Objective

• Learn to make the code robust

Discussion

In the previous article TDD Beyond Basics : Test Precisely and Concretely we worked on a simple stack implementation that deals with normal cases. To make that code robust, it's time to think about cases such as:

• Extreme cases: 0, negative, null, maximums, etc
• User error: What happens if the user passes in null or a negative value?

These cases do not seem to be relevant to our stack example but it's a good starting point. So, here is the list of relevant test cases:

• A stack is empty on construction
• After n pushes to an empty stack, n > 0, the stack is not empty and its size is n
• If one pushes x then pops, the value popped is x.
• If one pushes x then peeks, the value returned is x, but the size stays the same
• If the size is n, then after n pops, the stack is empty and has a size 0
• Popping from an empty stack throws an Exception
• Peeking into an empty stack throws an Exception

Steps

Step 1

it 'should be empty on construction' do
stack = Stack.new

expect(stack.empty?).to be(true)
end

Let's define empty? method to get past 'undefined method empty?' error.

Step 2

def empty?
@elements.empty?
end

The test passes. Split the second test case into two tests in order to test one thing at a time.

Step 3

it 'after n pushes to an empty stack, n > 0, the stack is not empty' do

end

it 'after n pushes to an empty stack, n > 0, its size is n' do

end

Step 4

it 'after n pushes to an empty stack, n > 0, the stack is not empty' do
stack = Stack.new
stack.push(1)
stack.push(2)

expect(stack.empty?).to be(false)
end

This test passes due to existing implementation.

Step 5

it 'after n pushes to an empty stack, n > 0, its size is n' do
stack = Stack.new
stack.push(1)
stack.push(2)

expect(stack.size).to eq(2)
end

passes.

Let's update the existing test as follows:

Step 6

it 'should push a given item' do
stack = Stack.new

stack.push(1)

expect(stack.pop).to eq(1)
end

Here we only have one assertion. We updated this test to eliminate overlap in the tests.

Step 7

it 'if one pushes x then peeks, the value returned is x, but the size stays the same' do
stack = Stack.new
stack.push(1)

expect(stack.peek).to eq(1)
expect(stack.size).to eq(1)
end

This fails.

def peek
@elements.last
end

Since I had two assertions, I made the test fail by changing the assertion to eq(2) to make the test fail for the second assertion.

Step 8

it 'If the size is n, then after n pops, the stack is empty and has a size 0' do
stack = Stack.new
stack.push(1)
stack.push(2)

stack.pop
stack.pop

expect(stack.empty?).to be(true)
expect(stack.size).to eq(0)
end

Exercises

Implement the last two test cases:

• Popping from an empty stack throws an Exception
• Peeking into an empty stack throws an Exception

Give clear message in the exception why the failure occurred and how to overcome the failure.

References

1. Cracking the Coding Interview by Gayle Laakmann McDowell
2. Stacks

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