What is interface class in android

Java — Interfaces

An interface is a reference type in Java. It is similar to class. It is a collection of abstract methods. A class implements an interface, thereby inheriting the abstract methods of the interface.

Along with abstract methods, an interface may also contain constants, default methods, static methods, and nested types. Method bodies exist only for default methods and static methods.

Writing an interface is similar to writing a class. But a class describes the attributes and behaviors of an object. And an interface contains behaviors that a class implements.

Unless the class that implements the interface is abstract, all the methods of the interface need to be defined in the class.

An interface is similar to a class in the following ways −

An interface can contain any number of methods.

An interface is written in a file with a .java extension, with the name of the interface matching the name of the file.

The byte code of an interface appears in a .class file.

Interfaces appear in packages, and their corresponding bytecode file must be in a directory structure that matches the package name.

However, an interface is different from a class in several ways, including −

You cannot instantiate an interface.

An interface does not contain any constructors.

All of the methods in an interface are abstract.

An interface cannot contain instance fields. The only fields that can appear in an interface must be declared both static and final.

An interface is not extended by a class; it is implemented by a class.

An interface can extend multiple interfaces.

Declaring Interfaces

The interface keyword is used to declare an interface. Here is a simple example to declare an interface −

Example

Following is an example of an interface −

Interfaces have the following properties −

An interface is implicitly abstract. You do not need to use the abstract keyword while declaring an interface.

Each method in an interface is also implicitly abstract, so the abstract keyword is not needed.

Methods in an interface are implicitly public.

Example

Implementing Interfaces

When a class implements an interface, you can think of the class as signing a contract, agreeing to perform the specific behaviors of the interface. If a class does not perform all the behaviors of the interface, the class must declare itself as abstract.

A class uses the implements keyword to implement an interface. The implements keyword appears in the class declaration following the extends portion of the declaration.

Example

This will produce the following result −

Output

When overriding methods defined in interfaces, there are several rules to be followed −

Checked exceptions should not be declared on implementation methods other than the ones declared by the interface method or subclasses of those declared by the interface method.

The signature of the interface method and the same return type or subtype should be maintained when overriding the methods.

An implementation class itself can be abstract and if so, interface methods need not be implemented.

When implementation interfaces, there are several rules −

A class can implement more than one interface at a time.

A class can extend only one class, but implement many interfaces.

An interface can extend another interface, in a similar way as a class can extend another class.

Extending Interfaces

An interface can extend another interface in the same way that a class can extend another class. The extends keyword is used to extend an interface, and the child interface inherits the methods of the parent interface.

The following Sports interface is extended by Hockey and Football interfaces.

Example

The Hockey interface has four methods, but it inherits two from Sports; thus, a class that implements Hockey needs to implement all six methods. Similarly, a class that implements Football needs to define the three methods from Football and the two methods from Sports.

Extending Multiple Interfaces

A Java class can only extend one parent class. Multiple inheritance is not allowed. Interfaces are not classes, however, and an interface can extend more than one parent interface.

The extends keyword is used once, and the parent interfaces are declared in a comma-separated list.

For example, if the Hockey interface extended both Sports and Event, it would be declared as −

Example

Tagging Interfaces

The most common use of extending interfaces occurs when the parent interface does not contain any methods. For example, the MouseListener interface in the java.awt.event package extended java.util.EventListener, which is defined as −

Example

An interface with no methods in it is referred to as a tagging interface. There are two basic design purposes of tagging interfaces −

Creates a common parent − As with the EventListener interface, which is extended by dozens of other interfaces in the Java API, you can use a tagging interface to create a common parent among a group of interfaces. For example, when an interface extends EventListener, the JVM knows that this particular interface is going to be used in an event delegation scenario.

Adds a data type to a class − This situation is where the term, tagging comes from. A class that implements a tagging interface does not need to define any methods (since the interface does not have any), but the class becomes an interface type through polymorphism.

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How to Create Interfaces in Android Studio?

Interfaces are a collection of constants, methods(abstract, static, and default), and nested types. All the methods of the interface need to be defined in the class. The interface is like a Class. The interface keyword is used to declare an interface.

public interface AdapterCallBackListener <

void onRowClick(String searchText);

public interface OnFragmentInteractionListener <

So basically in android , there are two types of interfaces we can create and we use frequently .

• Creating Java Interface
• Creating Kotlin Interface

So in this article, we are going to create both Java and Kotlin Interface in Android studio.

Creating Java Interface in Android Studio

Like a class, a Interface can have methods and variables, but the methods declared in an interface are by default abstract (only method signature, no body).

• Interfaces specify what a class must do and not how. It is the blueprint of the class.
• An Interface is about capabilities like a Player may be an interface and any class implementing Player must be able to (or must implement) move(). So it specifies a set of methods that the class has to implement.
• If a class implements an interface and does not provide method bodies for all functions specified in the interface, then the class must be declared abstract.
• A Java library example is Comparator Interface. If a class implements this interface, then it can be used to sort a collection.

// declare constant fields

// declare methods that abstract

To declare an interface, use the interface keyword. It is used to provide total abstraction. That means all the methods in an interface are declared with an empty body and are public and all fields are public, static , and final by default. A class that implements an interface must implement all the methods declared in the interface. To implement interface use implements keyword.

Step by Step Implementation

Step 1: Go to Android Studio and open the project in Android mode as shown in the below image.

