This should work. As mentioned in the comments. a is not a string but an array. So you need to iterate over it to call the split() method on it’s containing strings
String s = "How are you?"
String[] a = s.split("\\s");
for(String s2 : a){
    String[] a2 = s2.split("");
    // do your stuff with a2 in every iteration
}
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solved Split array into subarray java