Method reference basics
java introduced method reference in java8 version. Method reference allows the developer to call a method or constructor by their names. Method references are one more short way of writing lambda expression. Method references use the symbol "::" to separate class/object/constructor from method name Sometimes we used to get the compile-time warning like "The target type of this expression must be a functional interface" so this error is because the need to assign method reference or lambda expression to functional interface
method reference syntax
Classname:: methodname
Instancename:: methodname
Constructorname::new
- Static methods reference
- Instance method reference
- Constructor reference
Static method reference example
Static methods are global to class, They can directly call using Classname.staticmethod() with java8, You can call using Classname::static-methodpublic class MethodReferenceExample {
public static void main(String[] args) {
// Using lambda expression
BooleanTest lambdaExample = (n) -> PositiveNumberCheck.isPositive(n);
System.out.println(lambdaExample.test(2));
System.out.println(lambdaExample.test(-2));
// Using method reference
BooleanTest methodRefExample = PositiveNumberCheck::isPositive;
System.out.println(methodRefExample.test(2));
System.out.println(methodRefExample.test(-2));
}
}
@FunctionalInterface
interface BooleanTest {
boolean test(int n);
}
class PositiveNumberCheck {
public static boolean isPositive(int n) {
return n >= 0;
}
}
true
false
true
false
Instance method reference example
Instance methods work on instances of a class. Prior to java 8, we used to call instances method using new class(). method name with java8, You can call object:: instance-methodimport java.util.function.Function;
public class MethodReferenceExample {
public Integer IncrementByOne(Integer value) {
return value + 1;
}
public static void main(String[] args) {
Function<Integer, Integer> function = new MethodReferenceExample()::IncrementByOne;
System.out.println(function.apply(5));
}
}
6
Construction reference example
Construction reference can be called using the new operator. Reference to construction is executing via functional interface Here is an example for construction referenceimport java.util.function.Consumer;
public class MethodReferenceExample {
public static void main(String[] args) {
Consumer<String> function = Emp::new;
function.accept("Hello");
}
}
class Emp {
Emp(String msg) {
System.out.print(msg);
}
}
Hello
