In Java 8, we can refer a method from class or object using class::methodName type syntax. Let’s learn about different types of available method references in java 8.
Table of Contents 1. Types of Method References 2. Reference to static method - Class::staticMethodName 3. Reference to instance method from instance - ClassInstance::instanceMethodName 4. Reference to instance method from class type - Class::instanceMethodName 5. Reference to constructor - Class::new
1. Types of method references
Java 8 allows four types of method references.
| Method Reference | Description | Method reference example |
|---|---|---|
| Reference to static method | Used to refer static methods from a class | Math::max equivalent to Math.max(x,y) |
| Reference to instance method from instance | Refer to an instance method using a reference to the supplied object | System.out::println equivalent to System.out.println(x) |
| Reference to instance method from class type | Invoke the instance method on a reference to an object supplied by the context | String::length equivalent to str.length() |
| Reference to constructor | Reference to a constructor | ArrayList::new equivalent to new ArrayList() |
2. Method reference to static method – Class::staticMethodName
An example to use Math.max() which is static method.
List<Integer> integers = Arrays.asList(1,12,433,5); Optional<Integer> max = integers.stream().reduce( Math::max ); max.ifPresent(value -> System.out.println(value));
Output:
433
3. Method reference to instance method from instance – ClassInstance::instanceMethodName
In above example, we use System.out.println(value) to print the max value found. We can use System.out::println to print the value.
List<Integer> integers = Arrays.asList(1,12,433,5); Optional<Integer> max = integers.stream().reduce( Math::max ); max.ifPresent( System.out::println );
Output:
433
4. Method reference to instance method from class type – Class::instanceMethodName
In this example, s1.compareTo(s2) is referred as String::compareTo.
List<String> strings = Arrays
.asList("how", "to", "do", "in", "java", "dot", "com");
List<String> sorted = strings
.stream()
.sorted((s1, s2) -> s1.compareTo(s2))
.collect(Collectors.toList());
System.out.println(sorted);
List<String> sortedAlt = strings
.stream()
.sorted(String::compareTo)
.collect(Collectors.toList());
System.out.println(sortedAlt);
Output:
[com, do, dot, how, in, java, to] [com, do, dot, how, in, java, to]
5. Reference to constructor – Class::new
The first method can be updated to create a list of integers from 1 to 100. Using lambda expression is rather easy. To create a new instance of ArrayList, we have use ArrayList::new.
List<Integer> integers = IntStream .range(1, 100) .boxed() .collect(Collectors.toCollection( ArrayList::new )); Optional<Integer> max = integers.stream().reduce(Math::max); max.ifPresent(System.out::println);
Output:
99
That’s 4 type of method references in java 8 lambda enhancements.
Happy Learning !!
ArrayList::new equivalent to new ArrayList() this is not correct this applicable only if “The target type of this expression must be a functional interface”
I meant not always correct there is a condition.
Consumer con = System.out::println;
con.accept(“Hello”);
How do you come to know about the parameters that are taken by these static methods.
For Example:
If I want to write System.out.println(“Hello”);
How do i write it using method reference ?
Consumer con = System.out::println;
con.accept(“Hello”);
System.out::println