why array of strings can be assigned to array of objects but arraylist of strings cannot be assigned to arraylist of object?

Question

Why somethig like line#3 is allowed and line #6 is not allowed in java?

Object[] object= new Object[100];
String[] string= new String[100];
object=string; //line #3
List<Object> objectList = new ArrayList<>();
List<String> stringList = new ArrayList<>();
objectList=stringList;// line #6 [Error: Type mismatch: cannot convert from List<String> to List<Object>]

@ThomasKläger Hi, I am noob in Java, I understood the behavior in case of generics but i am not able understand it in terms of arrays? — implosivesilence, Mar 31 '20 at 06:20
For the question about arrays read https://stackoverflow.com/a/18666878/5646962 - it explains why arrays where defined this way. — Thomas Kläger, Mar 31 '20 at 06:29

score 2 · Accepted Answer · answered Mar 31 '20 at 06:19

That is because arrays are covariant and String[] is a sub type of Object[], hence the assignment object = string; is legal. But what if you do this:

object[0] = 1;

The statement compiles fine, but when you run it you get this error.

java.lang.ArrayStoreException: java.lang.Integer

On the contrary, parameterized types are invariant. So, List<String> is neither a subtype nor a supertype of List<Object> and your compiler starts complaining at the site of assignment objectList = stringList;. But, if you need to make parameterized types covariant while maintaining type safety, you can use wildcards. For an instance this compiles fine.

List<?> objectList = new ArrayList<>();
List<String> stringList = new ArrayList<>();
objectList = stringList;

why array of strings can be assigned to array of objects but arraylist of strings cannot be assigned to arraylist of object?

1 Answers1