What does “an Arbitrary Object of a Particular Type” mean in java 8?

Question

In Java 8 there is \"Method Reference\" feature. One of its kind is \"Reference to an instance method of an arbitrary object of a particular type\"

http://docs.oracl

Answer 1

Please see the below code sample which explains "Reference to an Instance Method of an Arbitrary Object of a Particular Type" category described in https://docs.oracle.com/javase/tutorial/java/javaOO/methodreferences.html

import java.util.Arrays;

class Person{
String name;

//constructor
public Person(String name){
    this.name = name;
}

//instance method 1
public int personInstanceMethod1(Person person){
    return this.name.compareTo(person.name);
}

//instance method 2
public int personInstanceMethod2(Person person1, Person person2){
    return person1.name.compareTo(person2.name);
}
}

class Test {
public static void main (String[] args) throws Exception{
    Person[] personArray = {new Person("A"), new Person("B")};

    // Scenario 1 : Getting compiled successfully
    Arrays.sort(personArray, Person::personInstanceMethod1);

    // Scenario 2 : Compile failure
    Arrays.sort(personArray, Person::personInstanceMethod2);

    // Scenario 3 : Getting compiled successfully. 
    Person personInstance = new Person("C");
    Arrays.sort(personArray, personInstance::personInstanceMethod2);

    // Scenario 4 : Getting compiled successfully. As the same way as "Scenario 1"
    String[] stringArray = { "Barbara", "James", "Mary", "John",
            "Patricia", "Robert", "Michael", "Linda" };
    Arrays.sort(stringArray, String::compareToIgnoreCase);
}

}

Scenario 1 and Scenario 4 describes "Reference to an Instance Method of an Arbitrary Object of a Particular Type" category described in https://docs.oracle.com/javase/tutorial/java/javaOO/methodreferences.html

If the method parameter takes a variable in same instance Type as the instance Type of the element, you can call that instance method using Type.(Person::personInstanceMethod1)

Compare "personInstanceMethod1" instance method in "Person" class with "compareToIgnoreCase" instance method in "String" class (https://docs.oracle.com/javase/8/docs/api/java/lang/String.html#compareToIgnoreCase-java.lang.String-) to see the similarity. Both are taking a single parameter with the same Type.

Compare Scenario 1 and Scenario 2 to see the difference.