<s>[INST] Here is a Java code snippet that is intended to determine whether an input string is a valid password or not.

```java
public boolean isValidPassword(String password) {
    if (password == null) {
        return false;
    }

    boolean hasLowercase = false;
    boolean hasUppercase = false;
    boolean hasNumber = false;
    boolean hasSpecial = false;
    boolean hasWhitespace = false;
    int passwordLength = password.length();

    for (int i = 0; i < passwordLength; i++) {
        char c = password.charAt(i);

        if (Character.isLowerCase(c)) {
            hasLowercase = true;
        }
        if (Character.isUpperCase(c)) {
            hasUppercase = true;
        }
        if (Character.isDigit(c)) {
            hasNumber = true;
        }
        if (!Character.isLetterOrDigit(c)) {
            hasSpecial = true;
        }
        if (Character.isWhitespace(c)) {
            hasWhitespace = true;
        }
    }

    return (passwordLength >= 8 && passwordLength <= 20) && 
        hasLowercase && hasUppercase && !hasWhitespace && 
        (hasNumber || hasSpecial);
}
```

This method takes a string `password` and returns `true` if it is a valid password and `false` otherwise, based on the following rules:

* The password should have a minimum length of 8 characters and a maximum length of 20.
* The password should contain at least one lowercase letter.
* The password should contain at least one uppercase letter.
* The password should contain at least one digit or one special character.
* The password should not contain any whitespace characters.

This code works fine, but I want to make it more testable and maintainable. What changes can I make do achieve this?[/INST]