mobile-java

Writes, reviews and modernizes Java in Android apps: Java 17 and 21 features that D8 supports, immutability, JSpecify nullability for Kotlin interop, leak-free threading without static Context or inner-class Handlers, ExecutorService, LiveData, ViewBinding, ListAdapter and DiffUtil, tests with JUnit, Mockito and Espresso, and incremental migration to Kotlin. Use when editing, reviewing or migrating .java files in an Android project.

Java on Android

Keep Java Android code correct, leak-free and Kotlin-friendly while it lives, and move it to Kotlin in small safe steps when the team decides to.

When to use

• Editing, fixing or reviewing .java files in an Android app or library.
• Bumping the Java language level, or using a JDK API that may not exist on the app's minSdk.
• Java code that Kotlin code calls (or will call) and needs nullability and interop annotations.
• Leaks, ANRs or crashes caused by Handlers, executors, Fragments, adapters or unclosed resources.
• Planning or executing an incremental Java-to-Kotlin migration.

Not for: Kotlin-first code (use mobile-kotlin), architecture choices (use mobile-mvvm, mobile-mvi).

Workflow

1. Read the module's build file: compileOptions source and target level, minSdk, and whether isCoreLibraryDesugaringEnabled is on. These decide which features and APIs are safe.
2. Load the reference that matches the task: Threading, Migration to Kotlin, Testing.
3. If the project already builds Kotlin and the team has not said otherwise, write new classes in Kotlin. Change existing Java minimally and in place.
4. Add nullability annotations to every public signature you touch.
5. Apply the rules below. Prefer fixing leaks and threading over stylistic rewrites.
6. Run lint (./gradlew lint) for NewApi, threading annotation and resource-leak checks, then the tests.
7. Keep conversion or modernization commits separate from behaviour changes.

Toolchain: what Java you can use

Feature	Java	On Android
Lambdas, method references, default and static interface methods	8	Desugared by D8 for every API level
var, switch expressions, text blocks, instanceof patterns	10-16	Compile-time features, every API level
Records	16	Supported by current AGP (D8 desugars them). Check reflection-based JSON libraries
Sealed classes and interfaces	17	Only on recent AGP / D8. Verify before relying on them
Pattern matching for switch, record patterns	21	Needs Java 21 source level and AGP / D8 support. Verify first
java.time (API 26), java.util.stream, Optional, java.util.function (API 24)	library	Below those API levels only with core library desugaring
Newer JDK methods (List.of, List.getFirst(), Stream.toList())	library	Tied to newer API levels. Trust lint NewApi; desugaring covers only part

The JDK that runs Gradle (17+ for current AGP) is separate from the language level and from the APIs available at runtime. The runtime APIs come from the device's Android version.

Rules

1. Language level is free, library APIs are not

java

// BAD: compiles against a recent compileSdk, crashes on older devices (added to Android in API 35)
Device first = devices.getFirst();

// GOOD
Device first = devices.get(0);

kotlin

// GOOD: java.time on minSdk below 26 through core library desugaring (app/build.gradle.kts)
android {
    compileOptions {
        isCoreLibraryDesugaringEnabled = true
        sourceCompatibility = JavaVersion.VERSION_17
        targetCompatibility = JavaVersion.VERSION_17
    }
}
dependencies { coreLibraryDesugaring(libs.desugar.jdk.libs) }

Never suppress NewApi without a Build.VERSION.SDK_INT check or confirmed desugaring.

2. Immutable by default

java

// BAD: public mutable fields, internal list shared with callers
public class Device {
    public String name;
    public List<String> tags;
}

// GOOD: record with a defensive copy (works on every API level)
public record Device(String id, String name, List<String> tags) {
    public Device {
        Objects.requireNonNull(id);
        tags = Collections.unmodifiableList(new ArrayList<>(tags));
    }
}

• No records? final class, private final fields, no setters, copies in and unmodifiable views out.
• static final constants only for truly immutable values. A static final List is still mutable.

3. Nullability is declared for Kotlin callers

java

// package-info.java: everything in the package is non-null unless marked
@NullMarked
package com.example.devices;

import org.jspecify.annotations.NullMarked;

java

// BAD: Kotlin sees String! (platform type) and skips null checks, NPE at runtime
public String findName(String id) { return names.get(id); }

// GOOD: inside @NullMarked, only nullable values are annotated
public @Nullable String findName(String id) { return names.get(id); }

• JSpecify @Nullable is a type-use annotation: List<@Nullable String>, String @Nullable [].
• Packages not yet @NullMarked use AndroidX @NonNull / @Nullable on every public parameter and return.
• Recent Kotlin compilers report JSpecify violations as errors, so wrong annotations break the Kotlin build.

