line = reader.readline（）结果与文字“F”的行为不同

Question

I was trying to develop a command line interface test first using a technique described in Jeff Langr's book Agile Java, but I can not figure out why:

line = reader.readline(); result does not behave same as a literal "F" in my code.

This works...rover moves forward to 1:

private void goForward() {
        rover.move("F");
    }

This does not work...rover remains at 0:

private void goForward() throws IOException {
    StringBuffer input = new StringBuffer();
    input.append(line("F"));
    byte[] buffer = input.toString().getBytes();
    InputStream inputStream = new ByteArrayInputStream(buffer);
    InputStreamReader inputStreamReader = new InputStreamReader(inputStream);
    BufferedReader reader = new BufferedReader(inputStreamReader);  
    String line = reader.readLine();
    rover.move(line);
}

private String line(String str) {
    return String.format("%s%n", str);
}

I have done everything I can think of:

I added several

System.out.println(line);

to look at the values for line before and after and everything looks okay.

But looks can be deceiving so I added an

if(line.equals("F"))

thinking this would prove they are actually different but it evaluates to True

And spent several hours with my debugger, Javadocs and searching the Internet. I have learned a lot along the way but nothing that explains what is happening.

Any assistance would be greatly appreciated. I am sure I am missing some fine nuance that plague novices like myself. Thanks.

Answer 1

Ok, after reading your comment, I have a better answer... (First one was completely wrong, sorry):

Strings should NEVER be compared via == , because that will only be true if it's the same OBJECT (and not the same STRING). Use equals() .

It only works with constant "F", because the compiler takes all constant "F"s and uses the same String for them. But as soon as you have a dynamic String, coming from the "outside" during Runtime, it will be a totally different object, for which == will be FALSE, while equals will be TRUE.

String x = "F";
// Might be true, because the compiler will use ONE object 
// for both instances of "F". It's still bad and should not be done.
boolean isF = x == "F";

String y = new Scanner(System.in).nextLine();
// Will NEVER be true, even if you entered "F", because
//  y will always be a NEW String object...
isF = y == "F"; 
// Will be true, if you entered "F"
isF = "F".equals(y);