Android Collection.sort根据Android API版本不一致地崩溃

Question

An interesting bug that essentially if you have this line of code in your Android code it crashes on Android API 21 but works on API 28

Collections.sort(Collections.singletonList(“1”));


java.lang.UnsupportedOperationException
at java.util.AbstractList.set(AbstractList.java:681)
at java.util.AbstractList$FullListIterator.set(AbstractList.java:143)
at java.util.Collections.sort(Collections.java:1869)
at com.davidcorrado.collectionssort.CollectionJavaUnitTest2.Collection_Sort_Immutable(CollectionJavaUnitTest2.java:16)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
at org.junit.runners.ParentRunner.run(ParentRunner.java:363)

I understand why it crashes as it does not allow immutable objects in Collection.sort but I really do not understand the inconsistency.

I have this code example: https://github.com/DavidCorrado/CollectionSortCrash/blob/master/app/src/androidTest/java/com/davidcorrado/collectionssort/CollectionJavaUnitTest2.java

So if you run the above test targeting API 21 emulator it crashes but on API 28 emulator it works. This also applies to real devices.

So I have 2 questions

1) What in different in these devices that I do not know about that causes this.

2) Is there a way to get this error in robo electric. I have a code example that succeeds.

Answer 1

It works with API 28 because the implementation of sort was replaced, and the new version has optimization that simply skips the sorting if list.size() <= 1 .

When sorting is skipped, it doesn't calls list.set() , and hence never triggers the UnsupportedOperationException of an immutable list.

Older API levels don't have that optimization.