Java is Compiled and Interpreted Both

It seems little confusing for java beginner to understand the exact difference between compilation and interpretation. Java is compiled language because java code is compiled to get byte code. Java is interpreted language because this byte code is interpreted into native code.

Actually getting output from java program is two steps process compilation and interpretation. The executable used for these steps are java compiler (javac) and java interpreter (java). First step is compilation in which the java compiler compiles the source code to generate byte code in the form of .class file. Actually this byte code is not machine code but it is just an intermediate code. In the second step java interpreter uses this byte code to generate native code for under lying operating system. The JIT (Just In Time) compiler comes into action in second interpretation step. The JIT reads the byte code to generate native code dynamically so that program runs fast.

Finally in short, java is compiled to byte code which is then used by JVM where it gets interpreted to native code. So java is compiled and interpreted both.

Setting Path Environment Variable in Java

For the beginner one of the difficult and confusing task is to set path and classpath environment variables.

Path is nothing but setting up an environment for operating system. Path is a system variable used to tell to Operating System all locations of executable files. Operating System will look in this path variable for executable files.

Classpath is nothing but setting up an environment for java. Classpath is pure java variable, which is used to tell the Java Virtual Machine or the Java compiler where to look for user-defined classes and packages.

For example,assume that Java is installed in C:\Program Files\Java\jdk1.7.0_25\ and your code is in C:\Sample\example.java, then path and classpath should be set to like this:

classpath=C:\Sample

path= C:\Program Files\Java\jdk1.7.0_25\bin;

Or

path=%JAVA_HOME%\bin;

where JAVA_HOME is pointing to Home directory “C:\Program Files\Java\jdk1.7.0_25”, where jdk is installed

Let us see what exactly means by setting path variable

Whenever you try to compile a program using javac, JVM will search for javac in current directory where your program is residing, If it does not find javac and other executables in current directory then it will search in the directories where the path variable is set to. So setting path variable allows you to execute your program from any directory as location of executables java and javac is retrieved through path variable.

If you have not set path variable then it will show error like “javac is not a recognized command” . hence to remove this error set path environment variable as follows:

• Command to set path variable from command prompt

·         Set path=”C:\Program Files\Java\jdk1.7.0_25\bin”

·         Set path=%path%;”C:\Program Files\Java\jdk1.7.0_25\bin”

Here in second command we are just adding our path value to existing value of path variable.

• Command to remove value from path variable

·         Set path=

This will remove directory path from environment variable.

• Check whether environment variables are set correctly or not

·         If your environment variables are set correctly, then windows should recognize these commands.

C:\> java OR

C:\> javac

·         you can see current value of path variable by following command

C:\> echo %path%