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When a Java class overrides the equals method, it should override the hashCode method as well. As defined in the method's contract: Whenever it is invoked on the same object more than once during an execution of a Java application, the hashCode method must consistently return the same integer, ...

Once we've gotten the Connection, we will mostly use it to create Statement objects. Statements represent a single SQL transaction; they are used to execute a query, and retrieve the results (if any). Let's look at some examples: public void useConnection() throws SQLException{ Connection ...

OptionalDouble, OptionalInt and OptionalLong work like Optional, but are specifically designed to wrap primitive types: OptionalInt presentInt = OptionalInt.of(value); OptionalInt absentInt = OptionalInt.empty(); Because numeric types do have a value, there is no special handling for null. Empt...

An enum cannot have a public constructor; however, private constructors are acceptable (constructors for enums are package-private by default): public enum Coin { PENNY(1), NICKEL(5), DIME(10), QUARTER(25); // usual names for US coins // note that the above parentheses and the constructor...

Colors are usually stored in a resource file named colors.xml in the /res/values/ folder. They are defined by <color> elements: <?xml version="1.0" encoding="utf-8"?> <resources> <color name="colorPrimary">#3F51B5</color> <c...

A simple example of producer-consumer problem solution. Notice that JDK classes (AtomicBoolean and BlockingQueue) are used for synchronization, which reduces the chance of creating an invalid solution. Consult Javadoc for various types of BlockingQueue; choosing different implementation may drastica...

Calendar, Date, and LocalDate Java SE 8 before, after, compareTo and equals methods //Use of Calendar and Date objects final Date today = new Date(); final Calendar calendar = Calendar.getInstance(); calendar.set(1990, Calendar.NOVEMBER, 1, 0, 0, 0); Date birthdate = calendar.getTime(); ...

format() from SimpleDateFormat class helps to convert a Date object into certain format String object by using the supplied pattern string. Date today = new Date(); SimpleDateFormat dateFormat = new SimpleDateFormat("dd-MMM-yy"); //pattern is specified here System.out.println(date...

parse() from SimpleDateFormat class helps to convert a String pattern into a Date object. DateFormat dateFormat = DateFormat.getDateInstance(DateFormat.SHORT, Locale.US); String dateStr = "02/25/2016"; // input String Date date = dateFormat.parse(dateStr); System.out.println(date.getYe...

A Stream will only be traversed when there is a terminal operation, like count(), collect() or forEach(). Otherwise, no operation on the Stream will be performed. In the following example, no terminal operation is added to the Stream, so the filter() operation will not be invoked and no output will...

You should be careful when comparing floating-point values (float or double) using relational operators: ==, !=, < and so on. These operators give results according to the binary representations of the floating point values. For example: public class CompareTest { public static void main...

Annotation @XmlAccessorType determines whether fields/properties will be automatically serialized to XML. Note, that field and method annotations @XmlElement, @XmlAttribute or @XmlTransient take precedence over the default settings. public class XmlAccessTypeExample { @XmlAccessorType(XmlAccessT...

Annotations @XmlElement, @XmlAttribute or @XmlTransient and other in package javax.xml.bind.annotation allow the programmer to specify which and how marked fields or properties should be serialized. @XmlAccessorType(XmlAccessType.NONE) // we want no automatic field/property marshalling public cla...

The + symbol can mean three distinct operators in Java: If there is no operand before the +, then it is the unary Plus operator. If there are two operands, and they are both numeric. then it is the binary Addition operator. If there are two operands, and at least one of them is a String, then i...

The Java language provides 7 operators that perform arithmetic on integer and floating point values. There are two + operators: The binary addition operator adds one number to another one. (There is also a binary + operator that performs string concatenation. That is described in a separate e...

Optional<String> optionalWithValue = Optional.of("foo"); optionalWithValue.ifPresent(System.out::println);//Prints "foo". Optional<String> emptyOptional = Optional.empty(); emptyOptional.ifPresent(System.out::println);//Does nothing.

Introduction All modern web browsers, NodeJs as well as almost every other JavaScript environments support writing messages to a console using a suite of logging methods. The most common of these methods is console.log(). In a browser environment, the console.log() function is predominantly used f...

A list comprehension creates a new list by applying an expression to each element of an iterable. The most basic form is: [ <expression> for <element> in <iterable> ] There's also an optional 'if' condition: [ <expression> for <element> in <iterable> if <c...

A dictionary comprehension is similar to a list comprehension except that it produces a dictionary object instead of a list. A basic example: Python 2.x2.7 {x: x * x for x in (1, 2, 3, 4)} # Out: {1: 1, 2: 4, 3: 9, 4: 16} which is just another way of writing: dict((x, x * x) for x in (1, 2...

The fundamental part of most classes is its constructor, which sets up each instance's initial state and handles any parameters that were passed when calling new. It's defined in a class block as though you're defining a method named constructor, though it's actually handled as a special case. cla...