break 语句的控制流如何在嵌套循环中工作？ （爪哇）

Question

I have been programming in Java for a while, however I ran into something today that I thought was odd. I recall that when a break statement is used in a nested loop the control flow returns to the header of the outer loop. As demonstrated by this graphic.

Control Flow Diagram (I know you guys like it inline, but per reputation requirements I am not allowed)

Perhaps, this graphic and my recollection is incorrect. I say this because when I run the following code, the control flow adds every token to the LinkedList listOfAllPalindromes. I have verified that the if-block that contains the break statement is being accessed when it is supposed to be. So in short, is my memory wrong, is the control flow when it hits the break statement supposed to jump to listOfAllPalindromes.add(token) like it appears to be doing?

 public static void main(String[] args) throws FileNotFoundException {

    File dictionary = new File("english3.txt");
    Scanner s = new Scanner(dictionary);
    String token;
    LinkedList<String> listOfPalindromes = new LinkedList<>();

    while (s.hasNext())
    {
        token = s.next().toLowerCase();

        for (int i = 0; i <= (token.length()-1)/2; i++)
        {
            if(token.charAt(i) != token.charAt(token.length()-(i+1)))
            {
                break;
            }
        }
        
        listOfPalindromes.add(token);
    }

    for(String word : listOfPalindromes)
    {
        System.out.println(word);
    }
}

Answer 1

You should use continue with a label instead of break like this.

Scanner s = new Scanner("lever level canoe kayak carrace racecar mademoiselle madam");
String token;
LinkedList<String> listOfPalindromes = new LinkedList<>();

L: while (s.hasNext()) {
    token = s.next().toLowerCase();

    for (int i = 0; i <= (token.length() - 1) / 2; i++) {
        if (token.charAt(i) != token.charAt(token.length() - (i + 1))) {
            continue L;
        }
    }

    listOfPalindromes.add(token);
}

for (String word : listOfPalindromes) {
    System.out.println(word);
}

output

level
kayak
racecar
madam

Answer 2

use labels to loops for better understanding on control flow of loops.

It should jump come out of for loop as it hits break statement

Answer 3

From jls-14.15 :

A break statement with no label attempts to transfer control to the innermost enclosing switch, while, do, or for statement; this enclosing statement, which is called the break target, then immediately completes normally.

So, answering your question the break statement will break your for loop, and the listOfPalindromes.add(token) line will be executed.

Answer 4

You can resolve your problem using other method to check is palindrome, and then from your first looop fire this method. Example:

public static void main(String[] args) throws FileNotFoundException {

    File dictionary = new File("english3.txt");
    Scanner s = new Scanner(dictionary);
    String token;
    LinkedList<String> listOfPalindromes = new LinkedList<>();

    while (s.hasNext())
    {
        token = s.next().toLowerCase();

        if(isPalindrome(token)) {
            listOfPalindromes.add(token);
        }
    }

    for(String word : listOfPalindromes)
    {
        System.out.println(word);
    }
}

private static boolean isPalindrome(String token) {
    for (int i = 0; i <= (token.length()-1)/2; i++)
    {
        if(token.charAt(i) != token.charAt(token.length()-(i+1)))
        {
            return false;
        }
    }
    return true;
}

I didn't check is code correctly, only resolve problem with break