Writing a custom function in Neo4j

If you are interested in writing your own custom procedure, Neo4j provides a Java API for this. In a Maven project, you will have to include the following dependency:

 <dependency>
     <groupId>org.neo4j</groupId>
     <artifactId>neo4j</artifactId>
     <version>${neo4j.version}</version>
     <scope>provided</scope>
 </dependency>

A user-defined function that simply multiplies two numbers together can be implemented with the following piece of code:

@UserFunction
@Description("packt.multiply(value, value)")
public Double multiply(
        @Name("number1") Double number1,
        @Name("number2") Double number2
) {
    if (number1 == null || number2 == null) {
        return null;
    }
    return number1 * number2;
}

Let's break down this code:

1. The @UserFunction annotation declares a user-defined function. Similarly, procedures can be declared with the @Procedure annotation...