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雙冒號運算子由兩個冒號(::)夾在類別名稱或物件參考和方法名稱之間。它用作Java中引用方法或建構函數的簡寫符號。
// A functional interface with a single abstract method
interface Printer {
void print(String message);
}
// A class that implements the Printer interface
class ConsolePrinter {
public static void printMessage(String message) {
System.out.println(message);
}
}
public class Main {
public static void main(String[] args) {
Printer printer = ConsolePrinter::printMessage;
printer.print("Hello, World!");
}
}
在上面的程式碼中，我們使用一個名為Printer的實用連接點來定義一個具有單一動態方法print()的類別。 ConsolePrinter類別實作了此連接點，並為printMessage()方法提供了實作。在Principal類別中，我們使用雙冒號運算子建立了一個Printer實例，以引用ConsolePrinter類別的printMessage()方法。最後，我們呼叫印表機實例的print()方法，該方法又呼叫printMessage()方法。
要在Java中使用雙冒號運算符，請依照下列步驟進行 -
定義一個具有單一抽象方法的函數式介面。
在一個類別中實作介面並提供方法的實作。
Use the double colon operator to refer to the method or constructor.
使用雙冒號運算元建立函數式介面的實例。
在實例上呼叫該方法，將呼叫引用的方法或建構子。
Approach 1 involves using the double colon operator to reference a static method of a class. This approach is useful when we want to pass a method reference that does not depend on any nceinstance v.
Example// A functional interface with a single abstract method
interface Calculator {
int calculate(int a, int b);
}
class MathUtils {
public static int add(int a, int b) {
return a + b;
}
}
public class Main {
public static void main(String[] args) {
Calculator calculator = MathUtils::add;
int result = calculator.calculate(5, 3);
System.out.println("Result: " + result);
}
}
Result: 8
方法2：使用實例變數使用雙冒號運算子進行方法參考
Example
import java.util.ArrayList;
import java.util.List;
class Person {
private String name;
public Person(String name) {
this.name = name;
}
public void printName() {
System.out.println(name);
}
}
public class Main {
public static void main(String[] args) {
List<Person> persons = new ArrayList<>();
persons.add(new Person("Alice"));
persons.add(new Person("Bob"));
persons.forEach(Person::printName);
}
}
Alice Bob
方法3: 使用雙冒號運算子引用任意物件的實例方法的方法。
Example
import java.util.Arrays;
import java.util.List;
class StringUtil {
public boolean isPalindrome(String s) {
String reversed = new StringBuilder(s).reverse().toString();
return s.equals(reversed);
}
}
public class Main {
public static void main(String[] args) {
List<String> words = Arrays.asList("level", "hello", "radar", "world");
StringUtil stringUtil = new StringUtil();
long count = words.stream().filter(stringUtil::isPalindrome).count();
System.out.println("Number of palindromic words: " + count);
}
}
Number of palindromic words: 2
Approach 4: Constructor referencing using double colon operator.
Example
import java.util.function.Supplier;
class Employee {
public String name;
public int age;
public Employee() {
// Default constructor
}
public Employee(String name, int age) {
this.name = name;
this.age = age;
}
public String toString() {
return "Employee [name=" + name + ", age=" + age + "]";
}
}
public class Main {
public static void main(String[] args) {
Supplier<employee> employeeSupplier = Employee::new;
Employee employee = employeeSupplier.get();
employee.name = "John Doe";
employee.age = 30;
System.out.println(employee);
}
}
</employee>Employee [name=John Doe, age=30]
結論
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