What are the most important hardware components needed for programming?

Java

That really depends on several factors, one of which is the language you are going to use. There are literally hundreds of programming languages, for different purposes; each has different requirements.

For example, Java is a programming language that works on different platforms - that is, it works on a variety of types of hardware, and operating systems; the most basic thing you need is the (free) JDK (Java Development Kit), which includes the Java compiler and the Java Runtime; and any basic text editor such as NotePad.