java8-cert

Some code for practicing for the Java 8 OCP / Java SE 8 Programmer (1Z0-809).

150min (2.5h), 85 questions, 65% to pass.

Hot topics

Lambdas
Streams
File IO
NIO
Generics
Collection API
File IO (Writer, Reader, ...)
NIO 2: Path, Files, String, Stringwriter
Exceptions
HashMap
Abstrakte Klassen
Annotations
Date/Time API
Concurrency Updates
Reflection

Resources

TODO

Lessons learned / pitfalls

toString on Enums will print String, not ordinal
"An abstract type can be an abstract class or interface."
"In a subclass, the compiler implicitly invokes a parameterless constructor in the superclass if a superclass constructor is not explicitly invoked."
"Top-level classes cannot be declared using the protected access modifier. Only the access modifier public is allowed for top-level classes. An inner class can be declared with the access modifier protected because an inner class is a member of the top-level class."
"Overriding methods can specify access modifiers that are less restrictive than the original method. Also, Java allows an overriding method to return a subtype of the return type of the original method."
Command line params start at args[0] and are always Strings.
"It is permitted, but discouraged as a matter of style, to redundantly specify the public and/or abstract modifier for a method declared in an interface."
"If you do not specify that the interface is public, then your interface is accessible only to classes defined in the same package as the interface."
"Static methods can not be overridden in the exact sense of the word, but they can hide parent static methods" (source)
Enums can not have a public constructor. All Enum values ("inner subclasses") will be constructed the first time a Enum value is used.
"The first time that we ask for any of the enum values (of a certain Enum) Java constructs all of the enum values."
"In Java 8, the 'effectively final' concept was introduced. If the code can still compile with the keyword final inserted before the local variable, the variable is effectively final." (study book)
Inner classes: "If the member or constructor is declared private, then access is permitted if and only if it occurs within the body of the top level class (§7.6) that encloses the declaration of the member or constructor." source
'local inner class' = nested class defined within a method
Floats (for example) as class variables (not local ones) will be initialized with 0.0, but not inside a method!
In a concrete class, all methods and Constructors must contain bodies.
A class cannot include package or import statements. (class itself not, but file that contains class)
(abstract) classes can be extended, interfaces can be implemented (or extended, if you're an interface yourself). In other words, valid examples are: interface extends interface / class extends class / class implements interface.
"A JavaBean is a design principle for encapsulating data in an object in Java."
A new public StringBuilder() has initial capacity of 16 characters.
Parentheses for Lambda parameter: can be omitted only if one parameter (not if 0 or 2+) and there is no type given.
"Tight coupling is the practice of developing coupled classes that are highly dependent, such that a minor change in one class may greatly impact the other class. Alternatively, loose coupling is the practice of developing coupled classes with minimum dependencies on one another."
"An instant is a point in time. A LocalDateTime does not contain a time zone, and is therefore not universally recognized around the world as the same moment in time."
"The CyclicBarrier class allows us to perform complex, multi-threaded tasks, while all threads stop and wait at logical barriers."

Random facts & tables

Access modifiers

Modifier	Class	Package	Subclass	World
public	Y	Y	Y	Y
protected	Y	Y	Y	N
no modifier	Y	Y	N	N
private	Y	N	N	N

Functional Interfaces

Functional Interface	# Parameters	Return Type	Single Abstract Method
`Supplier<T>`	0	T	get
`Consumer<T>`	1 (T)	void	accept
`BiConsumer<T, U>`	2 (T, U)	void	accept
`Predicate<T>`	1 (T)	boolean	test
`BiPredicate<T, U>`	2 (T, U)	boolean	test
`Function<T>`	1 (T)	R	apply
`BiFunction<T, U>`	2 (T, U)	R	apply
`UnaryOperator<T>`	1 (T)	T	apply
`BinaryOperator<T, U>`	2 (T, U)	T	apply

Generics naming conventions

E for an element
K for a map key
V for a map value
N for a number
T for a generic data type
S, U, V, and so forth for multiple generic types

Exceptions for OCP

Checked:

java.text.ParseException
java.io.IOException (generally all checked)
- java.io.FileNotFoundException
- java.io.NotSerializableException
java.sql.SQLException

Unchecked:

java.lang.ArithmeticException
java.lang.ArrayIndexOutOfBoundsException
java.lang.ClassCastException
java.lang.IllegalArgumentException
java.lang.NullPointerException
java.lang.NumberFormatException
java.lang.ArrayStoreException
java.lang.DateTimeException
java.lang.MissingResourceException
java.lang.IllegalStateException
java.lang.UnsupportedOperationException

Creational patterns

Singleton pattern: Only one instance of a Singleton class may exist at runtime.
Immutable pattern: Read-only objects.
Builder pattern: Enrich object with attributes through setters (chainable) and create instance with build()
Factory pattern: Return different object types, based on given criteria, for example PetFactory which returns either a Dog or Cat instance.

Immutable class

Steps to make a class immutable:

Use a constructor to set all properties of the object
Mark all instance variables as private and final
Don't define any setter methods
Don't allow referenced mutable objects to be modified or accessed directly
Prevent methods from being overriden

Picking a resource bundle

Rules for picking a resource bundle:

The requested locale as .java-File
The requested locale as .properties-File
Only requested language (no country) as .java-File
Only requested language (no country) as .properties-File
The default locale as .java-File
The default locale as .properties-File
Only default language (no country) as .java-File
Only default language (no country) as .properties-File
The default bundle with no locale as .java-File
The default bundle with no locale as .properties-File
A MissingResourceException is thrown.

akleemans/java8-cert