How to unit test a code with print statement?

Question

Many a times, technical interviewers ask questions such as 'print something'. Eg: http://www.geeksforgeeks.org/given-a-binary-tree-print-all-root-to-leaf-paths/

The following code will ask to print all paths from root to leaf.

How do I unit test a code like this ?

One solution would be to return a datastructure will all paths. But then interviewer would rebuke me for consuming huge space complexity ?

the data structure itself does not have to hold all data - you can return iterator that will be able to return all values but will compute them with each calling of `next()` method. This way you have nice decoupled and testable code and low space complexity. — sodik, Feb 03 '15 at 17:14

score 2 · Answer 1 · answered Feb 03 '15 at 16:57

One solution is to pass a PrintStream to your method. When you call it from main, pass System.out, like this:

public static void main(String[] args) {
    Tree tree = ...
    TreePrinter printer = ...
    printer.printTree(System.out);
}

When you call the method from your unit test code, pass it a subclass of PrintStream, which collects the output in memory, and compares it to the expected output:

@Test
public void testTreePrinter() {
    Tree tree = ...
    TreePrinter printer = ...
    MyTestStream testStream = ...
    printer.printTree(testStream);
    assertEquals(expectedOutput, testStream.collectedOutput());
}

Or even simpler to mock in Java 8: a `Consumer`, and you can pass `System.out::println` as argument. — JB Nizet, Feb 03 '15 at 17:05

score 0 · Answer 2 · edited May 23 '17 at 11:57

In the general case isolating a hard-to-test dependency (like console output) is the way to go. So in general, I'd do something along the lines of dasblinkenlight's suggestion. This also cleanly separates the responsibilities of calculating the tree and printing it out (be it after or during calculation), which makes your code arguably more maintainable.

In the very common case of System.out, there's an alternative that does not require changing the production code: you can capture the out stream in a string by using System.setOut. In the toy problem you provide, I would be tempted to take this shortcut.

How to unit test a code with print statement?

2 Answers2