The following is a code to replace vowels of a word with an underscore:

```
String[] vowelArray = arrayInput;
    for (int x = 0; x < vowelArray.length; x++){
        for (int a = 0; a < vowelArray[x].length(); a++){
            switch (vowelArray[x].charAt(x)){
                case 'A': vowelArray[x].charAt(x) = "_";
                case 'a': vowelArray[x].charAt(x) = "_";
                case 'E': vowelArray[x].charAt(x) = "_";
                case 'e': vowelArray[x].charAt(x) = "_";
                case 'I': vowelArray[x].charAt(x) = "_";
                case 'i': vowelArray[x].charAt(x) = "_";
                case 'O': vowelArray[x].charAt(x) = "_";
                case 'o': vowelArray[x].charAt(x) = "_";
                case 'U': vowelArray[x].charAt(x) = "_";
                case 'u': vowelArray[x].charAt(x) = "_";
                case 'Y': vowelArray[x].charAt(x) = "_";
                case 'y': vowelArray[x].charAt(x) = "_";
            }
        }
    }

```

However upon compilation, I receive this error:

> 
> Exception in thread "main" java.lang.Error: Unresolved compilation
>  problems: The left-hand side of an assignment must be a variable
> 
> 
> 

It occurs in the switch statement on each case.
What am I doing wrong?