Libgdx Collections JSON

libGDX Collections, Pooling, JSON & Utilities

Quick reference for com.badlogic.gdx.utils.* — libGDX's GC-friendly collections, object pooling, JSON serialization, and utility classes. These replace java.util.* equivalents to reduce garbage collection pressure, especially on Android.

Array<T> — ArrayList Replacement

Array<String> arr = new Array<>();                   // also: new Array<>(capacity), new Array<>(ordered, capacity)

Public fields: T[] items (backing array), int size (element count), boolean ordered.

Key methods: add, get, set, insert, swap, pop, peek, first, clear, sort, reverse, shuffle, truncate, shrink, ensureCapacity, isEmpty, notEmpty, select(predicate).

CRITICAL — identity parameter required: removeValue(item, identity), contains(item, identity), indexOf(item, identity), lastIndexOf(item, identity). identity=true uses ==, identity=false uses .equals(). There is no single-argument overload.

There is NO remove(Object) method. It is removeValue(value, identity) or removeIndex(int).

Gotchas:

• items array may be larger than size — iterate with for (int i = 0; i < arr.size; i++), never items.length.
• items[i] past size contains stale references — always check i < size.
• DO NOT modify Array during for-each iteration. Use DelayedRemovalArray, SnapshotArray, or manual index loop.
• When ordered is false, removeIndex() swaps the last element into the gap (O(1)) instead of shifting (O(n)).

DelayedRemovalArray<T> / SnapshotArray<T>

Both wrap begin()/end() around iteration. DelayedRemovalArray queues removals during iteration, applies at end() (returns void from begin()). SnapshotArray takes a snapshot; begin() returns T[] to iterate, modifications go to a copy. Both support nested begin()/end(). Used internally by Scene2D.

ObjectMap<K,V> — HashMap Replacement

ObjectMap<String, Integer> map = new ObjectMap<>();
map.put("hp", 100);                                  // returns old value or null
map.get("hp");  map.get("hp", 0);                    // null or default
map.remove("hp");  map.containsKey("hp");
map.containsValue(100, false);                        // identity param on values too

size is a public field (not a method). Iterate via for (Entry e : map), map.keys(), map.values(), or map.entries().

Gotchas:

• Same Entry instance is reused on each next() call. Do not store entry references.
• Iterators are pooled by default (Collections.allocateIterators = false). Nested iteration on the same map throws GdxRuntimeException. Set Collections.allocateIterators = true if needed.
• There is no entrySet()/keySet() — use entries(), keys(), values() (libGDX iterator types).

OrderedMap<K,V> — extends ObjectMap, maintains insertion order. orderedKeys() returns internal Array<K>.

Primitive-Key Maps (Avoid Boxing)

IntMap<V>, LongMap<V> — object values, get() returns null if missing. IntIntMap, ObjectIntMap<K>, ObjectFloatMap<K> — primitive values, get() MUST provide default (no null for primitives). ObjectIntMap has getAndIncrement(key, defaultValue, increment).

Primitive-value maps: put() returns void (not old value). Use put(key, value, defaultValue) overload to get old value.

Sets

ObjectSet<T> — add, remove, contains (NO identity param, unlike Array). first() throws if empty. OrderedSet<T> — maintains insertion order, orderedItems() returns internal Array<T>. IntSet — primitive int, does NOT implement Iterable. size is a public field on all sets.

Queue<T> — Double-Ended Queue

addLast/addFirst, removeFirst/removeLast, first()/last() (peek), get(index). size is a public field.

Pooling

Pool<T> and DefaultPool<T>

Pool<T> is abstract — override newObject(). Methods: obtain(), free(obj) (calls reset() if Pool.Poolable), fill(n), getFree(), clear(). Constructor: Pool(initialCapacity, max).

DefaultPool<T> (since 1.13.5) — preferred. Uses supplier: new DefaultPool<>(Bullet::new) or new DefaultPool<>(Bullet::new, 16, 100). No reflection, GWT-safe.

Implement Pool.Poolable for automatic reset() on free().

PoolManager — Shared Pool Registry (since 1.14.0)

PoolManager pools = new PoolManager();
pools.addPool(Bullet.class, Bullet::new);            // or pass a Pool directly
Bullet b = pools.obtain(Bullet.class);
pools.free(b);                                        // looks up pool by object's class

Deprecated Pooling APIs

• DO NOT use ReflectionPool (deprecated 1.13.5) — use DefaultPool.
• DO NOT use Pools (deprecated 1.14.0) — use DefaultPool or PoolManager.

JSON

Json — Serializer/Deserializer

Json json = new Json();
json.setOutputType(JsonWriter.OutputType.json);       // json | javascript | minimal
json.setIgnoreUnknownFields(true);                    // don't fail on extra fields
json.setUsePrototypes(false);                         // write all fields, not just changed
String str = json.toJson(myObject);                   // serialize
MyClass obj = json.fromJson(MyClass.class, jsonStr);  // deserialize (also accepts FileHandle)

There is no Json.parse() method. Use JsonReader for raw parsing or Json.fromJson() for typed deserialization.

Custom serializer: json.setSerializer(MyClass.class, new Json.Serializer<MyClass>() { ... }) — implement write(Json, T, Class) and read(Json, JsonValue, Class).

JsonReader — Raw Parsing to Tree

new JsonReader().parse(fileHandle) or .parse(string) returns JsonValue tree.

