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Trail: Java Native Interface
Lesson: Writing a Java Program with Native Methods

## Step 4: Write the Native Method Implementation

> Now, you can finally write the implementation for the native method in a language other than Java.

> > Note: Back to that "real world," C and C++ programmers may already have existing implementations of native methods. For those "real" methods, you need only ensure that the native method signature matches the signature generated on the Java side.

> The function that you write must have the same function signature as the one generated by `javah` in the `HelloWorld.h` file in Step 3: Create the .h File. Recall that the function signature generated for the `HelloWorld` class's `displayHelloWorld` native method looks like this:

> >     JNIEXPORT void JNICALL
> >         Java_HelloWorld_displayHelloWorld(JNIEnv *, jobject);

> Here's the C language implementation for the native method `Java_HelloWorld_displayHelloWorld`. This implementation is in the file named `HelloWorldImp.c`.

> >     #include <jni.h>
> >     #include "HelloWorld.h"
> >     #include <stdio.h>

> >     JNIEXPORT void JNICALL
> >     Java_HelloWorld_displayHelloWorld(JNIEnv *env, jobject obj)
> >     {
> >         printf("Hello world!\n");
> >         return;
> >     }

> The implementation for `Java_HelloWorld_displayHelloWorld` is straightforward. The function uses the `printf` function to display the string "Hello World!" and then returns.

> The `HelloWorldImp.c` file includes three header files:

> 1. `jni.h` - This header file provides information that the native language code requires to interact with the Java runtime system. When writing native methods, you must always include this file in your native language source files.
> 2. `HelloWorld.h` - The `.h` file that you generated in Step 3: Create the .h File.
> 3. `stdio.h` - The code snippet above includes `stdio.h` because it uses the `printf` function. The `printf` function is part of the `stdio.h` library.

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