How to be assertive

• Assertions were introduced in J2SE 1.4
• Rather than introducing it as a class library it was built into the language itself using the keyword assert
• Assertions can be used to test the following three roles:
  
  • Precondition - a condition that the caller of a method agrees to satisfy
  • Postcondition - a condition that the method itself promises to achieve
  • Invariant - a condition that should always be true for a specified segment or at a specified point of a program
• An assertion has a boolean expression that if evaluated as false indicates a bug in the code
  

  assert a !=b  : "Not Equal!!"

• The message part after the : is optional, if given it is passed to the stack trace which is printed
• When an assertion fails, it means that the application has entered an incorrect state
• When the expression evaluates to false an AssertionError is thrown
• A good practice is to terminate the application when the error is thrown, because the application may start functioning inappropriately after a failure
• Assertions are disabled by default - to run an application with assertions enabled we have to give the option "-ea" or "-enableassertion"
  

  java -ea AssertionTest

• The above command enables assertion for all classes except for the system classes
• To turn on assertion in system classes we have to use "-esa" or "-enablesystemassertions"
• To disable assertions we have to use "-da" or "-disableassertion", which is the default
• Also, we can enable or disable assertions for specific classes or packages
  

  java -ea:com.test.ui.UIClass MainClass
  java -ea:com.javacourses.tests... -da:com.javacourses.ui.phone MyClass

• The ellipsis ... is part of the syntax
• Following is a sample class that uses assertion to check for invalid user input
  

  import java.io.*;
  
  public class AssertionTest {
   public static void main(String[] args) throws IOException {
       System.out.print("Enter your marital status: ");
       int c = System.in.read();
       switch((char)c) {
           case 's': case 'S':
               System.out.println("Single");
               break;
           case 'm': case 'M':
               System.out.println("Married");
               break;
           case 'd': case 'D':
               System.out.println("Divorced");
               break;
           default:
               assert !true : "Invalid Option";
       }
   }
  }