Assertions.java

package exceptionsAndAssertions;

/**
 * @author chengfeili 
 * Jun 11, 2017 11:25:30 AM
 * 
 * An assertion is a Boolean expression that you place at a point in your code
 * where you expect something to be true.
 * 
 * assert boolean_expression; assert boolean_expression: error_message;
 * 
 * If assertions are disabled, Java skips the assertion and goes on in the code.
 * If assertions are enabled and the boolean expression is true, then our
 * assertion has been validated and nothing happens. The program continues to
 * execute in its normal manner. If assertions are enabled and the boolean
 * expression is false, then our assertion is invalid and a
 * java.lang.AssertionError is thrown.
 * 
 * 1. Internal Invariants. You assert that a value is within a certain
 * constraint. assert x < 0 is an example of an internal invariant.
 * 
 * 2.Class Invariants. You assert the validity of an object’s state. Class
 * invariants are typically private methods within the class that return a
 * boolean.
 * 
 * 3. Control Flow Invariants. You assert that a line of code you assume is
 * unreachable is never reached. The upcoming TestSeasons class demonstrates a
 * control of invariant.
 * 
 * 4. Preconditions. You assert that certain conditions are met before a method
 * is invoked.
 * 
 * 5. Post Conditions. You assert that certain conditions are met after a method
 * executes successfully.
 */
public class Assertions {
	public static void main(String[] args) {
		int numGuests = -5;
		// if booleanExpression evaluates to False, an exception of type
		// java.lang.AssertonError is thrown.
		assert numGuests > 0;
		if (!(numGuests > 0)) {
			throw new AssertionError();
		}
		assert numGuests > 0 : "Message";
		if (!(numGuests > 0)) {
			throw new AssertionError("Message");
		}
		System.out.println(numGuests);
	}
}