Calling C functions from Java code

The powerful JNI interface, as referred to before, is able to manage interaction in both directions, from Java to C and from C to Java.

A regular Java class declaring a method with the keyword native declares that the method behavior is implemented in native code. Like a regular Java method, the JNI native method is able to receive Java objects or primitive types as arguments and return primitive types and objects.

Let's see how a native method definition will look like in a Java class:

public class MyNativeActivity extends Activity {

  @Override
  protected void onCreate(Bundle savedInstanceState) {
    ...
    cTv.setText(isPrime(2) ? "true" : "false");
  }
  …
  private native boolean isPrime(int number );
}

The preceding activity will call the native code to check whether a number is prime or not and print the result on the UI.

Notice that our function receives a primitive as an argument and return a primitive boolean as a result and does not have any...