, and what you marked that was still reachable. Committing to an
        answer before seeing it is where the learning is.
      </p>

      <h2 id="edit">The teacher's playground — the <code>?edit</code> URL</h2>
      <p>
        There are two playgrounds, and which one you get is decided by the URL.
      </p>
      <div class="table-wrap">
        <table>
          <thead>
            <tr>
              <th>URL</th>
              <th>What you get</th>
            </tr>
          </thead>
          <tbody>
            <tr>
              <td>the app&rsquo;s address</td>
              <td>
                The student's playground: the whole diagram, every edit, and the
                steps of a trace to walk through.
              </td>
            </tr>
            <tr>
              <td>the same, with <code>?edit</code></td>
              <td>
                The teacher's: the same, plus class configuration and step
                authoring.
              </td>
            </tr>
          </tbody>
        </table>
      </div>
      <p>
        <code>/edit</code> as a path works too, where the host has a rewrite
        rule for it. The query flag <code>?edit</code> always works, because the
        app is served as static files.
      </p>
      <div class="note">
        <p>
          The split is about which tools are on screen, not about what a student
          is allowed to touch. A student still builds objects, connects
          references, walks the steps and runs the collector. What the teacher's
          playground adds is deciding <em>which classes exist</em> and
          <em>what the steps are</em> — the setup for an exercise, not the
          working of it.
        </p>
      </div>
      <p>
        The diagram itself lives in the part of the URL after the
        <code>#</code>, and the mode is deliberately kept out of it. That means
        one diagram has both an editing link and a working link: take a link you
        saved from the teacher's playground, drop the <code>?edit</code>, and
        you have the student's view of the same picture.
      </p>

      <h2 id="classes">Classes from Java source</h2>
      <p>
        The teacher's playground asks for your classes instead of making you
        describe each field through a dialog. Paste the Java:
      </p>
      <pre><code>class Node {
    int value;
    Node next;
}</code></pre>
      <p>
        Only the structure is read — the class names, and the name and type of
        each field. Method bodies are skipped and
        <strong>nothing is executed</strong>. Records work too, and their
        components are read as fields.
      </p>
      <p>
        Applying classes reaches every step of the diagram, so when a change
        would cost something already drawn, you are told what before it happens:
        which field is about to be deleted from how many objects, and which
        objects belong to a class that no longer exists. A field that holds
        nothing — an untouched <code>int</code> still at <code>0</code>, a
        reference still <code>null</code> — is not counted, because losing it
        costs nothing.
      </p>

      <h2 id="options">Options</h2>
      <p>
        Options are part of the diagram, so they travel with a shared link. The
        configuration view has presets — <em>References only</em>,
        <em>With the stack</em>, <em>Everything</em> — for the common shapes of
        lesson. The individual switches:
      </p>
      <div class="table-wrap">
        <table>
          <thead>
            <tr>
              <th>Option</th>
              <th>Default</th>
              <th>Effect</th>
            </tr>
          </thead>
          <tbody>
            <tr>
              <td><code>hideSidebar</code></td>
              <td>off</td>
              <td>Hide the palette, for a diagram meant only to be read.</td>
            </tr>
            <tr>
              <td><code>hideCallMethod</code></td>
              <td>off</td>
              <td>Drop <em>Call Method</em> from the palette.</td>
            </tr>
            <tr>
              <td><code>hideDeclareGlobalVariable</code></td>
              <td><strong>on</strong></td>
              <td>
                Drop <em>Declare Global Variable</em>. On by default, because
                Java has no globals.
              </td>
            </tr>
            <tr>
              <td><code>hideNewArray</code></td>
              <td>off</td>
              <td>Drop <em>new Array</em>.</td>
            </tr>
            <tr>
              <td><code>disableGarbageCollector</code></td>
              <td>off</td>
              <td>No collector button, and no tinting of unreachable objects.</td>
            </tr>
            <tr>
              <td><code>gcPrediction</code></td>
              <td>off</td>
              <td>Ask which objects are unreachable before collecting them.</td>
            </tr>
            <tr>
              <td><code>createNewOnEdgeDrop</code></td>
              <td>off</td>
              <td>
                Dropping an arrow on empty canvas creates the object it points
                at.
              </td>
            </tr>
            <tr>
              <td><code>inlineStrings</code></td>
              <td><strong>on</strong></td>
              <td>
                Draw a <code>String</code> inside its holder instead of as its
                own heap box.
              </td>
            </tr>
            <tr>
              <td><code>hideSteps</code></td>
              <td>off</td>
              <td>Hide the step bar, for a lesson about one picture.</td>
            </tr>
            <tr>
              <td><code>hideStepChanges</code></td>
              <td>off</td>
              <td>Stop marking what a step changed since the one before it.</td>
            </tr>
          </tbody>
        </table>
      </div>

