Java8 - Types of method references

Category : Java | Sub Category : Java8 Features | By Prasad Bonam Last updated: 2023-11-13 05:23:34 Viewed : 235

In Java, there are several types of method references, each corresponding to a specific situation where a method is invoked. Method references provide a more concise syntax when working with lambda expressions. The main types of method references are:

1. Static Method References:

• Refers to a static method.

• Syntax: ClassName::staticMethodName

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  // Lambda expression
  Function<Integer, String> toStringLambda = (num) -> String.valueOf(num);
  
  // Static method reference
  Function<Integer, String> toStringReference = String::valueOf;
  

2. Instance Method References on a Particular Instance:

• Refers to an instance method of a particular object.

• Syntax: instance::instanceMethodName

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  // Lambda expression
  Predicate<String> isEmptyLambda = (str) -> str.isEmpty();
  
  // Instance method reference on a particular instance
  String sampleString = "Hello, World!";
  Predicate<String> isEmptyReference = sampleString::isEmpty;
  

3. Instance Method References on an Arbitrary Instance:

• Refers to an instance method of the first parameter as an input.

• Syntax: ClassName::instanceMethodName

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  // Lambda expression
  BiFunction<String, String, Boolean> startsWithLambda = (str, prefix) -> str.startsWith(prefix);
  
  // Instance method reference on an arbitrary instance
  BiFunction<String, String, Boolean> startsWithReference = String::startsWith;
  

4. Constructor References:

• Refers to a constructor.

• Syntax:

  • For parameterless constructors: ClassName::new
  • For constructors with parameters: ClassName::new

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  // Lambda expression
  Supplier<List<String>> listSupplierLambda = () -> new ArrayList<>();
  
  // Constructor reference
  Supplier<List<String>> listSupplierReference = ArrayList::new;
  

5. Reference to an Instance Method of an Arbitrary Object of a Particular Type:

• Refers to an instance method of an arbitrary object of a particular type.

• Syntax: ClassName::instanceMethodName

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  // Lambda expression
  BiFunction<String, String, Boolean> endsWithLambda = (str, suffix) -> str.endsWith(suffix);
  
  // Instance method reference on an arbitrary instance of a particular type
  BiFunction<String, String, Boolean> endsWithReference = String::endsWith;
  

Example:

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import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.function.BiFunction;
import java.util.function.Function;
import java.util.function.Predicate;
import java.util.function.Supplier;

public class MethodReferencesExample {

    public static void main(String[] args) {
        // Static method reference
        Function<Integer, String> staticMethodRef = String::valueOf;

        // Instance method reference on a particular instance
        Predicate<String> instanceMethodRef1 = "Hello"::equals;

        // Instance method reference on an arbitrary instance
        Predicate<String> instanceMethodRef2 = String::isEmpty;

        // Constructor reference
        Supplier<List<String>> constructorRef = ArrayList::new;

        // Reference to an instance method of an arbitrary object of a particular type
        BiFunction<String, String, Boolean> arbitraryInstanceMethodRef = String::endsWith;

        // Print results
        System.out.println(staticMethodRef.apply(42)); // "42"
        System.out.println(instanceMethodRef1.test("Hello")); // true
        System.out.println(instanceMethodRef2.test("")); // true
        System.out.println(constructorRef.get()); // []
        System.out.println(arbitraryInstanceMethodRef.apply("Hello", "lo")); // true
    }
}

In this example, various types of method references are used, demonstrating the syntax for static methods, instance methods on particular instances, instance methods on arbitrary instances, constructor references, and references to instance methods of arbitrary objects of a particular type.