4. Nothing long-lived references an Activity, Fragment or View

java

// BAD: static field keeps the Activity alive for the process lifetime
private static Context context;

// BAD: non-static inner class holds the outer Activity while queued or running
private class Poller implements Runnable {
    @Override public void run() { statusView.setText(read()); }
}

// GOOD: application context where a Context must be stored; work moved to a ViewModel
public final class StatusRepository {
    private final Context appContext;
    public StatusRepository(Context context) { this.appContext = context.getApplicationContext(); }
}

• Anonymous classes and lambdas that use outer members capture the outer instance too.
• WeakReference is a last resort for legacy callbacks, not a design.

5. Main-thread work goes through an explicit Looper; no AsyncTask

java

// BAD: deprecated, binds to the calling thread's Looper
Handler handler = new Handler();

// BAD: AsyncTask is deprecated (API 30); leaks the Activity and loses results on rotation
new LoadTask().execute();

// GOOD
private final Handler main = new Handler(Looper.getMainLooper());

@Override protected void onDestroy() {
    main.removeCallbacksAndMessages(null);
    super.onDestroy();
}

Replace AsyncTask with an ExecutorService in a ViewModel that publishes through LiveData (see Threading).

6. Every executor has an owner that shuts it down; every Future can be cancelled

java

// BAD: new thread pool per call, never shut down
Executors.newFixedThreadPool(4).execute(this::sync);

// GOOD: owned by the ViewModel, cancelled with it
public final class DevicesViewModel extends ViewModel {
    private final ExecutorService io = Executors.newSingleThreadExecutor();
    private @Nullable Future<?> loading;

    public void load() {
        if (loading != null) loading.cancel(true);
        loading = io.submit(this::loadBlocking);
    }

    @Override protected void onCleared() { io.shutdownNow(); }
}

If the executor is injected and shared app-wide, cancel the Futures in onCleared() instead of shutting the executor down.

7. LiveData: right setter, read-only exposure, view lifecycle owner

java

// BAD: setValue from a worker thread throws; mutable type exposed
public MutableLiveData<UiState> state = new MutableLiveData<>();
io.execute(() -> state.setValue(load()));

// GOOD
private final MutableLiveData<UiState> state = new MutableLiveData<>(UiState.loading());
public LiveData<UiState> getState() { return state; }
io.execute(() -> state.postValue(load()));

java

// BAD (Fragment): observers pile up after each view recreation
viewModel.getState().observe(this, this::render);

// GOOD
viewModel.getState().observe(getViewLifecycleOwner(), this::render);

postValue keeps only the latest pending value. Use it for state, not for events that must all arrive.

8. ViewBinding is cleared in onDestroyView

java

// BAD: binding outlives the Fragment's view and leaks the whole view tree
private FragmentDevicesBinding binding;

// GOOD
private @Nullable FragmentDevicesBinding binding;

@Override
public void onViewCreated(@NonNull View view, @Nullable Bundle savedInstanceState) {
    binding = FragmentDevicesBinding.bind(view);
}

@Override
public void onDestroyView() {
    binding = null;
    super.onDestroyView();
}

Use the Fragment(R.layout.fragment_devices) constructor so onCreateView is not needed.

9. Lists use ListAdapter and DiffUtil

java

// BAD: full rebind, no animations, scroll state jumps
adapter.setItems(items);
adapter.notifyDataSetChanged();

// GOOD
public final class DeviceAdapter extends ListAdapter<Device, DeviceAdapter.Holder> {
    private static final DiffUtil.ItemCallback<Device> DIFF = new DiffUtil.ItemCallback<>() {
        @Override public boolean areItemsTheSame(@NonNull Device a, @NonNull Device b) { return a.id().equals(b.id()); }
        @Override public boolean areContentsTheSame(@NonNull Device a, @NonNull Device b) { return a.equals(b); }
    };

    public DeviceAdapter() { super(DIFF); }
    // onCreateViewHolder / onBindViewHolder use getItem(position)
}

adapter.submitList(items); // new list instance each time; never mutate a submitted list

10. Resources are closed with try-with-resources

java

// BAD: leaks on exception, StrictMode reports it
InputStream in = resolver.openInputStream(uri);
byte[] data = readAll(in);
in.close();

// GOOD
try (InputStream in = resolver.openInputStream(uri)) {
    if (in == null) throw new FileNotFoundException(uri.toString());
    return readAll(in);
}

Applies to Cursor, streams, ParcelFileDescriptor, TypedArray (API 31+ implements AutoCloseable; call recycle() below that).