      <h2 id="shortcuts">Keyboard shortcuts</h2>
      <div class="table-wrap">
        <table>
          <thead>
            <tr>
              <th>Shortcut</th>
              <th>Action</th>
            </tr>
          </thead>
          <tbody>
            <tr><td><code>Ctrl/Cmd + S</code></td><td>Save</td></tr>
            <tr><td><code>Ctrl/Cmd + Z</code></td><td>Undo</td></tr>
            <tr><td><code>Ctrl/Cmd + Y</code></td><td>Redo</td></tr>
            <tr><td><code>Ctrl/Cmd + ,</code></td><td>Toggle the configuration view</td></tr>
            <tr><td><code>Ctrl/Cmd + +</code></td><td>Zoom in</td></tr>
            <tr><td><code>Ctrl/Cmd + -</code></td><td>Zoom out</td></tr>
            <tr><td><code>Ctrl/Cmd + 0</code></td><td>Reset the zoom</td></tr>
            <tr><td><code>Shift + !</code></td><td>Fit the diagram to the window</td></tr>
          </tbody>
        </table>
      </div>
      <p>
        A shortcut is never taken from someone typing into a field. In an
        embedded playground the bindings can be changed, or unbound, through the
        <code>keyBindings</code> attribute.
      </p>

      <h2 id="sharing">Sharing a diagram</h2>
      <p>
        <strong>Save (URL)</strong> writes the whole diagram — classes, options,
        every step — into the address bar, compressed and encoded after the
        <code>#</code>. The link is the document: send it to a class, put it in
        a worksheet, or bookmark it. There is no account and nothing is stored
        on a server.
      </p>
      <p>
        Because the state is in the fragment, it never reaches the server, and a
        very large diagram makes a very long link. <strong>Download (PNG)</strong>
        exports the picture, and with more than one step,
        <strong>Download all steps</strong> gives one image containing the whole
        trace for a printed worksheet.
      </p>
      <p>
        <strong>B&amp;W</strong> switches the whole playground to black and
        white, and the downloads follow. It is not a filter over the colours:
        the heap turns white, a stack frame grey, and everything the diagram was
        saying in colour is said again in line style: an unreachable object is
        the box with a dotted border, and what a step changed is dashed —
        the ring around a node, or the reference itself. Photocopy it and it
        still reads.
      </p>

      <h2 id="embedding">Embedding</h2>
      <p>
        The playground is published as a framework-agnostic web component. Two
        elements are defined, matching the two playgrounds:
      </p>
      <pre><code>&lt;script src="path/to/index.umd.js"&gt;&lt;/script&gt;

&lt;!-- the student's --&gt;
&lt;java-memory-playground id="playground"&gt;&lt;/java-memory-playground&gt;

&lt;!-- the teacher's --&gt;
&lt;java-memory-playground-editor&gt;&lt;/java-memory-playground-editor&gt;

&lt;script&gt;
  const playground = document.getElementById("playground");
  playground.setAttribute("memory", JSON.stringify(diagram));
  playground.addEventListener("change", (e) =&gt; console.log(e.detail));
&lt;/script&gt;</code></pre>
      <p>
        The bundle is published as
        <code>@java-memory-playground/web-component</code>. An
        embedded playground never touches the URL of its host page, and several
        of them can share one page. Both elements take the same attributes:</p>
      <div class="table-wrap">
        <table>
          <thead>
            <tr>
              <th>Attribute</th>
              <th>Type</th>
              <th>Meaning</th>
            </tr>
          </thead>
          <tbody>
            <tr>
              <td><code>memory</code></td>
              <td>string</td>
              <td>The diagram, as JSON or as a saved link's fragment.</td>
            </tr>
            <tr>
              <td><code>options</code></td>
              <td>JSON</td>
              <td>The options above, overriding what the diagram carries.</td>
            </tr>
            <tr>
              <td><code>language</code></td>
              <td>string</td>
              <td>
                <code>en</code> or <code>de</code>. Follows the browser
                otherwise.
              </td>
            </tr>
            <tr>
              <td><code>step</code></td>
              <td>number</td>
              <td>
                Which step to show, counted from zero, so prose around the
                diagram can walk a reader through a trace.
              </td>
            </tr>
            <tr>
              <td><code>persistence</code></td>
              <td>boolean</td>
              <td>
                Off for an embedded playground: it must not take over the URL of
                the page hosting it.
              </td>
            </tr>
            <tr>
              <td><code>keyBindings</code></td>
              <td>JSON</td>
              <td>Overrides for the shortcuts above.</td>
            </tr>
          </tbody>
        </table>
      </div>
      <p>
        It reports back with two events: <code>change</code>, carrying the whole
        diagram when the user saves, and <code>stepchange</code>, carrying the
        step index whenever the shown step moves.
      </p>
      <p>
        For a React application, the component is also published on its own as
        <code>@java-memory-playground/java-memory-playground</code>.
      </p>

      <h2 id="files">.jmp files and VS Code</h2>
      <p>
        A <code>.jmp</code> file is exactly the JSON that a shared link carries,
        so a diagram moves between the app, an embedded playground and a file
        unchanged. It is an ordinary text file: diffable, reviewable, and fine
        to keep in a repository next to the code it illustrates.
      </p>
      <p>
        <strong>Java Memory Playground Studio</strong> opens those files in
        VS Code as diagrams rather than as JSON, with the editor's own undo and
        save. <em>Show Source</em> switches to the JSON and
        <em>Show Diagram</em> switches back.
      </p>
    </main>

    <footer>
      <a href="./">Playground</a>
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      <a href="https://openpatch.org">OpenPatch</a>
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