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

  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"
  
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  1. java -ea AssertionTest  

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

  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
  
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  1. import java.io.*;  
  2.   
  3. public class AssertionTest {  
  4.  public static void main(String[] args) throws IOException {  
  5.      System.out.print("Enter your marital status: ");  
  6.      int c = System.in.read();  
  7.      switch((char)c) {  
  8.          case 's': case 'S':  
  9.              System.out.println("Single");  
  10.              break;  
  11.          case 'm': case 'M':  
  12.              System.out.println("Married");  
  13.              break;  
  14.          case 'd': case 'D':  
  15.              System.out.println("Divorced");  
  16.              break;  
  17.          default:  
  18.              assert !true : "Invalid Option";  
  19.      }  
  20.  }  
  21. }  

  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";
       }
   }
  }