Step 2: Now go to the app > java > your package name > right-click > New > Java Class as shown in the below image.

Step 3: After completing step 2 a pop-up screen will arise like below. Here enter your interface name and choose the Interface and click the Enter button.

After completing the above steps successfully you can find your Java interface here. Go to the app > java > your package name > GeeksforGeeks.java. And you can write your own Java code here.

Creating Kotlin Interface in Android Studio

Interfaces are custom types provided by Kotlin that cannot be instantiated directly. Instead, these define a form of behavior that the implementing types have to follow. With the interface, you can define a set of properties and methods, that the concrete types must follow and implement. The interface definition in Kotlin begins with the interface keyword followed by the name of the interface, followed by the curly braces within which the members of the interface reside. The difference is that the members will have no definition of their own. These definitions will be provided by the conforming types.

Step by Step Implementation

Step 1: Go to Android Studio and open the project in Android mode as shown in the below image.

Step 2: Now go to the app > java > your package name > right-click > New > Kotlin File/Class as shown in the below image.

Step 3: After completing step 2 a pop-up screen will arise like below. Here enter your class name and choose the Interface and click the Enter button.

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Интерфейсы в Java

Интерфейсы

Интерфейсы Java созданы для поддержки динамического выбора (resolution) методов во время выполнения программы. Интерфейсы похожи на классы, но в отличие от последних у интерфейсов нет переменных представителей, а в объявлениях методов отсутствует реализация. Класс может иметь любое количество интерфейсов. Все, что нужно сделать — это реализовать в классе полный набор методов всех интерфейсов. Сигнатуры таких методов класса должны точно совпадать с сигнатурами методов реализуемого в этом классе интерфейса. Интерфейсы обладают своей собственной иерархией, не пересекающейся с классовой иерархией наследования. Это дает возможность реализовать один и тот же интерфейс в различных классах, никак не связанных по линии иерархии классового наследования. Именно в этом и проявляется главная сила интерфейсов.

Интерфейс — это структура данных, которая может содержать поля, представленные в виде именованных констант (ключевое слово final тут обычно не указывается, так как поля — это всегда именованные константы) и объявления методов. Интерфейсом могут расширяться многие классы. Интерфейс может сам расширяться несколькими интерфейсами. Заметим, что интерфейс могут использовать сразу несколько классов, независимых друг от друга.

У интерфейса могут быть следующие модификаторы.
• public (если он есть, то интерфейс доступен отовсюду, если его нет — доступен только в данном пакете).
• abstract (так как интерфейс всегда абстрактный, то модификатор обычно опускается).
• strictfp— все позже реализуемые методы должны будут работать с числами с плавающей точкой аналогично на всех машинах Java.
Приведем пример простого интерфейса, а также класса, который его наследует (листинг 4.13).

Листинг 4.13.
Пример простого интерфейса и класса, который его наследует

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Let’s practice interface and abstract class in android (java)

Halima Akhter Mitu

Sep 9, 2019 · 3 min read

Polymorphism is one of the important features of object-oriented programming. It means taking different roles in different environments. For example, a human can play different character s/he can be a child, father/mother, employee, etc. Abstract and interface come from this concept.

Abstract class and interface are the most used attributes in android. When we create an activity it indirectly extends an abstract class called Context and when we implement the interface View.OnClickListener it forces us to override the onClick function. These are the example of abstract class and interface which are innate attributes in android.

So, w h at are we gonna learn in this blog?

We gonna learn what is an abstract class and interface, why interface or abstract class need to be used, how to create our own interface and abstract and last but not the least how to use them.

A class which has “abstract” declaration after access modifier (public, protected, private) is called an abstract class. An abstract class can have any kind of method with the method body, it also can have methods with the only function declaration, but those methods must be declared as an abstract method. When we extend an abstract class all the abstract methods must be overridden.

public abstract class MyAbstractClass <

public void myFunctionOne()<

//Abstract methods don’t have any body

protected abstract void myFunctionWhichisAbstract(String s);

public class UseAbstractClass extends MyAbstractClass <

protected void myFunctionWhichisAbstract(String s) <

// use this in any override function of the activity

UseAbstractClass useAbstractClass = new UseAbstractClass();

The interface is a feature in java which declared as interface and by default it is public. Methods in an interface are declared by definition they don’t have a body. We can implement multiple interfaces. An interface also can implement single or multiple interfaces.

For example here is an interface called TaskComplete

public interface TaskComplete <

void doTask(String asyncClassName, int defaultValue);

We can use this interface in different ways. I am showing three of them.

* define this in outside of any overridden method

TaskComplete taskComplete = new TaskComplete() <

public void doTask(String asyncClassName, int defaultValue) <

//do your functionality

Then call this from any override function in your activity

//In my case, I called it in the onCreate override function of MainActivity.java

Sometimes we need to update UI after network call, this kind of functionality can be a great help on this.

In MainActivity.java class implement the TaskComplete interface then we bound to use doTask method.

public class MainActivity extends AppCompatActivity implements TaskComplete <

public void doTask(String asyncClassName, int defaultValue) <

// do your functionality

Make a class named Event

public class Event <

private TaskComplete mTaskComplete;

public void setOnTaskCompleteListener(TaskComplete listener) <

Then use this class’s object in MainActivity class

// in the onCreate function of MainActivity.java file

new Event().setOnEventListener(new OnEventListener() <

public void onEvent(String er) <

//do functionality here

I hope this information will be helpful for some of you.

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