How can I unit test a method that uses a Scanner and an InputStream?

Question

So here is the code:

public int foo(InputStream in) {
    int counter = 0;
    Scanner scanner = new Scanner(in);
    while(scanner.hasNextLine()) {
        counter++;
        scanner.nextLine();
    }
    return counter;
}

Normally I will be passing a FileInputStream to this method, however I want to be able to test it without accessing a physical file.

Should I mock the File object? How to I implement

@Test
public void shouldReturnThree(){

}

How about a [`ByteArrayInputStream`](http://docs.oracle.com/javase/7/docs/api/java/io/ByteArrayInputStream.html)? — RealSkeptic, Mar 09 '15 at 21:29

score 5 · Accepted Answer · edited May 23 '17 at 12:06

5

You can pass a test String into the method as an InputStream like this (:

InputStream stream = new ByteArrayInputStream(exampleString.getBytes());

(Shamelessly stolen from this answer)

For instance, this code will print 3:

String str = "\n\n\n";
InputStream stream = new ByteArrayInputStream(str.getBytes());
System.out.println(foo(stream));

edited May 23 '17 at 12:06

Community

1
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answered Mar 09 '15 at 21:28

Malt

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How do I make this stream return 3? – Koray Tugay Mar 09 '15 at 21:30
@KorayTugay added an example – Malt Mar 09 '15 at 21:34
@JBNizet you're right. I removed the custom charset. – Malt Mar 09 '15 at 21:34
Why did you remove the custom charset? – Koray Tugay Mar 09 '15 at 21:37
@KorayTugay I removed the charset because your method works with Java's default charset. So I assumed that's what you're trying to test. – Malt Mar 09 '15 at 21:38
1

@KorayTugay as I said in my - now deleted - comment: the code uses the default charset to read from the stream. So if that is the correct thing to do, the string should also be transformed to bytes using this default charset. – JB Nizet Mar 09 '15 at 21:38

How can I unit test a method that uses a Scanner and an InputStream?

1 Answers1