:: (double colon) operator in Java 8

2021-6-3 anglehua

I was exploring the Java 8 source and found this particular part of code very surprising:

//defined in IntPipeline.java
public final OptionalInt reduce(IntBinaryOperator op) {
    return evaluate(ReduceOps.makeInt(op));

public final OptionalInt max() {
    return reduce(Math::max); //this is the gotcha line

//defined in Math.java
public static int max(int a, int b) {
    return (a >= b) ? a : b;

Is Math::max something like a method pointer? How does a normal static method gets converted to IntBinaryOperator?

Usually, one would call the reduce method using Math.max(int, int) as follows:

reduce(new IntBinaryOperator() {
    int applyAsInt(int left, int right) {
        return Math.max(left, right);

That requires a lot of syntax for just calling Math.max. That's where lambda expressions come into play. Since Java 8 it is allowed to do the same thing in a much shorter way:

reduce((int left, int right) -> Math.max(left, right));

How does this work? The java compiler "detects", that you want to implement a method that accepts two ints and returns one int. This is equivalent to the formal parameters of the one and only method of interface IntBinaryOperator (the parameter of method reduce you want to call). So the compiler does the rest for you - it just assumes you want to implement IntBinaryOperator.

But as Math.max(int, int) itself fulfills the formal requirements of IntBinaryOperator, it can be used directly. Because Java 7 does not have any syntax that allows a method itself to be passed as an argument (you can only pass method results, but never method references), the :: syntax was introduced in Java 8 to reference methods:


Note that this will be interpreted by the compiler, not by the JVM at runtime! Although it produces different bytecodes for all three code snippets, they are semantically equal, so the last two can be considered to be short (and probably more efficient) versions of the IntBinaryOperator implementation above!

(See also Translation of Lambda Expressions)

:: is called Method Reference. It is basically a reference to a single method. I.e. it refers to an existing method by name.

Short Explanation:
Below is an example of a reference to a static method:

class Hey {
     public static double square(double num){
        return Math.pow(num, 2);

Function<Double, Double> square = Hey::square;
double ans = square.apply(23d);

square can be passed around just like object references and triggered when needed. In fact, it can be just as easily used as a reference to "normal" methods of objects as static ones. For example:

class Hey {
    public double square(double num) {
        return Math.pow(num, 2);

Hey hey = new Hey();
Function<Double, Double> square = hey::square;
double ans = square.apply(23d);

Function above is a functional interface. To fully understand ::, it is important to understand functional interfaces as well. Plainly, a functional interface is an interface with just one abstract method.

Examples of functional interfaces include Runnable, Callable, and ActionListener.

Function above is a functional interface with just one method: apply. It takes one argument and produces a result.

The reason why ::s are awesome is that:

Method references are expressions which have the same treatment as lambda expressions (...), but instead of providing a method body, they refer an existing method by name.

E.g. instead of writing the lambda body

Function<Double, Double> square = (Double x) -> x * x;

You can simply do

Function<Double, Double> square = Hey::square;

At runtime, these two square methods behave exactly the same as each other. The bytecode may or may not be the same (though, for the above case, the same bytecode is generated; compile the above and check with javap -c).

The only major criterion to satisfy is: the method you provide should have a similar signature to the method of the functional interface you use as object reference.

The below is illegal:

Supplier<Boolean> p = Hey::square; // illegal

square expects an argument and returns a double. The get method in Supplier returns a value but does not take an argument. Thus, this results in an error.

A method reference refers to the method of a functional interface. (As mentioned, functional interfaces can have only one method each).

Some more examples: the accept method in Consumer takes an input but doesn't return anything.

Consumer<Integer> b1 = System::exit;   // void exit(int status)
Consumer<String[]> b2 = Arrays::sort;  // void sort(Object[] a)
Consumer<String> b3 = MyProgram::main; // void main(String... args)

class Hey {
    public double getRandom() {
        return Math.random();

Callable<Double> call = hey::getRandom;
Supplier<Double> call2 = hey::getRandom;
DoubleSupplier sup = hey::getRandom;
// Supplier is functional interface that takes no argument and gives a result

Above, getRandom takes no argument and returns a double. So any functional interface that satisfies the criteria of: take no argument and return double can be used.

Another example:

Set<String> set = new HashSet<>();
Predicate<String> pred = set::contains;
boolean exists = pred.test("leo");

In case of parameterized types:

class Param<T> {
    T elem;
    public T get() {
        return elem;

    public void set(T elem) {
        this.elem = elem;

    public static <E> E returnSame(E elem) {
        return elem;

Supplier<Param<Integer>> obj = Param<Integer>::new;
Param<Integer> param = obj.get();
Consumer<Integer> c = param::set;
Supplier<Integer> s = param::get;

Function<String, String> func = Param::<String>returnSame;

Method references can have different styles, but fundamentally they all mean the same thing and can simply be visualized as lambdas:

  1. A static method (ClassName::methName)
  2. An instance method of a particular object (instanceRef::methName)
  3. A super method of a particular object (super::methName)
  4. An instance method of an arbitrary object of a particular type (ClassName::methName)
  5. A class constructor reference (ClassName::new)
  6. An array constructor reference (TypeName[]::new)

For further reference, see http://cr.openjdk.java.net/~briangoetz/lambda/lambda-state-final.html.


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