How to sort MutableList<File> using Collator?

Question

I want to sort the output from listFiles() using Collator.getInstance(Locale.ENGLISH). How do I do it?

    val path = File(Os.getenv("EXTERNAL_STORAGE"))
    val filefilter = FileFilter { file -> file.isDirectory && !file.isHidden }
    val fileList = path.listFiles(filefilter)!!.toList()
    Collections.sort(fileList, Collator.getInstance(Locale.ENGLISH))

The error I get is

java.lang.ExceptionInInitializerError
    at com.praps.myapplication.ui.list.ListFragment.onCreateView(ListFragment.kt:43)
    at androidx.fragment.app.Fragment.performCreateView(Fragment.java:2698)
    at androidx.fragment.app.FragmentStateManager.createView(FragmentStateManager.java:320)
    at androidx.fragment.app.FragmentManager.moveToState(FragmentManager.java:1187)
    at androidx.fragment.app.FragmentManager.addAddedFragments(FragmentManager.java:2224)
    at androidx.fragment.app.FragmentManager.executeOpsTogether(FragmentManager.java:1997)
    at androidx.fragment.app.FragmentManager.removeRedundantOperationsAndExecute(FragmentManager.java:1953)
    at androidx.fragment.app.FragmentManager.execPendingActions(FragmentManager.java:1849)
    at androidx.fragment.app.FragmentManager$4.run(FragmentManager.java:413)
    at android.os.Handler.handleCallback(Handler.java:883)
    at android.os.Handler.dispatchMessage(Handler.java:100)
    at android.os.Looper.loop(Looper.java:221)
    at android.app.ActivityThread.main(ActivityThread.java:7520)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:539)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:950)
 Caused by: java.lang.ClassCastException: java.io.File cannot be cast to java.lang.String
    at java.text.Collator.compare(Collator.java:277)
    at java.util.TimSort.countRunAndMakeAscending(TimSort.java:355)
    at java.util.TimSort.sort(TimSort.java:220)
    at java.util.Arrays.sort(Arrays.java:1492)
    at java.util.ArrayList.sort(ArrayList.java:1470)
    at java.util.Collections.sort(Collections.java:206)
    at com.praps.myapplication.ui.list.dummy.DummyContent

Nicolas · Accepted Answer · 2020-09-30 21:48:02Z

1

You're trying to sort a Array<File> with a Comparator<String>, that's why. There's no type warning because Collator is actually a Comparator<Any>, but its implementation RuleBasedCollator isn't, it only compares strings.

You need to make an intermediate comparator that takes the name of the files and compare that:

val files = dir.listFiles().orEmpty()
val collator = Collator.getInstance(Locale.ENGLISH);
files.sortWith { a, b -> collator.compare(a.name, b.name) }

answered Sep 30, 2020 at 21:48

Nicolas

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