Java what is an interface definition

Java Interfaces

Repeatedly, code that works in dev breaks down in production. Java performance issues are difficult to track down or predict.

Simply put, Digma provides immediate code feedback. As an IDE plugin, it identifies issues with your code as it is currently running in test and prod.

The feedback is available from the minute you are writing it.

Imagine being alerted to any regression or code smell as you’re running and debugging locally. Also, identifying weak spots that need attending to, based on integration testing results.

Of course, Digma is free for developers.

As always, the writeup is super practical and based on a simple application that can work with documents with a mix of encrypted and unencrypted fields.

We rely on other people’s code in our own work. Every day.

It might be the language you’re writing in, the framework you’re building on, or some esoteric piece of software that does one thing so well you never found the need to implement it yourself.

The problem is, of course, when things fall apart in production — debugging the implementation of a 3rd party library you have no intimate knowledge of is, to say the least, tricky.

Lightrun is a new kind of debugger.

It’s one geared specifically towards real-life production environments. Using Lightrun, you can drill down into running applications, including 3rd party dependencies, with real-time logs, snapshots, and metrics.

Learn more in this quick, 5-minute Lightrun tutorial:

Slow MySQL query performance is all too common. Of course it is. A good way to go is, naturally, a dedicated profiler that actually understands the ins and outs of MySQL.

The Jet Profiler was built for MySQL only, so it can do things like real-time query performance, focus on most used tables or most frequent queries, quickly identify performance issues and basically help you optimize your queries.

Critically, it has very minimal impact on your server’s performance, with most of the profiling work done separately — so it needs no server changes, agents or separate services.

Basically, you install the desktop application, connect to your MySQL server, hit the record button, and you’ll have results within minutes:

DbSchema is a super-flexible database designer, which can take you from designing the DB with your team all the way to safely deploying the schema.

The way it does all of that is by using a design model, a database-independent image of the schema, which can be shared in a team using GIT and compared or deployed on to any database.

And, of course, it can be heavily visual, allowing you to interact with the database using diagrams, visually compose queries, explore the data, generate random data, import data or build HTML5 database reports.

Get started with Spring 5 and Spring Boot 2, through the Learn Spring course:

> CHECK OUT THE COURSE

1. Overview

In this tutorial, we’re going to talk about interfaces in Java. We’ll also see how Java uses them to implement polymorphism and multiple inheritances.

2. What Are Interfaces in Java?

In Java, an interface is an abstract type that contains a collection of methods and constant variables. It is one of the core concepts in Java and is used to achieve abstraction, polymorphism and multiple inheritances.

Let’s see a simple example of an interface in Java:

public interface Electronic < // Constant variable String LED = "LED"; // Abstract method int getElectricityUse(); // Static method static boolean isEnergyEfficient(String electtronicType) < if (electtronicType.equals(LED)) < return true; >return false; > //Default method default void printDescription() < System.out.println("Electronic Description"); >> 

We can implement an interface in a Java class by using the implements keyword.

Next, let’s also create a Computer class that implements the Electronic interface we just created:

public class Computer implements Electronic < @Override public int getElectricityUse() < return 1000; >> 

2.1. Rules for Creating Interfaces

In an interface, we’re allowed to use:

We also should remember that:

• we can’t instantiate interfaces directly
• an interface can be empty, with no methods or variables in it
• we can’t use the final word in the interface definition, as it will result in a compiler error
• all interface declarations should have the public or default access modifier; the abstract modifier will be added automatically by the compiler
• an interface method can’t be protected or final
• up until Java 9, interface methods could not be private; however, Java 9 introduced the possibility to define private methods in interfaces
• interface variables are public, static, and final by definition; we’re not allowed to change their visibility

3. What Can We Achieve by Using Them?

3.1. Behavioral Functionality

We use interfaces to add certain behavioral functionality that can be used by unrelated classes. For instance, Comparable, Comparator, and Cloneable are Java interfaces that can be implemented by unrelated classes. Below is an example of the Comparator interface that is used to compare two instances of the Employee class:

public class Employee < private double salary; public double getSalary() < return salary; >public void setSalary(double salary) < this.salary = salary; >> public class EmployeeSalaryComparator implements Comparator  < @Override public int compare(Employee employeeA, Employee employeeB) < if (employeeA.getSalary() < employeeB.getSalary()) < return -1; >else if (employeeA.getSalary() > employeeB.getSalary()) < return 1; >else < return 0; >> > 

For more information, please visit our tutorial on Comparator and Comparable in Java.

3.2. Multiple Inheritances

Java classes support singular inheritance. However, by using interfaces, we’re also able to implement multiple inheritances.

For instance, in the example below, we notice that the Car class implements the Fly and Transform interfaces. By doing so, it inherits the methods fly and transform:

public interface Transform < void transform(); >public interface Fly < void fly(); >public class Car implements Fly, Transform < @Override public void fly() < System.out.println("I can Fly!!"); >@Override public void transform() < System.out.println("I can Transform!!"); >> 

3.3. Polymorphism

Let’s start with asking the question: what is polymorphism? It’s the ability for an object to take different forms during runtime. To be more specific it’s the execution of the override method that is related to a specific object type at runtime.