11. equals and hashCode are overridden together

java

// BAD: equals without hashCode breaks HashSet, HashMap and DiffUtil
@Override public boolean equals(Object o) { return o instanceof Device d && id.equals(d.id); }

// GOOD: both, over the same fields (or use a record)
@Override public boolean equals(@Nullable Object o) {
    return o instanceof Device d && id.equals(d.id) && name.equals(d.name);
}
@Override public int hashCode() { return Objects.hash(id, name); }

12. Exceptions and interrupts are never swallowed

java

// BAD
try { worker.join(); } catch (InterruptedException e) { }
try { sync(); } catch (Exception e) { e.printStackTrace(); }

// GOOD
try {
    worker.join();
} catch (InterruptedException e) {
    Thread.currentThread().interrupt();
    return;
}
try {
    sync();
} catch (IOException e) {
    state.postValue(UiState.offline());
}

13. Interop annotations follow the direction of calls

kotlin

// Kotlin code called from Java
class Units {
    companion object {
        @JvmStatic fun celsius(f: Double): Double = (f - 32) / 1.8 // Units.celsius(..) from Java
        const val MAX_MTU = 517                                     // compile-time constant
    }
}
class ScanSpec @JvmOverloads constructor(val timeoutMs: Long = 10_000, val lowLatency: Boolean = false)

@JvmField exposes a non-const property as a field. Details and ordering are in Migration to Kotlin.

14. Tests are deterministic

java

// BAD: real threads and sleeps
viewModel.load();
Thread.sleep(500);

// GOOD: inject a direct executor; InstantTaskExecutorRule runs LiveData synchronously
@Rule public InstantTaskExecutorRule liveData = new InstantTaskExecutorRule();

DevicesViewModel vm = new DevicesViewModel(fakeRepo, Runnable::run);

See Testing for Mockito, Espresso and IdlingResource.

Anti-patterns

• Rewriting a working Java class in a review that was about a bug fix.
• static fields holding Context, Activity, View, Fragment, adapters or listeners.
• new Handler(), AsyncTask, Executors.new... inside an Activity or Fragment.
• observe(this, ...) in a Fragment; ViewBinding fields never cleared.
• notifyDataSetChanged() for list updates; mutating a list already passed to submitList.
• Public signatures without nullability annotations in code Kotlin calls.
• catch (Exception e) { e.printStackTrace(); } and empty InterruptedException handlers.
• Raising the Java level and assuming the JDK library on the device matches.
• Converting a file to Kotlin and changing its behaviour in the same commit.
• Logging tokens, device identifiers or personal data with Log.d (escalate to mobile-security).

Checklist

• Language level, minSdk and core library desugaring checked before using a JDK API
• Lint NewApi clean; no blanket suppressions
• Value types immutable (records or final fields, defensive copies)
• Package @NullMarked or public signatures annotated with @Nullable / @NonNull
• No static or long-lived references to Activity, Fragment, View or non-application Context
• Handlers use Looper.getMainLooper() and are cleared; no AsyncTask in new code
• Executors shut down by their owner; Futures cancelled in onCleared()
• setValue on main thread, postValue from workers; LiveData exposed read-only
• Fragments observe with getViewLifecycleOwner(); binding nulled in onDestroyView()
• RecyclerView uses ListAdapter + DiffUtil with id-based areItemsTheSame
• Closeables in try-with-resources
• equals and hashCode overridden together
• No swallowed exceptions; interrupt status restored
• Tests use direct executors and InstantTaskExecutorRule; Espresso uses IdlingResource, not sleeps

Related skills

• mobile-kotlin -- target idioms when migrating, and Kotlin-side interop rules
• mobile-mvvm -- ViewModel and LiveData or StateFlow architecture
• mobile-design-patterns -- Builder, Factory and Strategy where Java needs them
• mobile-code-review -- review workflow and severity levels
• mobile-security -- storage, logging and network rules for legacy Java code
• mobile-ci-cd -- lint and test gates in the build pipeline
• mobile-testing -- cross-platform test strategy