Decisions4J

Decisions4J is a Java API that allows you to define a set of rules to apply to an object and execute the appropriate action based on the first matching rule. It's useful when you need to make a decision based on a complex set of conditions.

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Usage

To use Decisions4J, create a new instance of it by passing an object of the type that you want to evaluate as its argument:

DecisionsTree<String> tree = new DecisionsTree<>("test"); // in this case we're evaluating a String

Adding rules

Once you have a Decisions4J instance, you can add rules to it. A rule consists of a condition and an action. The condition is a Predicate that evaluates to true if the rule should be applied, and the action is a Consumer that specifies what should be done if the rule is applied.

To add a rule to the DecisionsTree, call the caseCondition() method with the condition and the action as arguments. Here's an example:

tree.caseCondition(s -> s.equals("test"), s -> System.out.println("test"));

In this example, we're adding a rule that says "if the string is 'test', then print 'test'".

Adding a default rule

If none of the rules match, you can specify a default action to be executed by calling the defaultCase() method:

tree.defaultCase(s -> System.out.println("default"));

In this example, we're specifying that if none of the other rules match, the action should be to print "default".

Applying the rules

Once you've added all the rules, you can apply them to the object by calling the decide() method:

tree.decide();

This will evaluate the object and execute the first matching rule.

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Example

Here's an example of using Decisions4J to evaluate a Person object:

// Create a new DecisionsTree with a Person object
Person person = new Person("Alice", 30);
DecisionsTree<Person> tree = new DecisionsTree<>(person);

// Add a rule
tree.caseCondition(p -> p.getAge() > 18, p -> System.out.println(p.getName() + " is an adult"));

// Add a default rule
tree.defaultCase(p -> System.out.println(person.getName() + " is a minor"));

// Apply the rules
tree.decide();

In this example, we're creating a new DecisionsTree instance with a Person object. We're adding a rule that says "if the person's age is greater than 18, then print 'Alice is an adult'" (where "Alice" is the person's name). We're also adding a default rule that says "print 'Alice is a minor' if none of the other rules match". Finally, we're applying the rules to the person object by calling decide().

You may also use this syntax:

// Create a Person object
Person person = new Person("Alice", 30);

// Create a DecisionsTree object
DecisionsTree<Person> tree = new DecisionsTree<>(person)
        .caseCondition(p -> p.getAge() > 18, p -> System.out.println(p.getName() + " is an adult"))
        .defaultCase(p -> System.out.println(person.getName() + " is a minor"));

// Apply the rules
tree.decide();

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Error handling

Decisions4J provides some basic error handling. If you don't add any rules to the DecisionsTree, calling decide() will result in an IllegalStateException. Similarly, if you don't call decide() after adding rules, you'll get an IllegalStateException as well. If you call defaultCase() with a null action, you'll get a NullPointerException.

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check out the javascript version here and the python version here :)