created simple JNI demo

Author: Juraj Veverka

simple-jni-demo/.gitignore

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+Display.h
+Display.class
+libDisplay.so
+

simple-jni-demo/Display.c

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+#include <jni.h>        // JNI header provided by JDK
+#include <stdio.h>      // C Standard IO Header
+#include "Display.h"   // Generated
+ 
+// Implementation of the native method sayHello()
+JNIEXPORT void JNICALL Java_Display_setPixel(JNIEnv *env, jobject thisObj, jint x, jint y, jint r, jint g, jint b) {
+   printf("setting pixel\n");
+   return;
+}
+

simple-jni-demo/Display.java

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+public class Display {
+
+	static {
+		System.out.println("Loading Library...");
+	        System.loadLibrary("Display");
+		System.out.println("Library loaded."); 
+	}
+
+	public native void setPixel(int x, int y, int r, int g, int b);
+
+	public static void main(String[] args) {
+	       Display display = new Display();
+	       display.setPixel(0,0, 0, 255, 0);
+	       System.out.println("done.");
+	}
+
+}
+

simple-jni-demo/README.md

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+# Very simple Java JNI demo
+
+## create JNI bindings
+`javac -h . Display.java`
+
+## compile c
+`gcc -I"/opt/java11/include/" -I"/opt/java11/include/linux" -lc -shared -o libDisplay.so Display.c`
+
+## compile java
+`javac Display.java`
+
+## run test
+`java -Djava.library.path=. Display`
+
+## clean 
+`rm Display.h libDisplay.so Display.class`
+