What does :: mean in java

The double colon (::) symbol in Java represents the following concepts: Static method reference Class method reference Constructor reference Type parameter inference

Double colon (::) in Java

The double colon (::) symbol represents a variety of concepts in the Java language, depending on the context in which it is used.

1. Static method reference

Double colon can be used to obtain a reference to a static method. For example:

<code class="java">List<String> names = List.of("Alice", "Bob", "Carol");
names.forEach(System.out::println);</code>

The above code uses a double colon to reference the System.out.println() static method.

2. Class method reference

Double colon can also be used to obtain a reference to a class method. For example:

<code class="java">class Person {
    public void introduceYourself() {
        System.out.println("Hello, my name is " + name);
    }
}

List<Person> people = List.of(new Person(), new Person());
people.forEach(Person::introduceYourself);</code>

The above code uses a double colon to reference the Person.introduceYourself() class method.

3. Constructor reference

Double colon can also be used to obtain a reference to the constructor. For example:

<code class="java">List<String> names = List.of("Alice", "Bob", "Carol");
Map<String, Integer> nameLengths = names.stream()
    .collect(Collectors.toMap(String::length));</code>

The above code uses a double colon to reference the String() constructor.

4. Type parameter inference

In Java 8 and higher, double colons can be used for type parameter inference. For example:

<code class="java">List<String> names = List.of("Alice", "Bob", "Carol");
Map<String, Integer> nameLengths = names.stream()
    .collect(Collectors.toMap(Function.identity(), String::length));</code>

The above code uses double colons to infer the type parameters of Function.identity() and String::length.

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