4. Importing Java classes into R code

A true testament of the level of integration of Java and R in Renjin is the ability to directly access (public!) Java classes and methods from R code. Renjin introduces the import() function which adds a Java class to the environment from which it is called. In the section Pushing data from Java to R in the previous chapter we had already seen how a Java class could be put into the global environment of the R session.

Consider the following sample R script:

import(java.util.HashMap)

# create a new instance of the HashMap class:
ageMap <- HashMap$new()

# call methods on the new instance:
ageMap$put("Bob", 33)
ageMap$put("Carol", 41)

print(ageMap$size())

age <- ageMap$get("Carol")
cat("Carol is ", age, " years old.\n", sep = "")

# Java primitives and their boxed types
# are automatically converted to R vectors:
typeof(age)

As we showed in the Introduction, we can execute this script using the java.io.FileReader interface:

engine.eval(new java.io.FileReader("import_example.R"));

Output:

[1] 2
Carol is 41 years old.

The first line in the output is the output from the print(ageMap$size()) statement.

4.1. Bean classes

For Java classes with accessor methods that conform to the getXX() and setXX() Java bean convention, Renjin provides some special sauce to make access from R more natural.

Take the following Java bean as an example:

package beans;

public class Customer {
    private String name;
    private int age;

    public String getName() { return name; }
    public void setName(String name) { this.name = name; }

    public int getAge() { return age; }
    public void setAge(int age) { this.age = age; }
}

You can construct a new instance of the Customer class and provide initial values with named arguments to the constructor. For example:

import(beans.Customer)

bob <- Customer$new(name = "Bob", age = 36)
carol <- Customer$new(name = "Carol", age = 41)
cat("'bob' is an ", typeof(bob), ", bob$name = ", bob$name, "\n", sep = "")

If the previous R code is stored in a file bean_example.R then the following Java code snippet runs this example:

// required import(s):
import beans.Customer;

engine.eval(new java.io.FileReader("bean_example.R"));

Output:

'bob' is an externalptr, bob$name = Bob