In Java, we can achieve polymorphism using interfaces. For example, the Shape interface can take different forms — it can be a Circle or a Square.

Let’s start by defining the Shape interface:

Now let’s also create the Circle class:

public class Circle implements Shape < @Override public String name() < return "Circle"; >> 

And also the Square class:

public class Square implements Shape < @Override public String name() < return "Square"; >> 

Finally, it’s time to see polymorphism in action using our Shape interface and its implementations. Let’s instantiate some Shape objects, add them to a List, and, finally, print their names in a loop:

List shapes = new ArrayList<>(); Shape circleShape = new Circle(); Shape squareShape = new Square(); shapes.add(circleShape); shapes.add(squareShape); for (Shape shape : shapes)

4. Default Methods in Interfaces

Traditional interfaces in Java 7 and below don’t offer backward compatibility.

What this means is that if you have legacy code written in Java 7 or earlier, and you decide to add an abstract method to an existing interface, then all the classes that implement that interface must override the new abstract method. Otherwise, the code will break.

Java 8 solved this problem by introducing the default method that is optional and can be implemented at the interface level.

5. Interface Inheritance Rules

In order to achieve multiple inheritances thru interfaces, we have to remember a few rules. Let’s go over these in detail.

5.1. Interface Extending Another Interface

When an interface extends another interface, it inherits all of that interface’s abstract methods. Let’s start by creating two interfaces, HasColor and Shape:

public interface HasColor < String getColor(); >public interface Box extends HasColor

In the example above, Box inherits from HasColor using the keyword extends. By doing so, the Box interface inherits getColor. As a result, the Box interface now has two methods: getColor and getHeight.

5.2. Abstract Class Implementing an Interface

When an abstract class implements an interface, it inherits all of its abstract and default methods. Let’s consider the Transform interface and the abstract class Vehicle that implements it:

public interface Transform < void transform(); default void printSpecs()< System.out.println("Transform Specification"); >> public abstract class Vehicle implements Transform <> 

In this example, the Vehicle class inherits two methods: the abstract transform method and the default printSpecs method.

6. Functional Interfaces

Java has had many functional interfaces since its early days, such as Comparable (since Java 1.2) and Runnable (since Java 1.0).

Java 8 introduced new functional interfaces such as Predicate, Consumer, and Function. To learn more about these, please visit our tutorial on Functional Interfaces in Java 8.

7. Conclusion

In this tutorial, we gave an overview of Java interfaces, and we talked about how to use them to achieve polymorphism and multiple inheritances.

As always, the complete code samples are available over on GitHub.

Slow MySQL query performance is all too common. Of course it is. A good way to go is, naturally, a dedicated profiler that actually understands the ins and outs of MySQL.

The Jet Profiler was built for MySQL only, so it can do things like real-time query performance, focus on most used tables or most frequent queries, quickly identify performance issues and basically help you optimize your queries.

Critically, it has very minimal impact on your server’s performance, with most of the profiling work done separately — so it needs no server changes, agents or separate services.

Basically, you install the desktop application, connect to your MySQL server, hit the record button, and you’ll have results within minutes:

Источник

What Is an Interface?

As you’ve already learned, objects define their interaction with the outside world through the methods that they expose. Methods form the object’s interface with the outside world; the buttons on the front of your television set, for example, are the interface between you and the electrical wiring on the other side of its plastic casing. You press the «power» button to turn the television on and off.

In its most common form, an interface is a group of related methods with empty bodies. A bicycle’s behavior, if specified as an interface, might appear as follows:

To implement this interface, the name of your class would change (to a particular brand of bicycle, for example, such as ACMEBicycle ), and you’d use the implements keyword in the class declaration:

class ACMEBicycle implements Bicycle < int cadence = 0; int speed = 0; int gear = 1; // The compiler will now require that methods // changeCadence, changeGear, speedUp, and applyBrakes // all be implemented. Compilation will fail if those // methods are missing from this class. void changeCadence(int newValue) < cadence = newValue; >void changeGear(int newValue) < gear = newValue; >void speedUp(int increment) < speed = speed + increment; >void applyBrakes(int decrement) < speed = speed - decrement; >void printStates() < System.out.println("cadence:" + cadence + " speed:" + speed + " gear:" + gear); >>

Implementing an interface allows a class to become more formal about the behavior it promises to provide. Interfaces form a contract between the class and the outside world, and this contract is enforced at build time by the compiler. If your class claims to implement an interface, all methods defined by that interface must appear in its source code before the class will successfully compile.

Note: To actually compile the ACMEBicycle class, you’ll need to add the public keyword to the beginning of the implemented interface methods. You’ll learn the reasons for this later in the lessons on Classes and Objects and Interfaces and Inheritance.

Источник