Review the Comparator class; it has two abstract methods, why it can be a function interface?
package java.util;
@FunctionalInterface
public interface Comparator<T> {
// abstract method
int compare(T o1, T o2);
// abstract method
boolean equals(Object obj);
// few default and static methods
}
Definition of function interface
Conceptually, a functional interface has exactly one abstract method. Since default methods have an implementation, they are not abstract. If an interface declares an abstract method overriding one of the public methods of java.lang.Object, that also does not count toward the interface’s abstract method count since any implementation of the interface will have an implementation from java.lang.Object or elsewhere.
Answer
Yes, Comparator is a functional interface. The equals is an abstract method overriding one of the public methods of java.lang.Object, this doesn’t count as an abstract method.
boolean equals(Object obj);
The Comparator only has one abstract method int compare(T o1, T o2), and it meet the definition of functional interface.
package java.util;
@FunctionalInterface
public interface Comparator<T> {
// abstract method
int compare(T o1, T o2);
// abstract method, overriding public methods of `java.lang.Object`
// this doesn't count!
boolean equals(Object obj);
// few default and static methods
}