Is Java Compiled or Interpreted Programming language?

One of the first question a graduate C or C++ programmer, who has just started learning Java ask is, whether Java is a compiled language or an interpreted one? On academic courses or during college, students learn a lot of languages e.g. VB, C, C++ and they happily categories them as either compiled or interpreted, but with Java it's tricky. It's not clear whether Java is compiled or interpreted, because it neither generate machine language code after compiling source file,  neither interpreted source file to execute instruction line by line. In order to answer this question you need to fist know that Java is a platform independent language? Which means you can run a Java program to any platform, which includes hardware + operating system, without any modification. Knowing how Java achieves platform independence is key to answer this question. If anyone ask this question during interview, then your answer should be both i.e. Java is both compiled and interpreted programming language. Java code is written in .java files (also known as source file), which is compiled by javac, a Java compiler into class files. Unlike C or C++ compiler, Java compiler doesn't generate native code. These class files contains byte-code, which is different than machine or native code. Java virtual machine or JVM interprets byte codes during execution of Java program. So, you can see it's both compiled and interpreted language, but this answer is incomplete until you mention about JIT (Just in time compiler) which does another round of compilation to produce native code, which can directly be executed by corresponding platform. We will learn about how JIT works in next section.

Java is both Compiled and Interpreted Language but how?

Above answer is absolutely correct but its not complete. What javac (Java compiler, which comes along JDK) does is pseudo compilation, it doesn't convert Java source code into native code, which can directly be executed by CPU, real compilation into native code is done by another program called Just in Time compiler, also known as JIT. This is actually an optimization done on JVM by Java platform engineers. When JVM interprets Java byte code, it also gathers useful statistics, like which part of code is hot and always run. Once JVM has enough data to make such decision, JIT can compile that part of code e.g. method or block into native code. This native code will then directly be executed by machine, without interpreting by JVM. JIT provides immense performance boost to Java application and this is one more reason why Java is also used to write high performance application like electronic trading systems, algorithmic gateways etc, along with native languages like C and C++.



In short, Java is both compiled and interpreted language. It compiles using javac and JIT, and interprets using Java virtual machine. Here is the sequence of things normally happens from writing to execution of a Java program :



1) Programmer write source code and store that into a .java file. Always remember name of your Java source file must be same with the public class declared inside that file, for example if there is public class called Order inside Java file, then it name must be Order.java.



2) javac compiles .java fie and creates class files, which contains byte code. This byte code is main reason of Java being platform independent.



3) JVM executes these class files and gathers statistics of execution run. These statics are used to determine hot spot i.e. part of your code which executes 90% of time.



4) After certain threshold, when JVM has enough data to make decision, JIT compiles frequently used byte codes into native code, which is then directly executed by platform. This provides performance boost to Java application.



That's all folks, You can now say that Java is both compiled and interpreted language. Some people even called Java as dynamically compiled language, referring to compiling byte code into native code during runtime. While answering this question during interview, always remember to mention about JIT compilation as converting Java source code to class files is not really compilation in true C and C++ programming sense, where compiler produce native code.

Source:http://javarevisited.blogspot.com/2014/06/is-java-interpreted-or-compiled-programming-language.html