从命令提示符编译 java 及其问题

Question

i'm very new to java and only started working two days ago... i decided to work it through command line rather than IDE.. and it gave issues they are

while setting up path variable...

1)what exactly is PATH variable i got away from it by command "set path=%path%;C:\\Program Files\\Java\\jdk1.8.0_45\\bin" and i don't know what i did could u explain it in layman terms

mean while ,in searching solution for PATH variable i found there is another variable called CLASSPATH variable..

2)so what is CLASSPATH variable do need to find solution for that too..

after setting up just PATH variable i compiled program and it got following errors..

3)could explain what are the following errors and correct me

meanwhile my encounter with files in 'java' i found out there are jdk and jre and my question is

4)why do we choose only 'jdk' while setting up path variable

i have read differences between jdk and jre but i didn't understand it clearly they said that

" jre doesn't have compiler " and even though we can run our programs and i didn't get it.. explain me that too..

thanks in advance....

Answer 1

1)what exactly is PATH variable

The PATH variable is a Windows system variable that tells Windows where to look when you enter a command without path information. So entering "java.exe" can work from anywhere when you enter it at a command prompt if you have the full path to it in the PATH variable. Read the following page about how the PATH and CLASSPATH are used in Java.

2)so what is CLASSPATH variable

I'm not sure since you don't give any context, but I think you are referring to the Java Virtual Machine's (JVM) variable that tells the JVM where to look for classes and resources.

3)could explain what are the following errors and correct me

If you mean the compile error in the screen shot, the error is because you did not use the proper name System . Java is case sensitive. There is no object called system .

4)why do we choose only 'jdk' while setting up path variable

Some definitions:

• JRE = Java Runtime Environment. Bare minimum binaries and libraries for running the core Java classes.
• JDK = Java Development Kit. It contains all core binaries and libraries needed to do basic developement (javac, jar, etc). A JDK includes a JRE.
• JVM = Java Virtual Machine. The runtime (java.exe) that executes Java code.

Answer 2

在字符串中 S 必须是大写，在 System.out.println 中 S 必须是大写

Answer 3

1)what exactly is PATH variable

PATH is an environment variable specifying a set of directories where executable programs are located, so if you sent a command that is the name of a executable, your OS will search those dirs for the program to run

2)so what is CLASSPATH variable do need to find solution for that too..

This is where java looks for the class files. Usually specified by .cp cmd line argument

3)could explain what are the following errors and correct me

My guess is "system.out.println()" should be "System.out.println()"

4)why do we choose only 'jdk' while setting up path variable

JDK has a compiler. Compiler turns your source code .java into .class files which the computer (running the JVM) can actually understand. JRE is good enough if you only want to run the programs, but you need a JDK to compile them from source code.

Answer 4

1) PATH is the environment variable that says where to look for commands that you enter on the command line. If you type the command "java" at the command prompt, the OS will search in each directory listed in PATH, in order, until it finds an executable named "java" (for example, java.exe or java.cmd). If PATH were empty, the OS would only look in the current directory.

2) CLASSPATH is where the JVM will search for Java library files (.jar files) at both compile time and at run time. It should be set to something like c:\\jdk\\lib (where "jdk" is where you installed the JDK).

3) In your screenshot, it looks like your code is lowercase where it should be uppercase: change string --> String and system --> System

4) The JDK is the Java Developer's kit, used when you are developing Java programs. JRE is the runtime environment, needed by anyone who will run Java programs. The JDK usually contains the JRE, but if someone who is not developing can just use the JRE.