c_6_6_workingWithAssertions

🧠 6.6 Working With Assertions

• An assertion is a expressions which you place in your code to state something which is true.
• In this section, we will learn the syntax for assertion and how to turn them on/off.

🟥 6.6.1 The assert Statement

• The syntax for an assert statement has two forms:

assert boolean_expression;
assert boolean_expression: error_message;

• The error_message must be a String which is used in the AssertionError (if thrown)
• There are 3 possible outcomees of an assert statement:

1. If assertions are disable, Java will skip over
2. If the boolean expression is true, nothing happens
3. If the boolean expression is false, then a java.lang.AssertionError is thrown

• You enable assertions by running java -ea className.
• Here is an example:

public class Assertions {
    public static void main(String[] args) {
        int numGuests = -5;
        assert numGuests > 0;
        System.out.println(numGuests);
    }
}

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🟥 6.6.2 Enabling Assertions

• By default, assert statements are ignored by JVM. You can enable assertions using:

java -enableassertions ClassName
java -ea ClassName

• We can also target the assertions for a specific class or package. E.g. if you wanted to enable assertions only for classes in com.wiley.demos:

java -ea:com.wiley.demos... my.prgrams.Main

• The elipsis means any class in com.wiley.demos and its subpackages.

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🟥 6.6.3 Using Assertions

• You can use assertion for many reasons, such as the following:

1. Internal Invariants: used to assert the validity of an object's state.
2. Internal Invariants: used to assert that a value is within a certain constraint.
3. Control Flow Invariants: used to assert a line is indeed unreachable

• Suppose we have a zoo which is closed in Winter:

public enum Seasons {
    SPRING, SUMMER, FALL
}
public class TestSeasons {
    public static void test(Seasons s) {
        switch (s) {
            case SPRING:
            case FALL:
                System.out.println("Shorter hours");
                break;
            case SUMMER:
                System.out.println("Longer hours");
                break;
            default:
                assert false: "Invalid season";
        }}}

4. Preconditions: ensuring conditions are met before method is invoked
5. Post Conditions: ensuring conditions are met after method is invoked

• Assertions should not be used to check for valid arguments passed into a method