Abstract class(Abstract class) and interface are opposite side of same coin. You can declare interface by interface interface_name.

Following are the some important points about interface:

•  Interface contain only method signarute.
• As like abstract class,interface can not be instantiated.
• In interface by default member variables are public static final.
• Interface does not have constructor.
• One interface can extend multiple interfaces at once. As this is not applicable for classes.
• To implement a interface we use "implements" keyword.
• Java contain some empty interfaces like Serialzable, Cloanable...etx. They don't have any method signature.

 Now we will see the difference between abstract class and interface :

•  Interface contains only method signature however abstract class may have method body.
• Member variable inside interface are by default public static final where as you can declare different type of variable in abstract class .
• Multiple interfaces can be implemented at once where as multiple inheritance is not allowed in abstract classes.