getting incompatible types when using .length method

Question

I am working on an exercises from Objects first with java for self review. The part of the exercises that I am having trouble with is where it asks me to find the length of the refNumber string..if the length of the string is zero, then print out a line saying "zzz". I have tried doing this by making a local variable, and making it equal to refNumber.length(). However in my conditional statement bluejay indicates that I have an incompatible type. Ugh, please help. Thanks in advance.

class Book
{
// The fields.
private String author;
private String title;
private int pages;
private String refNumber;

/**
 * Set the author and title fields when this object
 * is constructed.
 */
public Book(String bookAuthor, String bookTitle, int numberOfPages)
{
    author = bookAuthor;
    title = bookTitle;
    numberOfPages = pages;
    refNumber = "";
}

public String getAuthor()
{
    return author;
}

public String getTitle()
{
    return title;
}

public int getPages()
{
    return pages;
}

public String getRefNumber()
{
    return refNumber;
}

public void setRefNumber(String ref){
    ref = refNumber;
}

public void printTitle() {
    System.out.println("Book desciption: " + title);

}

public void printAuthor() {
    System.out.print(" by " + author);

}

public void printPages(){
    System.out.println("Pages: " + pages);
}

public void printRef(){
    int count = refNumber.length();
    if (count = 0){                        //incompatible type wtf?
    System.out.println("zzz");
    }
    else {
        System.out.println("Reference Number: " + referenceNumber);
    }

}

Answer 1

Most programming languages use the single equals sign = to be an assignment operator. What you are trying to do is compare the two numbers, which uses the double equal sign == .

So, effectively, your code is trying to assign count with the value 0 , then check if the value is true or false . And since the result of an assignment operation is neither true nor false , it throws the error.

As other people are saying, use count == 0 .

Answer 2

try it ..get out put

 if (refNumber.length() == 0){                        
    System.out.println("zzz");
    }

Answer 3

Use this:

if (count == 0){ 
    ....
}

'=' is assignment operator and '==' is comparison operator

Answer 4

It should be

if(count == 0)

In your code it's like you are assigning 0 to count which cannot happen in the if clause, since it expects a boolean based on the condition. So here you should check if count is equals (using equality operator == ) and in return it returns true if they are equal or false otherwise.

Answer 5

count=0代表赋值，而count==0代表比较。