This is an in-action case of Creating a Generic List Wrapper in JAXB. The Wrapper class is final, its no-args constructor is private and even the factory method is private. public final Wrapper<T> { private static <T> of(final List<T> elements) { Objects.requireNonNull(elements); final Wrapper<T> instance = new Wrapper<>(); instance.elements …

I’ve encountered, quite a few times, into situations that I need to convert database strings into LocalDateTime instances. Let’s write an abstract class for that. abstract class TemporalAccessorStringConverter< T extends TemporalAccessor> implements AttributeConverter<T, String> { } Two constructors are added. protected TemporalAccessorConverter( final…

Distilled as short as possible — Links for Books Effective Java 3rd Edition 이펙티브 자바 제3판 Foreword Preface Acknowledgments 1 Introduction 2 Creating and Destroying Objects Item 1: Consider static factory methods instead of constructors Item 2: Consider a builder when faced with many constructor parameters Item 3: Enforce the singleton property with a private constructor or an enum type Item 4: Enforce noninstantiability with a private constructor

Recently I had to define some utility methods for accessing fields with their names. The basic utility method that following codes based is ReflectionUtils#findField(Class<?>, String). Let’s create a cache for (already found) fields for increasing re-usability. private static Map<Class<?>, Map<String, Field>> FIELDS = synchronizedMap(new WeakHashMap<>()); And here comes the method…

I just found my own anti-pattern regarding the try-with-resources statements. Whenever I encountered code like this. try (InputStream is = getSome()) { } The kind IDE tells me that I need to handle the IOException which is obviously from the close() method. And I instinctively add an outer try-catch block like this. try { try (InputStream is = getSome()) { } } catch (final IOException ioe) { ... }

java.lang.Iterable<T> The description of java.lang.Iterable<T> is fairly simple. Implementing this interface allows an object to be the target of the enhanced for statement (sometimes called the "for-each loop" statement). Simply, with given instance of Iterable<T>, we can do this. for (final T i : iterable) { // for-each element, as…

Sometimes we need to create temporary files and use them in a unit test. And usually, I do like this. @Test void testSome() { File file; try { file = File.createTempFile("tmp", null); } catch…

Check this out. // returns an array of given length with random values. int[] randomArray(@Min(1) final int length) { return range(0, length).map(current()::nextInt).toArray(); } I, simply, shocked when I saw the following stack trace. java.lang.IllegalArgumentException: bound must be positive at java.base/java.util.concurrent.ThreadLocalRandom.nextInt(ThreadLocalRandom.java:310) ... So, what was wrong?

With my following result type, @Data public class Some { ... private List<Sting> keywords; } MyBatis with following ResultMap complains about creating results. <collection property="keywords" ofType="string" javaType="list"> <result column="keyword"/> </collection> That’s because @Data annotation works as @RequiredArgsConstructor , MyBatis couldn’t create the value. And that’s why I changed the class like this.