Tutorial by Examples

Declaration of an interface using the interface keyword: public interface Animal { String getSound(); // Interface methods are public by default } Override Annotation @Override public String getSound() { // Code goes here... } This forces the compiler to check that we are overri...

A Java class can implement multiple interfaces. public interface NoiseMaker { String noise = "Making Noise"; // interface variables are public static final by default String makeNoise(); //interface methods are public abstract by default } public interface FoodEater { ...

An interface can extend another interface via the extends keyword. public interface BasicResourceService { Resource getResource(); } public interface ExtendedResourceService extends BasicResourceService { void updateResource(Resource resource); } Now a class implementing ExtendedR...

Let's say you want to define an interface that allows publishing / consuming data to and from different types of channels (e.g. AMQP, JMS, etc), but you want to be able to switch out the implementation details ... Let's define a basic IO interface that can be re-used across multiple implementations...

Interfaces can be extremely helpful in many cases. For example, say you had a list of animals and you wanted to loop through the list, each printing the sound they make. {cat, dog, bird} One way to do this would be to use interfaces. This would allow for the same method to be called on all of th...

A method defined in an interface is by default public abstract. When an abstract class implements an interface, any methods which are defined in the interface do not have to be implemented by the abstract class. This is because a class that is declared abstract can contain abstract method declaratio...

Introduced in Java 8, default methods are a way of specifying an implementation inside an interface. This could be used to avoid the typical "Base" or "Abstract" class by providing a partial implementation of an interface, and restricting the subclasses hierarchy. Observer patte...

The Oracle Java Style Guide states: Modifiers should not be written out when they are implicit. (See Modifiers in Oracle Official Code Standard for the context and a link to the actual Oracle document.) This style guidance applies particularly to interfaces. Let's consider the following code ...

Bounded type parameters allow you to set restrictions on generic type arguments: class SomeClass { } class Demo<T extends SomeClass> { } But a type parameter can only bind to a single class type. An interface type can be bound to a type that already had a binding. This is achieve...