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Welcome to part four, going back to requirements text, a checking account is taxable.

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That means you can draw a line between checking vs. savings and loan by making checking taxable.

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In other words, by having, checking, implement the taxable interface.

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So inside taxable, we're going to create the method signature.

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For a tax method that expects an income parameter.

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OK, and now from checking, we're going to implement the taxable interface implements.

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Taxable.

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Remember that an interface is a contract of behavior, a class that implements an interface must override

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all of its methods.

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So in this case, we have to override tax.

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And before we do anything, we want to create a unit test for this piece of behavior.

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So back inside of account tests.

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We're going to create another unit test that test called income tax.

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Public void income tax.

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And we'll just assume that person's total income ended up being three thousand.

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So we'll just deposit that.

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Or, you know what, make it 4000.

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The president.

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Accounts at Index zero dark deposit 4000 into the checking account.

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And now, if you try to say accounts at index zero tax, you're not going to get any results because

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right now every element in the accounts array is behaving as an account is taking the form of an account,

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and the account class does not have a tax method.

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So here it's basically telling you, what are you talking about?

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But we can typecast the element at index zero to taxable because we know that checking account is taxable.

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And now we can tax an income of 4000.

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Don't tax.

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Four thousand.

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OK.

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Now we're going to make an assertion.

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After the taxes are deducted, the resulting balance should be five thousand three hundred and seventy

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four point five one.

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After we get the accounts, updated balance accounts and in zero, don't get balance.

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OK.

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Run the tests.

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And it fails now we can write code to make it pass.

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Inside of checking Java.

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I'm going to create a static final variable private static.

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Final double.

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Taxable income.

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Is equal to three thousand.

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And according to the requirements, the tax rate is 15 percent.

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So what I'm going to do is create another private static final.

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Double.

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Tax rate constant is equal to zero point one five.

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All right.

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And here what I'll do.

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Is it going to set double tax?

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Is equal to math a doubt, Max.

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Because either the amount that can be taxed is zero or it's the income minus the taxable income.

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The idea is that if the income is smaller than what's taxable, this is going to be negative.

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So Max returns zero, which means we don't want to tax them anything.

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Otherwise, if the income is bigger than what's taxable, then we're going to get some number that we

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can multiply by the tax rates.

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And that's going to be our tax.

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Now we can tax the person's account by saying super upset balance, and we'll update the balance by

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rounding whatever the result is.

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And that result is going to be based on us getting the current balance and deducting the tax.

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OK, if we rerun our test.

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Test Java.

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Everything works out.

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Let's rerun all of our tests.

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And we're good.