Java APIs

In the code below why do the first two casts

  (String[]) oa;

succeed while the third one fails ?

import java.util.ArrayList;
import java.util.Arrays;

class cast {

    public static void  main(String[] args) {
	Object[] oa = args;
	//we can cast an Object[] to a String[] if the elements of the Object[] are strings
	String[] sa = (String[]) oa;
	
	ArrayList<String> als = new ArrayList<String>(Arrays.asList((String [])oa));
	als.toArray((String[]) oa);
	oa = als.toArray();
	//why does this get a ClassCastException given that the same casts above works
	sa = (String[]) oa; 

	
    }
}

$ javac -g -Xlint:unchecked cast.java
$ java cast hello world
Exception in thread "main" java.lang.ClassCastException: [Ljava.lang.Object;
at cast.main(cast.java:15)