JsonValue — DOM Tree Navigation

Navigation: get("name") / get(index) returns child JsonValue (null if not found). has("name"), size (public field). Type checks: isObject(), isArray(), isString(), isNumber(), isBoolean(), isNull().

as* vs get* — CRITICAL distinction:

• asString(), asInt(), asFloat(), asBoolean() — converts THIS node's value.
• getString("name"), getInt("hp") — finds child by name, returns its value. Throws IllegalArgumentException if child not found. Use getString("name", "default") for safe defaults.

Iterate children: for (JsonValue entry : root) { entry.name; entry.asString(); } or for (JsonValue c = root.child; c != null; c = c.next).

I18NBundle — Localization

I18NBundle.createBundle(fileHandle) or createBundle(fileHandle, locale). Methods: get("key"), format("key", args...). Resolution: messages_de_DE.properties → messages_de.properties → messages.properties.

There is no I18NBundle.load() — use the static factory I18NBundle.createBundle().

Timer

// Static convenience (uses global Timer.instance())
Timer.schedule(new Timer.Task() {
    public void run() { /* runs on GL thread */ }
}, 2f);                                               // delay seconds
Timer.schedule(task, 1f, 0.5f);                       // delay, then repeat every 0.5s forever
Timer.schedule(task, 1f, 0.5f, 5);                    // delay, interval, repeatCount

task.cancel();                                         // cancel a scheduled task
task.isScheduled();                                    // boolean

Timer.Task is abstract — override run(). run() executes on the GL/render thread (posted via Gdx.app.postRunnable()), safe for libGDX API calls.

Instance methods use scheduleTask(task, ...) (not schedule). The timer also has stop(), start(), clear().

Other Utilities

TimeUtils:

long start = TimeUtils.nanoTime();
// ... work ...
long elapsed = TimeUtils.timeSinceNanos(start);       // nanoTime() - start
long ms = TimeUtils.millis();                          // System.currentTimeMillis()
long elapsedMs = TimeUtils.timeSinceMillis(startMs);

Align — bit-flag constants for layout alignment:

Align.center      // 1       Align.topLeft      // top|left
Align.top         // 2       Align.topRight     // top|right
Align.bottom      // 4       Align.bottomLeft   // bottom|left
Align.left        // 8       Align.bottomRight  // bottom|right
Align.right       // 16

Scaling — abstract class with static instances (NOT an enum): Scaling.fit, Scaling.fill, Scaling.contain, Scaling.stretch, Scaling.fillX, Scaling.fillY, Scaling.stretchX, Scaling.stretchY, Scaling.none.

AsyncExecutor — simple thread pool:

AsyncExecutor executor = new AsyncExecutor(4);         // 4 threads
AsyncResult<String> result = executor.submit(() -> computeExpensiveThing());
if (result.isDone()) {
    String value = result.get();                       // blocks if not done, throws on error
}
executor.dispose();                                    // MUST dispose

AsyncTask<T> interface has a single T call() method. Executor must be disposed.

ScreenUtils:

ScreenUtils.clear(0, 0, 0, 1);                        // r, g, b, a
ScreenUtils.clear(Color.BLACK);
ScreenUtils.clear(0, 0, 0, 1, true);                  // also clear depth buffer

GWT Compatibility

libGDX collections are GWT-compatible while many java.util features are not. Prefer libGDX collections in cross-platform projects. For reflection on GWT, use com.badlogic.gdx.utils.reflect.* instead of java.lang.reflect.*.

Common Mistakes

1. Using java.util.ArrayList / HashMap / HashSet — Use Array, ObjectMap, ObjectSet instead. libGDX collections reduce GC pressure and are GWT-compatible.
2. Omitting the identity parameter — Array.contains(), indexOf(), removeValue(), and ObjectMap.containsValue() require a boolean identity parameter. true = use ==, false = use .equals(). There is no single-argument overload.
3. Using array.items.length instead of array.size — items.length is the backing array capacity. Elements past size are stale. Always use size.
4. Calling Array.remove(object) — This method does not exist. Use removeValue(object, identity) or removeIndex(int).
5. Iterating ObjectMap with map.entrySet() — Does not exist. Use map.entries(), map.keys(), map.values(), or for (Entry e : map) directly. Same Entry instance is reused each iteration — do not store references.
6. Nested iteration on the same map — By default, iterators are pooled. Nested iteration throws GdxRuntimeException. Set Collections.allocateIterators = true to allow it.
7. Using Pools.obtain() / ReflectionPool — Both deprecated (1.14.0 / 1.13.5). Use new DefaultPool<>(MyClass::new) for local pools or PoolManager for shared pools.
8. Calling pool.obtain() without pool.free() — Defeats the purpose of pooling. Always pair obtain/free, ideally in try/finally.
9. Calling Json.parse() — Does not exist. Use JsonReader.parse() for raw JsonValue tree, or Json.fromJson() for typed deserialization.
10. Confusing getString("name") with asString() on JsonValue — getString("name") finds a child named "name" and returns its string value. asString() returns THIS node's value. Calling getString() on a node without matching children throws IllegalArgumentException.
11. Assuming Timer runs on a background thread — Timer.Task.run() executes on the GL/render thread via postRunnable(). Safe for libGDX API calls but do not block it.
12. Using I18NBundle.load() — Does not exist. Use the static factory I18NBundle.createBundle().

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