Java
default methods
Learn default methods inside interfaces.
default methods
A default method is a method with a body inside an interface. The implementing class may override it; if not, the interface body is used. It starts with the default keyword.
Example: interface Logger { default void info(String msg) { System.out.println(msg); } }. Here info has a body. Even if the implementing class does not write info, this body runs. This is used to add methods to existing interfaces without breaking old classes. It is the safe way to add behavior to a contract. The class may still override the method if it wants.
Adding methods without breaking the interface
Default methods let you add behavior to existing interfaces. You can add new methods without changing implementing classes. This is critical for backward compatibility.
Example: if you add default void warn(String msg) to Logger, old classes work even without writing the method. New classes may override it if they want. Without default methods, adding a method to an interface breaks all implementing classes. This feature, added in Java 8, gives great flexibility to library authors. It is the official way to grow an interface without breaking old code.
Exercise Add a default method to the Logger interface
Define an interface Logger. Add a default void info(String msg) method; its body runs System.out.println("Info: " + msg);. Then write class ConsoleLogger implements Logger; leave the body empty without writing info. In main call new ConsoleLogger().info("Logging started").
Show example solution
interface Logger {
default void info(String msg) {
System.out.println("Info: " + msg);
}
}
class ConsoleLogger implements Logger {
}
class Main {
public static void main(String[] args) {
new ConsoleLogger().info("Logging started");
}
}
Test yourself
-
What happens if a default method is not written in the implementing class?
- It gives a compile error
- The method is deleted
- The interface body runs
- It gives a runtime error
Show answer
The interface body runs — The interface body is used.
-
Add the default keyword
interface Logger {
____ void info(String msg) {
System.out.println(msg);
}
}
Show answer
interface Logger {
default void info(String msg) {
System.out.println(msg);
}
}
-
Match the default method concepts
- default
- Bodyful method keyword
- info(String msg)
- Method signature
- override
- Rewrite
- backward compatible
- Backward compatible
Next lesson: static methods
