如何在简单算法上应用单元测试？

Question

I have a maven project that draws Christmas tree. I need to implement Unit tests on it but i have no idea how to do it :/

I already set up , JUnit on my Maven project

for (int i = 0; i < 4; i++) {
   for (int j = 0; j < 10 - i; j++)
    System.out.print(" ");
   for (int k = 0; k < (2 * i + 1); k++)
    System.out.print("*");
   System.out.println();
  }

Answer 1

Assuming your class that draws Christmas tree looks more or less like that:

class ChristmasTreeDrafter {

    void draw() {
        for (int i = 0; i < 4; i++) {
            for (int j = 0; j < 10 - i; j++)
                System.out.print(" ");
            for (int k = 0; k < (2 * i + 1); k++)
                System.out.print("*");
            System.out.println();
        }
    }
}

You can test it in that way:

public class ChristmasTreeDrafterTest {

    private final ByteArrayOutputStream out = new ByteArrayOutputStream();

    @Before
    public void setup() {
        System.setOut(new PrintStream(out));
    }

    @Test
    public void shouldDrawChristmasTree() {
        // given
        ChristmasTreeDrafter christmasTreeDrafter = new ChristmasTreeDrafter();

        // when
        christmasTreeDrafter.draw();

        // then
        Assert.assertEquals("          *\r\n" +
                "         ***\r\n" +
                "        *****\r\n" +
                "       *******\r\n", out.toString());
    }
}

In the setup method standard output stream is redirected to the out object and thanks to this you can verify its content in then block. Unfortunately Java has no multiline strings, so this code looks ugly. In order to improve readability you can extract this content to the file under test/resources (assuming default maven project structure).

Answer 2

At a high level: the purpose of that code is to print a tree or triangle made of asterisks. The test is simple, run the code and see of it generates the desired output.

Since the code is printing to System.out you will need to redirect System.out to a file or a string (see this ) then compare the generated output to the desired output to see of the code is operating properly.