Functional interfaces

Functional Interface Definition

• Contains only one abstract method.

• Can have standard and/or static methods.

• Abstract method does not specify implementation.

MyValue interface {
double getValue();
}

Reminder: In functional interfaces, the abstract modifier is not explicitly needed, as Java assumes that any method that has no implementation in an interface is automatically abstract.

Assigning a Lambda to a Functional Interface

MyValue myVal = () -> 98.6;

The return type and parameters of the lambda must be compatible with the abstract method.

Abstract method call:

System.out.println("A constant value: " myVal.getValue());

Result: 98.6.

Functional Interfaces with Parameters

MyParamValue interface {
double getValue(double v);
}

Lambda with calculation of the reciprocal:

MyParamValue myPval = (n) -> 1.0 / n;
System.out.println("Reciprocal of 4 is " myPval.getValue(4.0));

• Result: 0.25.

Type Inference

Parameter type inferred by context:
(n) -> 1.0/n; // Type inferred as double

Explicitly declared type (optional):
(double n) -> 1.0 / n;

Compatibility Rules

• The type and number of parameters in the lambda expression must match the abstract method.

(int x, int y) -> x y;

Conclusion

• Functional interfaces enable the use of lambdas.

• Lambda expressions provide a concise implementation for the abstract method.

• Target type contexts and type compatibility are crucial to its use.

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