The challenge

Write a function that accepts a string, and returns true if it is in the form of a phone number.
Assume that any integer from 0-9 in any of the spots will produce a valid phone number.

Only worry about the following format:
(123) 456-7890 (don’t forget the space after the close parentheses)

Examples:

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"(123) 456-7890"  => true
"(1111)555 2345"  => false
"(098) 123 4567"  => false

The solution in Java code

Option 1:

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public class Solution {
  public static boolean validPhoneNumber(String phoneNumber) {
    return phoneNumber.matches("\\(\\d{3}\\) \\d{3}-\\d{4}");
  }
}

Option 2:

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public class Solution {
  public static boolean validPhoneNumber(String phoneNumber) {
      char[] cStr = phoneNumber.toCharArray();
      if(cStr[0] != '(') return false;
      if(cStr[4] != ')') return false;
      if(cStr[5] != ' ') return false;
      if(cStr[9] != '-') return false;
            
      for(int i = 1; i < cStr.length; i++){
         if(i != 4 && i != 5 && i != 9){
            if(!Character.isDigit(cStr[i])) return false;
         }
      }
      return true;
  }
}

Option 3:

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public class Solution {
  public static boolean validPhoneNumber(String phoneNumber) {
    return phoneNumber.equalsIgnoreCase("(123) 456-7890");
  }
}

Test cases to validate our solution

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import org.junit.Test;
import static org.junit.Assert.assertEquals;

public class PhoneExampleTests {
  @Test
  public void tests() {
    assertEquals(true, Solution.validPhoneNumber("(123) 456-7890"));
    assertEquals(false, Solution.validPhoneNumber("(1111)555 2345"));
    assertEquals(false, Solution.validPhoneNumber("(098) 123 4567"));
  }
}

Additional test cases

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import org.junit.Test;
import static org.junit.Assert.assertEquals;

public class PhoneTests {
  @Test
  public void basicTests() {
    String msg = "Follow the formatting instructions carefully";
    assertEquals(msg, true, Solution.validPhoneNumber("(123) 456-7890"));
    assertEquals(msg, false, Solution.validPhoneNumber("(1111)555 2345"));
    assertEquals(msg, false, Solution.validPhoneNumber("(098) 123 4567"));
    assertEquals(msg, false, Solution.validPhoneNumber("(123)456-7890"));
  }
  
  @Test
  public void formCharTests() {
    String msg = "Pay attention to the formatting of the string and surrounding characters";
    assertEquals(msg, false, Solution.validPhoneNumber("abc(123)456-7890"));
    assertEquals(msg, false, Solution.validPhoneNumber("(123)456-7890abc"));
    assertEquals(msg, false, Solution.validPhoneNumber("abc(123)456-7890abc"));
  }
  
  @Test
  public void charTests() {
    String msg = "Be careful with characters surrounding the phone number";
    assertEquals(msg, false, Solution.validPhoneNumber("abc(123) 456-7890"));
    assertEquals(msg, false, Solution.validPhoneNumber("(123) 456-7890abc"));
    assertEquals(msg, false, Solution.validPhoneNumber("abc(123) 456-7890abc"));
  }
  
  @Test
  public void badCharTests() {
    String msg = "Be careful with non-digit characters inside phone number";
    assertEquals(msg, false, Solution.validPhoneNumber("(123) 456-78f0"));
    assertEquals(msg, false, Solution.validPhoneNumber("(123) 4e6-7890"));
    assertEquals(msg, false, Solution.validPhoneNumber("(*23) 456-7890"));
  }
}