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This article mainly introduces "what methods are quoted in Java". In daily operation, I believe many people have doubts about what methods are quoted in Java. The editor consulted all kinds of materials and sorted out simple and easy-to-use operation methods. I hope it will be helpful for you to answer the doubts about "what methods are quoted in Java?" Next, please follow the editor to study!

One: simplicity

There are three kinds of method references. Method references are represented by a pair of double colons::. Method references are another way of writing a functional interface.

Static method reference, through class name:: static method name, such as Integer::parseInt

Instance method reference, through instance object:: instance method, such as str::substring

Constructor reference, through the class name:: new, such as User::new

Two: method reference

Public final class Integer {

Public static int parseInt (String s) throws NumberFormatException {

Return parseInt (SJ10)

}

}

Through the method reference, you can assign the reference of the method to a variable, and by assigning a value to Function, it shows that the method reference is also a kind of functional interface writing, Lambda expression is also a kind of functional interface, Lambda expression is generally used to provide its own method body, and method reference generally directly references off-the-shelf methods.

Public class User {

Private String username

Private Integer age

Public User () {

}

Public User (String username, Integer age) {

This.username = username

This.age = age

}

@ Override

Public String toString () {

Return "User {" +

"username='" + username +'\'+

", age=" + age +

'}'

}

/ / Getter&Setter

}

Public static void main (String [] args) {

/ / use double colons:: to construct static function references

Function fun = Integer::parseInt

Integer value = fun.apply ("123")

System.out.println (value)

/ / use double colons:: to construct non-static function references

String content = "Hello JDK8"

Function func = content::substring

String result = func.apply (1)

System.out.println (result)

/ / Constructor reference

BiFunction biFunction = User::new

User user = biFunction.apply ("mengday", 28)

System.out.println (user.toString ())

/ / function reference is also a functional interface, so you can also use function reference as a parameter to a method.

SayHello (String::toUpperCase, "hello")

}

/ / the method has two parameters, one is

Private static void sayHello (Function func, String parameter) {

String result = func.apply (parameter)

System.out.println (result)

}

Three: optional values for Optional

There is Optional in Google Guava and a similar syntax in Swift. In Swift, optional values are treated as a data type, and the status is equal to the basic type, with a very high status.

Package java.util

Import java.util.function.Consumer

Import java.util.function.Function

Import java.util.function.Predicate

Import java.util.function.Supplier

/ * *

* @ since 1.8

, /

Public final class Optional {

Private static final Optional EMPTY = new Optional ()

Private final T value

Private Optional () {

This.value = null

}

/ / return an empty Optional instance

Public static Optional empty () {

@ SuppressWarnings ("unchecked")

Optional t = (Optional) EMPTY

Return t

}

Private Optional (T value) {

This.value = Objects.requireNonNull (value)

}

/ / returns the Optional with the current non-null value of Optional

Public static Optional of (T value) {

Return new Optional (value)

}

/ / returns an Optional with a specified value of Optional, or an empty Optional if it is not empty

Public static Optional ofNullable (T value) {

Return value = = null? Empty (): of (value)

}

/ / if there is a value in Optional, the value is returned, otherwise NoSuchElementException is thrown.

Public T get () {

If (value = = null) {

Throw new NoSuchElementException ("No value present")

}

Return value

}

/ / returns true if there is a value, otherwise false

Public boolean isPresent () {

Return value! = null

}

/ / if a value exists, the specified consumer is called with that value, otherwise no action is performed.

Public void ifPresent (Consumer

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