Java is a simple computer language which runs programs in a "virtual machine". This makes all Java-programs inherantly slow and resource-demanding, since loading this virtual machine requires at least 220 MB virtual and 40 MB resident memory on Linux system. This is the requirement for a simple Java program which says "hello world".
 Write once, debug everywhere
Java's slogan is that can you supposedly write once, run everywhere. The truth is that you can write once and have to debug everywhere, since differnt OS running Java and different Java virtual machines give very different errors.