The difference between isEmpty method, null and "" in Java

I have always been very dizzy about these three empty spaces in String. My colleague just asked me and I will learn it too.

What I saw on someone else’s blog is this:
    isEmpty()
Memory space is allocated, the value is empty, it is absolutely empty, it is a kind of value (value = empty)
    ""
Memory space is allocated, the value is an empty string, which is relatively empty, and is a value (value = empty string)
    null
It is unallocated memory space, no value, it is a kind of unvalued (value does not exist)

However, I was still quite dizzy, so I wrote a demo to verify it.

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(.activity_main);

    String a = new String();
    String b = "";
    String c = null;
    testString(a);
    testString(b);
    testString(c);
}

private void testString(String str){
    if (str == null){
        Log.e("testNull", "null");
    }
    if (()){
        Log.e("testNull", "isEmpty");
    }
    if (("")){
        Log.e("testNull", "quotation marks");
    }
    Log.e("testNull", "----------------");
}

Since I only know how to play Android, I wrote an Android demo. The effect is the same ~
The result of logging is:
02-15 14:41:10.655 31875-31875/ E/testNull: isEmpty
02-15 14:41:10.655 31875-31875/ E/testNull: quotation marks
02-15 14:41:10.655 31875-31875/ E/testNull: ----------------
02-15 14:41:10.655 31875-31875/ E/testNull: isEmpty
02-15 14:41:10.655 31875-31875/ E/testNull: quotation marks
02-15 14:41:10.655 31875-31875/ E/testNull: ----------------
02-15 14:41:10.655 31875-31875/ E/testNull: null

Then str reports a null pointer error~ Here you can find that the effect of isEmpty and "" is the same.
I looked at the source code of isEmpty again:

public boolean isEmpty() {
    return count == 0;
}

That is to say, as long as the number of texts in String is 0, it returns true, and the number of texts in "" is 0.

In summary:
null is the "empty" of unallocated memory space~
The two "empty" of isEmpty and quotes are opposite to null (that is, they != null), and they are allocated memory (it seems to be almost the same meaning)

(PS: It is not clear whether isEmpty and "" are different. If you know, I hope you can comment and let me know. Thank you~)