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# 11.11 Practice

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## Practice

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Note: Assume all methods that you write are public static.

### Average

Code a method called average that takes two double parameters, a and b. The method should return the average of the two doubles. Test if your average method works in the main method by calling it at least 3 times with different arguments and printing the results. Bonus points if you write a JavaDoc comment for the average method.
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### Temperature

Write a class that contains the following two methods:
Convert from Celsius to Fahrenheit `public static double celsiusToFahrenheit(double celsius)`
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Convert from Fahrenheit to Celsius `public static double fahrenheitToCelsius(double fahrenheit)`
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The formula for the conversion is: Fahrenheit = (9.0 / 5) * Celsius + 32 Celsius = (5.0 / 9) * (Fahrenheit ā 32)
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Write a test program that asks the user for a temperature in Fahrenheit and prints out the Celsius conversion. Then ask the user for a temperature in Celsius and print out the Fahrenheit conversion.
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Adapted from Exercise 6.8, Introduction to Java Programming (Comprehensive), 10th ed. by Y. Daniel Liang
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### Print Evens

Code a method printEvens that takes an integer parameter n. The method should print the first n even numbers to the console. (Note that 0 is considered an even number.) Test if your printEvens method works in the main method by calling it at least 3 times with different arguments. Bonus points if you write a JavaDoc comment for the printEvens method.
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### Triangle

Create a class named Triangle that contains the following two methods:
Return true if the sum of any two sides is greater than the third side. (This is the definition of a valid triangle.) `public static boolean isValid(double side1, double side2, double side3)`
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Return the area of the triangle. `public static double area(double side1, double side2, double side3)`
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Write a test program that reads three sides for a triangle and computes the area if the sides form a valid triangle. Otherwise, it displays that the input is invalid. Use Heron's Formula to calculate the area of a triangle given 3 sides.
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Adapted from Exercise 6.19, Introduction to Java Programming (Comprehensive), 10th ed. by Y. Daniel Liang
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### Sum Digits

Write a method that computes the sum of the digits in an integer. Use the following method header:
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`public static int sumDigits(long n)`
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For example, sumDigits(234) returns 9 (because 2 + 3 + 4).
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Hint: Use the % operator to extract digits, and the / operator to remove the extracted digit. For instance, to extract 4 from 234, use 234 % 10 (= 4). To remove 4 from 234, use 234 / 10 (= 23).
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Use a loop to repeatedly extract and remove the digit until all the digits are extracted.
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Write a test program that prompts the user to enter an integer and displays the sum of all its digits.
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Bonus points for writing a JavaDoc comment for the sumDigits method.
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Adapted from Exercise 6.2, Introduction to Java Programming (Comprehensive), 10th ed. by Y. Daniel Liang
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### Is Prime

Write a method called isPrime which returns true if the given integer is prime and false otherwise. Bonus points for writing JavaDoc comments for the method.
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Test your isPrime method in the main method by calling it at least 3 times and printing the result.
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Solution from Listing 6.7, Introduction to Java Programming (Comprehensive), 10th ed. by Y. Daniel Liang
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Some websites impose certain rules for passwords. Write a method that checks whether a string is a valid password.
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Suppose the password rules are as follows: ā  A password must have at least eight characters. ā  A password consists of only letters and digits. ā  A password must contain at least two digits.
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Write a program that prompts the user to enter a password and displays Valid Password if the rules are followed or Invalid Password otherwise. To do this, use a method called isValid which returns true if the password is valid and false otherwise.
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You may also find it helpful to write a method called isAlphanumeric which returns true if the given string is only made up of letters and digits.
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You may also find it helpful to write a method called getNumberOfDigits which returns the number of digits in a given string.
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You may also find it helpful to use the Character.isDigit(`char`) and Character.isDigit(`char`) methods. You can read more about them here.
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If you don't want to use those methods, you can also use regular expressions (aka regex) and the matches(regex) String method. You can read about that here and here.
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