Java中的雙冒號(::)運算符

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Syntax

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!");
   }
}

語法解釋

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。

演算法

• 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。

方法一：使用雙冒號運算子的方法參考

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.

// 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);
   }
}

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#

Result: 8

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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 persons = new ArrayList<>();
      persons.add(new Person("Alice"));
      persons.add(new Person("Bob"));

      persons.forEach(Person::printName);
   }
}

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#

Alice
Bob

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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 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);
   }
}

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#

Number of palindromic words: 2

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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 employeeSupplier = Employee::new;
      Employee employee = employeeSupplier.get();

      employee.name = "John Doe";
      employee.age = 30;

      System.out.println(employee);
   }
}

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#

Employee [name=John Doe, age=30]

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Explanation

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