java - JUnit-NoSuchElementException : No line found

Question

我正在尝试学习单元测试和 Maven，为此我使用 JUnit 并编写简单的随机名称生成器。我有以下类(class):

public class Database {

  public String readRandomName(String url) throws FileNotFoundException {
     int sum = calculateFileLines(url);
     int lines = (int) (Math.random()*sum);
     File file = new File(url);
     Scanner scanner = new Scanner(file);
     for (int i=0; i<lines;i++){
         scanner.nextLine();
     }
     return scanner.nextLine();
  }

  public int calculateFileLines(String url) throws FileNotFoundException {
    int sum = 0;
    try (Scanner scanner = new Scanner(new File(url))){
        while(scanner.hasNextLine() && scanner.nextLine().length()!=0){
            ++sum;
        }
    }
    return sum;
  }
}

当我运行这样的简单测试时:

public static void main(String[] args) throws FileNotFoundException {
    Database database = new Database();
    database.readRandomName("names/maleNamesPL.txt");
}

它工作得很好，但是当我尝试使用断言编写 JUnit 测试时，出现了我不明白的意外失败。这是测试代码:

@Test
public void readRandomNameTest() throws FileNotFoundException {
    Database database = new Database();
    Assert.assertNotNull("Should be some name", database.readRandomName("names/maleNamesPL.txt"));
}

结果:

Tests in error: 
readRandomNameTest(DatabaseTest): No line found

非常感谢您的帮助，谢谢!

Answer 1

您正在调用 nextLine() ，并且当没有行时它会抛出异常，正如 javadoc 所描述的那样。它永远不会返回null

http://download.oracle.com/javase/1,5.0/docs/api/java/util/Scanner.html

使用Scanner，您需要使用hasNextLine()检查是否有下一行

所以循环变成了

while(scanner.hasNextLine()){
    String str=scanner.nextline();
    //...
}