Pros and cons of package private classes in Java?

Question

I am learning Java recently, and I came across the notion of package-private classes, which is the default if we don\'t specify anything. But then I realized:

Answer 1

The package-private access level is more restrictive than protected: protected attributes and methods can still be accessed by simply subclassing a class. Protected members are (or may be) intended for inheritance while package-private members are not.

Package-private members are often used so multilpe classes inside a package can access implementation-specific attributes or (utility) methods.

Good examples to this are the package-private constructor of String and the StringBuilder.value char array:

/*
* Package private constructor which shares value array for speed.
* this constructor is always expected to be called with share==true.
* a separate constructor is needed because we already have a public
* String(char[]) constructor that makes a copy of the given char[].
*/
String(char[] value, boolean share) {
    // assert share : "unshared not supported";
    this.value = value;
}

So classes inside the java.lang package can efficiently create new Strings if the content is already present in a char[] without compromising security. You cannot do this from your application because if you could, you would have access (reference) to the internal char array of a String which is immutable (reflection not counted!).

In StringBuilder (or rather AbstractStringBuilder where the implementation comes from) the char array holding the current value char[] value and an accessor method to this char[] getValue() are also package-private so various utility methods of String like contentEquals(StringBuffer sb) and contentEquals(CharSequence cs) can utilize this for efficiency and faster comparisons without exposing the internal char array to the "world".