android-idioms

name	android-idioms
description	Idiomatic Kotlin for Android in this codebase — decomposing oversized lifecycle functions, scope functions (run/apply/with), when and partition instead of switch, extension functions and AndroidX KTX over verbose Java utilities, null safety instead of platform types, and companion-object constants. Use when writing any new Kotlin or converting a Java class to Kotlin.

Android + Kotlin Idioms

The shape new Kotlin should take here, and the transformations to apply when converting Java. Every one is behaviour-preserving. Examples are drawn from a real fragment conversion in the wider nextcloud/android codebase — the principles transfer, but the class names (FileDetailSharingFragment, OCFile, fileActivity) are from that project, not this one.

1. Decompose Oversized Functions

The IDE keeps the Java structure: one enormous onViewCreated/setupView that inflates, themes, wires listeners, and kicks off loading in a single 80-line block. Split by intent into small private functions. The lifecycle callback becomes a readable table of contents.

// BEFORE: onViewCreated does everything inline (adapters, layout managers, listeners, fetch)

// AFTER
override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
    super.onViewCreated(view, savedInstanceState)
    fileActivity ?: return
    fileDataStorageManager = fileActivity?.storageManager
    fileOperationsHelper = fileActivity?.fileOperationsHelper

    startAnimation()
    val userId = getUserId()
    setupInternalShares(userId)
    setupExternalShares(userId)
    binding?.pickContactEmailBtn?.setOnClickListener { checkContactPermission() }
    fetchSharees()
    setupView()
}

Rules:

• One function = one reason to change. Name it for what it accomplishes (setupInternalShares, themeView, disableE2EEShareForV1), not how.
• Factor duplicated blocks into a parameterized helper (createShareListAdapter(userId, SharesType.INTERNAL)).
• Keep files ≤300 lines (project rule). Heavy decomposition sometimes means splitting a god-class into collaborators — raise that with the developer rather than exceeding 300.

2. Scope Functions Over Repetition

Replace repeated binding.x / viewThemeUtils.material.y chains with run/apply/with.

// BEFORE
viewThemeUtils.material.themeSearchCardView(binding.searchCardWrapper);
viewThemeUtils.material.colorMaterialButtonPrimaryOutlined(binding.sendCopyBtn);
viewThemeUtils.material.colorMaterialButtonPrimaryBorderless(binding.sharesListInternalShowAll);

// AFTER
binding.run {
    viewThemeUtils.material.run {
        themeSearchCardView(searchCardWrapper)
        colorMaterialButtonPrimaryOutlined(sendCopyBtn)
        colorMaterialButtonPrimaryBorderless(sharesListInternalShowAll)
    }
}

Use apply {} when configuring and returning the receiver:

ShareeListAdapter(fileActivity!!, ArrayList(), this, userId, user, viewThemeUtils, encrypted, type)
    .apply { setHasStableIds(true) }

3. switch → when / filter + partition

Collapse a switch that sorts items into buckets into a declarative pipeline with a constant Set.

// BEFORE: for-loop with switch(shareType) adding to internalShares / externalShares

// AFTER
private val externalShareTypes = setOf(
    ShareType.PUBLIC_LINK, ShareType.FEDERATED_GROUP, ShareType.FEDERATED, ShareType.EMAIL
)

val (external, internal) = shares
    .filter { it.shareType != null }
    .partition { it.shareType in externalShareTypes }

4. Extension Functions & KTX

Import members directly and lean on AndroidX KTX instead of verbose Java utilities.

Java / verbose	Idiomatic Kotlin
TextUtils.isEmpty(s)	s.isNullOrEmpty()
BundleExtensionsKt.getParcelableArgument(b, k, T.class)	b.getParcelableArgument(k, T::class.java)
for (int i = 0; i < vg.getChildCount(); i++)	for (i in 0..<view.size) (androidx.core.view.size)
manual getter/setter methods	Kotlin property access (view.visibility = View.GONE)
private int x; public int getX() (read-only to callers)	var columnsCount = 0; private set
empty override method body	= Unit single-expression body
free-standing util call	receiver extension (externalShares.mergeDistinctByToken(publicShares))

Domain-specific extensions read best as receivers on the relevant type:

private fun OCCapability?.isPasswordEnforced(): Boolean =
    this?.filesSharingPublicPasswordEnforced?.isTrue == true &&
        filesSharingPublicAskForOptionalPassword.isTrue

5. Null Safety Instead of Platform Types

The IDE leaves ! platform types and defensive Java null-checks. Replace with ?., ?:, and Kotlin's require/requireNotNull. A nullable binding (cleared in onDestroyView) is the canonical Android case — guard it with ?. / ?: return.

// BEFORE
if (binding == null) return;
final LinearLayout shimmer = binding.shimmerLayout.getRoot();
shimmer.clearAnimation();

// AFTER
binding?.run {
    shimmerLayout.root.run {
        clearAnimation()
        visibility = View.GONE
    }
    shareContainer.visibility = View.VISIBLE
}

6. Constants & Companion Object

Move static final and magic literals into a companion object; use const val for compile-time constants. Add @JvmStatic to factory methods still called from Java.

companion object {
    private const val TAG = "FileDetailSharingFragment"
    private const val ARG_FILE = "FILE"
    private const val MIN_SHOW_ALL_VISIBLE_ITEM_COUNT = 3
    private const val INTERNAL_LINK_PATH_PRETTY = "/f/"

    @JvmStatic
    fun newInstance(file: OCFile?, user: User?) = FileDetailSharingFragment().apply {
        arguments = Bundle().apply {
            putParcelable(ARG_FILE, file)
            putParcelable(ARG_USER, user)
        }
    }
}

7. Decompose; Do Not Suppress

If a legacy god-class cannot be split within the change's scope, say so and propose the split — do not paper over it with @Suppress("TooManyFunctions", "LargeClass", ...). Those are detekt rule names and this repository has no detekt, so the annotation suppresses nothing; it only tells the next reader that someone knew the file was too big and left it.

8. // region Organization

For large classes, grouping members under // region <name> / // endregion (lifecycle, private methods, overrides, companion) aids IDE folding. This is IDE structure, not a decorative divider. Match the surrounding file's existing style; do not introduce ASCII banner comments (// ==== ====), which the project forbids.