length() giving Nullpointerexception

Question

I am creating a program that will receive multiple inputs from the user and, when receiving a blank string, should stop receiving entries and continue with the program.

The code runs perfectly through my computer but the college’s automated correction system reports an error:

Exception in thread "main" java.lang.NullPointerException
    at SentencaDancante.main(SentencaDancante.java:20)

Line 20 corresponds to while of the code below:

BufferedReader input = new BufferedReader(new InputStreamReader(System.in));

String entrada = new String(input.readLine());

int contador = 0;

Frases[] objetos = new Frases[100];


while (entrada.length() != 0){

    //the object constructor gets the length and the char[], converted from the "entrada" string.

    objetos[contador] = new Frases(entrada.length(), entrada.toCharArray());

    // defined method in the Frases class
    objetos[contador].converter();

    entrada = input.readLine();

    contador++;

}

I’ve tried to replace the while by a while(entrada != null || entrada.length != 0). But then Vscode starts reporting nullpointeraccess.

Answer 1

The right thing would be

while(entrada != null && entrada.length() != 0)

because case entrada is null will not attempt to execute the function length and shall not enter into the snare which also makes use of the entrada. It is not possible to perform a function of a null object.

Answer 2

An alternative is to change the way you read the data:

String entrada;
int contador = 0;
while ((entrada = input.readLine()) != null) { // enquanto conseguir ler alguma coisa
    if (entrada.length() == 0) // string vazia, sai do loop
        break;

    objetos[contador] = new Frases(entrada.length(), entrada.toCharArray());
    ... faz o que precisar com objetos[contador]

    contador++;
    if (contador == objetos.length) // já encheu o array, sai do loop
        break;
}

Inside the while we have (entrada = input.readLine()) != null - note the parentheses: first I assign the return of readLine in the variable entrada, and then compare if he is null. If so, it’s out of the loop.

Inside the loop I check if the string size is zero (and if so, exits the loop).

I also included a check at the end to see if the counter has already exceeded the size of the array (otherwise it would give a ArrayIndexOutOfBoundsException when the meter reached 100).

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On your computer it worked because the System.in is read from the console and you always need to give a ENTER, then the entrada is never null (the readLine removes the line break for ENTER, leaving the empty string).

Already in the automated correction system, probably it takes the entries of some file and passes to your program via pipe or something like that (there is no ENTER typed), then when it comes to the end of stream data, the readLine returns null.