J8Validate is a small validation library that allows lambda expressions and method chaining to write concise and decoupled validation.

####Validation can remain stateless and safe to use dependancy injection or threads.

####J8ValidationResult's message property is now generic, so if one needs more than what is supplied out of the box, a custome class can be passed as the message

#####Creating a validator class: There are a few types to be aware of:

Type	Description
AbstractJ8Validator	any validator class should inherit from this class
J8ValidationResult	holds errors and has various methods to be returned
J8When	returned by AbstractJ8Validator's from method. Filters objects to validate by predicate
J8Must	returned by J8When's methods. Takes a predicate to test objects validity
J8WithMessage	returned by J8Must. Adds a message to the errors if test fails.
J8WithSeverity	returned by J8WithMessage. Adds levels of severity to errors
J8ToValidate	returned by J8WithSeverity. Returns a J8ValidationResult object

Step one: create a class to validate a type by inheriting from AbstractJ8Validator.

public class MyValidator extends AbstractJ8Validator<MyType>

And create a method that returns a result:

	public J8ValidationResult<String> validate(List<MyType> items, J8ValidationResult<String> result) {
		return result;
	}

Step two: use the from method to validate the arguments passed into the validate method.

	public J8ValidationResult<String> validate(List<MyType> items, J8ValidationResult<String> result) {
		return
			from(items, result)
			.when(item -> item.isSomething() && item.values.Size() > 0)
			.must(item -> item.getParent() != null)
			.withMessage("Error: item(s) not valid.")
			.critical()
			.toValidate();
	}

#####Validation methods:

Method	Description
from	object(s) to validate
when	filter based on a predicate or all
all	no filter
must	all objects should pass a predicate check
mustNot	No object should be true for the predicate
customMust	takes a boolean. Should be true
withMessage	add error message if failure
info	adds info severity if failure
warning	adds warning severity if failure
critical	adds critical severity if failure
fatal	adds fatal severity if failure
toValidate	returns a validation result

customMust allows for more complicate validation that takes multiple arguments. Example:

from(list, result).all().customMust(veryComplexValidation(list, 3, 87, true, "propName"))
.withMessage("Warning: failed").warning().toValidate()

##Example console program:

###Validator class

public class SnakeValidator extends AbstractJ8Validator<Snake> {
	private final String dangerousSnakes = "Warning: dangerous snakes.";
	
	public J8ValidationResult validateLists(List<Snake> snakes, List<Snake> petSnakes, J8ValidationResult<String> validationResult) {
		J8ValidationResult<String> result = 
				from(petSnakes, validationResult)
				.all()
				.must(snake -> !snake.isVenomous())
				.withMessage("Danger: pet snakes should not be venomous.")
				.critical()
				.toValidate();
		
		result =
				from(snakes, result)
				.when(snake -> snake.isConstrictor())
				.mustNot(snake -> snake.isVenomous())
				.withMessage("Alert: snakes should not be both constrictors and venomous.")
				.fatal()
				.toValidate();
		
		result =
				from(snakes, result)
				.all()
				.mustNot(this::validateDangerousSnakes)
				.withMessage("Warning: some snakes extremely lethal.")
				.warning()
				.toValidate();
				
		result =
				from(snakes, result)
				.all()
				.mustNot(snake -> snakes.size() + petSnakes.size() > 15)
				.withMessage("I HAVE HAD IT WITH THESE m%th#rf^ck^&g SNAKES ON THIS m%th#rf^ck^&g PLANE")
				.fatal()
				.toValidate();
		
		result = from(snakes, result)
				.all()
				.mustNot(snake -> snake.isVenomous())
				.withMessage("Warning: some poisonous snakes.")
				.warning()
				.toValidate();
		
		return result;
	}

	private boolean validateDangerousSnakes(Snake snake) {
		if(snake.getLength() >= 10 && snake.isVenomous() || snake.isConstrictor()) return true;
		return false;
	}
	
}

###Entry Point

public class Program {
	
	public static void main(String[] args) throws Exception {
		Program program = new Program();
		
		List<Snake> snakes = new ArrayList<Snake>();
		List<Snake> petSnakes = new ArrayList<Snake>();
		
		for (int i = 0; i < 20; i++) {
			Snake snake = new Snake();
			snakes.add(new Snake());
			if(snake.isVenomous()) continue;
			petSnakes.add(snake);
		}
		
		SnakeValidator validator = new SnakeValidator();
		
		J8ValidationResult<String> result = validator.validateLists(snakes, petSnakes, new J8ValidationResult<String>());
		
		result.getErrorMessages().forEach(err -> System.out.println(err));
		
		if(result.anyFatal()) {
			handleSnakeProblem();
		}
	}

	private static void handleSnakeProblem() {
		System.out.println("eject...");
	}
}

###Snake class

public class Snake {
	private boolean isVenomous;
	private boolean isConstrictor;
	private int length;
	
	public boolean isVenomous() {
		return isVenomous;
	}
	
	public boolean isConstrictor() {
		return isConstrictor;
	}
	
	public int getLength() {
		return length;
	}
	
	public Snake() {
		Random rand = new Random();
		length = rand.nextInt(12);
		isVenomous = rand.nextInt(10) > 5? true : false;
		isConstrictor = !isVenomous && rand.nextInt(10) > 5? true : false;
	}
}