Lesson 1 / 38

What is Java?

Learn what Java is, how source compiles to bytecode, how the JVM runs it anywhere, and the difference between the JDK, JRE and JVM.

The idea

Java is a statically typed, object-oriented language released by Sun Microsystems in 1995 and now stewarded by Oracle with an open-source reference implementation (OpenJDK). You write .java source, compile it to platform-neutral bytecode (.class), and the JVM runs that bytecode anywhere — "write once, run anywhere".

JDK vs JRE vs JVM

JVM executes bytecode and manages memory. JRE = JVM + the standard class library needed to run apps. JDK = JRE + developer tools (javac, jar, jshell, debugger) needed to build apps. You install a JDK.

Which version

Prefer a current LTS release (e.g. Java 17 or 21). LTS versions get years of security updates, which is what production code needs.