What Is Double Colon (::) Operator Inwards Coffee Eight - Example

The double colon (::) operator is known equally the method reference inward Java 8. Method references are expressions which induce got the same handling equally a lambda expression, but instead of providing a lambda body, they refer to an existing method past times name. For example, to impress all elements of the listing yous tin sack purpose the forEach() method equally follows

list.stream.forEach( s-> System.out.println(s));

but hither yous tin sack supersede lambda human face alongside method reference to amend readability equally shown below:

list.stream.forEach( System.out::println);

You tin sack purpose method reference or double colon operator to refer a static method, an instance method, or a constructor.


For referring a static method, yous tin sack either purpose className or instanceRef, but to refer an event method of a detail object, yous postulate instanceRef.

Here are to a greater extent than or less examples of method reference inward Java 8:

1. A static method (ClassName::methodName) similar Person::getAge
2. An event method of a detail object (instanceRef::methodName) similar System.out::println
3. A super method of a detail object (super::methodName)
4. An event method of an arbitrary object of a detail type (ClassName::methodName)
5. A course of report constructor reference (ClassName::new) like ArrayList::new
6. An array constructor reference (TypeName[]::new) like String[]:new 

The sentiment is that if yous are passing a lambda human face e.g. a business office which takes a value too prints inward the console than instead of passing lambda expression, merely piece of job past times the println() method of PrintStream course of report which yous can access as System.out::println.

Remeber at that spot is no parenthesis when yous refer a method using method reference or lambda human face inward Java 8.  If yous desire to larn more, The Complete Java MasterClass is a skillful house to start with.

Real basis Example of Double Colon Operator inward Java 8

Here is a real-world illustration fo method reference using double colon operator inward Java 8. In this example, nosotros are creating a Comparator which compares somebody past times their age. In the starting fourth dimension example, nosotros induce got used a lambda human face to piece of job past times the logic to compare age, piece inward the minute business of code nosotros induce got used double colon operator or method reference.

You tin sack run into that the minute illustration is much to a greater extent than readable too clean:

Btw, If yous are using IDE similar Eclipse, NetBeans, or IntelliJ IDEA, the IDE volition besides advise you convert your lambda human face to method reference equally shown in the following screenshot.

This confirms that it's a best practise inward Java 8 to convert lambda human face to a method reference or constructor reference using double colon operator. See this articles for to a greater extent than tips on when too how yous tin sack convert a lambda human face to method reference inward Java 8.

That's all almost double colon operator of Java 8. As I said, it's known equally method reference too tin sack hold out used to refer an existing method past times mention instead of using a lambda expression.  It's a  Java 8 best practise to favor method reference over lambda human face equally it results inward to a greater extent than cleaner code.

Further Reading
example)

How to purpose filter() method inward Java 8 (tutorial)

20 Examples of Date too Time inward Java 8 (tutorial)

How to purpose Stream course of report inward Java 8 (tutorial)

How to form HashMap inward Java 8? (example)

How to purpose forEach() method inward Java 8 (example)

5 Books to Learn Java 8 from Scratch (books)

How to convert List to Map inward Java 8 (solution)

How to purpose peek() method inward Java 8 (example)

10 Java 8 Tutorials You must read (tutorials)

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