Multiple interfaces
- Java course
- Intermediate
- Lesson 51/116
- 5 min
Learn to implement multiple interfaces.
Implementing multiple interfaces
A class can implement more than one interface. Interface names are separated by commas. The class must write all methods of all interfaces.
Example: class Document implements Printable, Savable { ... }. Here Document implements both the Printable and Savable contracts. It must write the methods of both interfaces. Java lets classes inherit from a single class; but they can implement multiple interfaces. This meets the need for multiple inheritance safely. As long as there is no conflict, a class can implement as many interfaces as it wants. It is key to flexible design.
The advantage of multiple interfaces
Implementing multiple interfaces lets a class take on different roles. The same object can be used as different contracts in different contexts. This increases code reuse.
Example: if Document is both Printable and Savable, it can be passed to a printAll(Printable[] items) method and also to a saveAll(Savable[] items) method. One class plays a role in two different workflows. This keeps the class hierarchy flexible. Each role is represented by an interface; the class combines roles. This approach helps large systems stay clean. New roles can be added as needed.
Practice Implement two interfaces on the Document class
Define an interface Printable; add the void print() signature. Define a second interface Savable; add the void save() signature. Then write class Document implements Printable, Savable; write both methods as public void with bodies. The print body runs System.out.println("Printing");, the save body runs System.out.println("Saving");. In main create Document d = new Document(); then call d.print() and d.save().
Starter code
// Define the Printable and Savable interfaces
// Write Document implements Printable, Savable
// Write print and save methods with bodies
// Call d.print() and d.save() in mainSample solution
interface Printable {
void print();
}
interface Savable {
void save();
}
class Document implements Printable, Savable {
public void print() {
System.out.println("Printing");
}
public void save() {
System.out.println("Saving");
}
}
class Main {
public static void main(String[] args) {
Document d = new Document();
d.print();
d.save();
}
}Expected output
Printing Saving
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 How are multiple interface names separated?
- With a comma Correct answer
- With a dot
- With a plus
- With and
Explanation: They are separated with a comma.
2 Match the multiple interface roles
| Term | Meaning |
|---|---|
Printable |
Printing role |
Savable |
Saving role |
Document |
Class combining roles |
implements |
Implementation keyword |
3 Complete the multiple implements line
class Document implements Printable, ____ {
public void print() { }
public void save() { }
}class Document implements Printable, Savable {
public void print() { }
public void save() { }
}Explanation: The second interface is added with a comma.