What method references are there in Java
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
