assert

• No libraries needed to be imported
• They are disabled by default in java
• To enable, you need -ea or -enableassertions at runtime in java command line
• Hence they can be used in development
• Syntax:

Connection conn = getConnection();
 
// expression1: boolean, expression2: Object/String/int/long/...
assert expression1 : expression2;
 
// without message
assert conn != null;
 
// with message
assert conn != null : "Connection is null";

• Throws AssertionError if condition fails
  • extends Error < Throwable
• AssertionError is same error thrown from usual assertion libraries
• expression2 will be used as new AssertionException(expression2) and thrown

Best Practices

• Assertions are great for places in the code that will never be executed, such as:
  • the default case of a switch statement
  • after a loop that never finishes
• Do not test arguments of public methods
  • Use some other runtime exception like IllegalArgumentException
• Don’t call methods in assert expression
  • save result and assert it