Java Predefined-Functional Interfaces
Java provides predefined functional interfaces to deal with functional programming by using lambda and method references.

You can also define your own custom functional interface. Following is the list of functional interface which are placed in java.util.function package.
Supplier, Consumer , Function Predicate etc.

An abstract method is a method that is declared without an implementation (without braces, and followed by a semicolon), like this:Abstract classes are classes that contain one or more abstract methods.

Example:
public abstract class vehicle{

//Non abstract method
void int speedUp(int speed){

spedd = speed + 10 ;

}

abstract void stopVehicle(String commandToStop);

}

public class thunderBird extends vehicle{

void stopVehicle(String commandToStop)
{

commandToStop;

}

}

So in the above abstract class vehicle there are two methods one is speedUp which is non-abstract method and one abstract method stopVehicle . So the classes that extend this abstract class must define the abstract methods declared in abstract class other wise the class itself must declared as abstract class again.

An abstract method is a method that is declared, but contains no implementation.
Abstract classes may not be instantiated, and require subclasses to provide implementations for the abstract methods.
An Abstract class without any implementation just looks like an Interface.

Abstract class can implement interface ..

abstract class jumbo implements vehicle {
  void startVehicle(Sting commandToStart){

start ;

}
void stopVehicle(String commandToStop){

stop ;

}
void speedUp(int speed){

speed = speed + 20;

}
}

Refer some more pages on the same topic here [1][2][3][4]