There can be only abstract methods in an interface, that is there is no method body inside these abstract methods. The class that implements the interface should be abstract, otherwise, we need to define all the methods of the interface in the class.<\/p>\n
Before moving ahead in this article it is recommended for you to take a quick revision on Classes in Java<\/a> with Techvidvan.<\/strong><\/em><\/p>\n Syntax of declaring Interfaces in Java:<\/strong><\/p>\n To declare an interface, the interface keyword is used. Here is a syntax to declare an interface:<\/p>\n Example:<\/strong><\/p>\n Following is an example of an interface:<\/p>\n Example:<\/strong><\/p>\n \/\/Filename : Animal.java<\/p>\n An Interface has the following properties \u2212<\/p>\n Hold On! <\/strong><\/em>It’s the right time to take a deep dive into the concept of Inheritance in Java with some real-life examples.<\/a><\/strong><\/em><\/p>\n Note:<\/strong> The Java compiler automatically adds the public and abstract keywords before the methods of an interface. It also adds public, static and final keywords before the data members. It is illustrated in the image below:<\/em><\/p>\n A class implementing an interface can be thought of as the class assigning a contract. This means that the class agrees to perform the specific behaviors of the interface. Unless a class is declared as abstract, it should perform all the behaviors of the interface.<\/p>\n In order to implement an interface, a class uses the implements keyword. The implements keyword appears in the class declaration after the extends portion of the declaration.<\/p>\n Code to understand Interfaces in Java:<\/strong><\/p>\n Output:<\/strong><\/p>\n As a class can extend another class, similarly an interface can extend another interface. The extends keyword is used to extend or inherit an interface. The derived interface inherits the methods of the parent interface.<\/p>\n The following Person interface is extended by Student and Teacher interfaces.<\/p>\n The Teacher interface has four methods, but it inherits two from the Person interface; thus, a class that implements the Teacher interface needs to implement all the six methods. Similarly, a class that implements the Student interface needs to define the three methods from Student and the two methods from Person.<\/p>\n We know that a Java class can only extend one parent class, as Multiple inheritances is not possible with the classes. Interfaces are similar to Java classes but an interface can extend more than one parent interface.<\/p>\n The multiple parent interfaces are declared in a comma-separated list after using the keyword extends.<\/strong><\/p>\n For example, if the Dog interface extended both Animal and Pet, it would be declared as:<\/strong><\/p>\nImportant points about Interface in Java<\/h3>\n
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Differences between Interface and Class in Java<\/h3>\n
![\"Difference](\"https:\/\/techvidvan.com\/tutorials\/wp-content\/uploads\/sites\/2\/2020\/02\/difference-between-class-and-interface-in-java.jpg\")
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interface interface-name\n{\n \/\/abstract methods\n}<\/pre>\n\/\/Filename: NameOfInterface.java\n\n import java.lang.*;\n \/\/ Any number of import statements\n\n interface NameOfInterface\n {\n \/\/ Any number of final, static fields\n \/\/ Any number of abstract method declarations\n }<\/pre>\ninterface Animal\n{\n public void eat();\n public void travel();\n}<\/pre>\nProperties of a Java Interface<\/h3>\n
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Advantages of an Interface in Java<\/h3>\n
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![\"Conversion](\"https:\/\/techvidvan.com\/tutorials\/wp-content\/uploads\/sites\/2\/2020\/02\/Automatic-Conversion-of-Java-Interface-Methods-by-Compiler.jpg\")
<\/a><\/p>\nImplementing Interfaces in Java<\/h3>\n
package com.techvidvan.interfaces;\ninterface Polygon\n{\n \/\/declaring variables of the interface\n public static final int side = 5,length = 4,breadth = 8;\n \/\/declaring interface methods(without a method body)\n public void getName();\n public void getNumberOfSides();\n public void getArea();\n public void getPerimeter();\n}\n\n\/\/ Rectangle class \"implements\" the Polygon interface\nclass Rectangle implements Polygon\n{\n public void getName()\n {\n \/\/ The body of getName() is provided here\n System.out.println(\"The name of the Polygon is: Rectangle\");\n }\n public void getNumberOfSides()\n {\n \/\/ The body of getNumberOfSides() is provided here\n System.out.println(\"There are 4 sides in a Rectangle\");\n }\n public void getArea()\n {\n \/\/ The body of getArea() is provided here\n System.out.println(\"The Area of Rectangle is: \" +length*breadth);\n }\n public void getPerimeter()\n {\n \/\/ The body of getPerimeter() is provided here\n System.out.println(\"The Perimeter of Rectangle is: \" +2*(length + breadth));\n }\n}\n\n\/\/Square class \"implements\" the Polygon interface\nclass Square implements Polygon\n{\n public void getName()\n {\n \/\/ The body of getName() is provided here\n System.out.println(\"\\nThe name of the Polygon is: Square\");\n }\n public void getNumberOfSides()\n {\n \/\/ The body of getNumberOfSides() is provided here\n System.out.println(\"There are 4 sides in a Rectangle\");\n }\n public void getArea()\n {\n \/\/ The body of getArea() is provided here\n System.out.println(\"The Area of Square is: \" +side * side);\n }\n public void getPerimeter()\n {\n \/\/ The body of getPerimeter() is provided here\n System.out.println(\"The Perimeter of Square is: \" +4*side);\n }\n}\n\nclass InterfaceDemo\n{\n public static void main(String[] args)\n {\n Rectangle rectangle = new Rectangle(); \/\/ Create a Rectangle object\n Square square=new Square(); \/\/ Create a Square object\n\n \/\/calling methods of class Rectangle\n rectangle.getName();\n rectangle.getNumberOfSides();\n rectangle.getArea();\n rectangle.getPerimeter();\n\n \/\/ calling methods of class Square\n square.getName();\n square.getNumberOfSides();\n square.getArea();\n square.getPerimeter();\n }\n}<\/pre>\n
\nThere are 4 sides in a Rectangle
\nThe Area of Rectangle is: 32
\nThe Perimeter of Rectangle is: 24
\nThe name of the Polygon is: Square
\nThere are 4 sides in a Rectangle
\nThe Area of Square is: 25
\nThe Perimeter of Square is: 20<\/div>\nExtending Interfaces<\/h3>\n
\/\/ Filename: Person.java\npublic interface Person\n{\n public void setName(String name);\n public void setAge(int age);\n}\n\n\/\/ Filename: Student.java\npublic interface Student extends Person\n{\n public void marks(int marks);\n public void getEnrollmentNumber(int roll);\n public void yearOfPassing(int year);\n}\n\n\/\/ Filename: Teacher.java\npublic interface Teacher extends Person\n{\n public void teacherId(int id);\n public void salary(int salary);\n public void assignedClasses(int number);\n public void subject(String subject);\n}<\/pre>\nExtending Multiple Interfaces<\/h3>\n
public interface Dog extends Pet, Animal<\/pre>\n
Relationship Between Classes and Interface<\/h3>\n
![\"\"](\"https:\/\/techvidvan.com\/tutorials\/wp-content\/uploads\/sites\/2\/2020\/02\/Relationship-between-Class-and-Interface-in-Java.jpg\")
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