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Traits are a structural construct of the language which allow −
- Composition of behaviors.
- Runtime implementation of interfaces.
- Compatibility with static type checking/compilation
They can be seen as interfaces carrying both default implementations and state. A trait is defined using the trait keyword.
An example of a trait is given below −
trait Marks {
void DisplayMarks() {
println("Display Marks");
}
}
One can then use the implement keyword to implement the trait in the similar way as interfaces.
class Example {
static void main(String[] args) {
Student st = new Student();
st.StudentID = 1;
st.Marks1 = 10;
println(st.DisplayMarks());
}
}
trait Marks {
void DisplayMarks() {
println("Display Marks");
}
}
class Student implements Marks {
int StudentID
int Marks1;
}
Implementing Interfaces
Traits may implement interfaces, in which case the interfaces are declared using the implements keyword.
An example of a trait implementing an interface is given below. In the following example the following key points can be noted.
-
An interface Total is defined with the method DisplayTotal.
-
The trait Marks implements the Total interface and hence needs to provide an implementation for the DisplayTotal method.
class Example {
static void main(String[] args) {
Student st = new Student();
st.StudentID = 1;
st.Marks1 = 10;
println(st.DisplayMarks());
println(st.DisplayTotal());
}
}
interface Total {
void DisplayTotal()
}
trait Marks implements Total {
void DisplayMarks() {
println("Display Marks");
}
void DisplayTotal() {
println("Display Total");
}
}
class Student implements Marks {
int StudentID
int Marks1;
}
The output of the above program would be −
Display Marks Display Total
Properties
A trait may define properties. An example of a trait with a property is given below.
In the following example, the Marks1 of type integer is a property.
class Example {
static void main(String[] args) {
Student st = new Student();
st.StudentID = 1;
println(st.DisplayMarks());
println(st.DisplayTotal());
}
interface Total {
void DisplayTotal()
}
trait Marks implements Total {
int Marks1;
void DisplayMarks() {
this.Marks1 = 10;
println(this.Marks1);
}
void DisplayTotal() {
println("Display Total");
}
}
class Student implements Marks {
int StudentID
}
}
The output of the above program would be −
10 Display Total
Composition of Behaviors
Traits can be used to implement multiple inheritance in a controlled way, avoiding the diamond issue. In the following code example, we have defined two traits – Marks and Total. Our Student class implements both traits. Since the student class extends both traits, it is able to access the both of the methods – DisplayMarks and DisplayTotal.
class Example {
static void main(String[] args) {
Student st = new Student();
st.StudentID = 1;
println(st.DisplayMarks());
println(st.DisplayTotal());
}
}
trait Marks {
void DisplayMarks() {
println("Marks1");
}
}
trait Total {
void DisplayTotal() {
println("Total");
}
}
class Student implements Marks,Total {
int StudentID
}
The output of the above program would be −
Total Marks1
Extending Traits
Traits may extend another trait, in which case you must use the extends keyword. In the following code example, we are extending the Total trait with the Marks trait.
class Example {
static void main(String[] args) {
Student st = new Student();
st.StudentID = 1;
println(st.DisplayMarks());
}
}
trait Marks {
void DisplayMarks() {
println("Marks1");
}
}
trait Total extends Marks {
void DisplayMarks() {
println("Total");
}
}
class Student implements Total {
int StudentID
}
The output of the above program would be −
Total
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