[`System.getenv`][1] gets an *environment variable*. [`System.getProperty`][2] gets a Java property. Environment variables are specified at the OS level. Java properties are specified by passing the `-D` option to the JVM (and can be set programmatically).


  [1]: http://docs.oracle.com/javase/7/docs/api/java/lang/System.html#getenv%28java.lang.String%29
  [2]: http://docs.oracle.com/javase/7/docs/api/java/lang/System.html#getProperty%28java.lang.String%29

[`System.getenv()`](http://docs.oracle.com/javase/6/docs/api/java/lang/System.html#getenv(java.lang.String)) is for Operating System environment variables, whereas [`System.getProperty()`](http://docs.oracle.com/javase/6/docs/api/java/lang/System.html#getProperty(java.lang.String)) is for JVM arguments which are passed as `-DpropName=value` to Java application launcher ([`java`](http://docs.oracle.com/javase/6/docs/technotes/tools/windows/java.html)).

We know that `sizeof` is an operator used for calculating the size of any datatype and expression, and when the operand is an expression, the parentheses can be omitted.

    int main()
    {
            int a;
    
            sizeof int;
            sizeof( int );
            sizeof a;
            sizeof( a );
    
            return 0;
    }

the first usage of `sizeof` is wrong, while others are right.

When it is compiled using gcc, the following error message will be given:

    main.c:5:9: error: expected expression before ‘int’

My question is why the C standard does not allow this kind of operation. Will `sizeof int` cause any ambiguity? 

Why sizeof int is wrong, while sizeof(int) is right?

I was wondering how to sign(`-s`) off previous commits that I have made in the past in git?

CLARIFICATION: Git has confusingly [similarly named concepts](
https://medium.com/@MarkEmeis/git-commit-signoff-vs-signing-9f37ee272b14)
  `git commit -s, --signoff` (lower case -s) and `git commit -S, --gpg-sign` (upper case -S).  
This question is about signoff(lower case -s).  
 However some answers here are about gpg-sign(upper case -S), that better discussed in dedicated questions on superUser [Can you GPG sign old commits?](
https://superuser.com/questions/397149/can-you-gpg-sign-old-commits#)
and stackoverflow https://stackoverflow.com/questions/41882919/is-there-a-way-to-gpg-sign-all-previous-commits

Git sign off previous commits?

&gt; **Possible Duplicate:**  
&gt; [What’s the difference between a System property and environment variable](https://stackoverflow.com/questions/2863674/whats-the-difference-between-a-system-property-and-environment-variable)  

&lt;!-- End of automatically inserted text --&gt;

    
What is the difference between System.getenv() &amp; System.getProperty()?

When we run any command using Processbuilder, we can set the environment variables ie:

    String[] vCmd = { System.getenv(&quot;ANT_HOME&quot;) + &quot;/bin/ant&quot;, &quot;-f&quot;,
    				ANT_BUILD_FILE, TARGET };
    		ProcessBuilder pb = new ProcessBuilder(vCmd);
    		Map&lt;String, String&gt; env = pb.environment();		
    		env.put(&quot;CLASSPATH&quot;,
    				antHome+&quot;/lib/ant.jar:&quot;
    						+ antHome+&quot;/lib/ant-launcher.jar:&quot;
    						+ antHome+&quot;/lib/ant-nodeps.jar:&quot;
    						);
    		try{
    			
    			Process process = pb.start();
    			InputStream is = process.getInputStream();
    			InputStreamReader isr = new InputStreamReader(is);
    			BufferedReader br = new BufferedReader(isr);
    			String line;
    			while ((line = br.readLine()) != null) {
    				System.out.println(line);
    			}
    			pb.wait();
    		}
    		catch(Exception e)
    		
    			System.out.println(e.getMessage());
    		 	
    
    		}

If I set some properties using System.setProperties() before this method , is it available to this process started by ProcessBuilder?


Difference between System.getenv() &amp; System.getProperty()

> Possible Duplicate: 
> <a href="https://stackoverflow.com/questions/2863674/whats-the-difference-between-a-system-property-and-environment-variable" target="_blank" rel="noopener noreferrer">What’s the difference between a System property and environment variable</a>

What is the difference between System.getenv() &#x26; System.getProperty()?
When we run any command using Processbuilder, we can set the environment variables ie:
<pre><code class="hljs language-ini">String[] vCmd = { System.getenv("ANT_HOME") + "/bin/ant", "-f",
				ANT_BUILD_FILE, TARGET };
		ProcessBuilder pb = new ProcessBuilder(vCmd);
		Map&#x3C;String, String> env = pb.environment();		
		env.put("CLASSPATH",
				antHome+"/lib/ant.jar:"
						+ antHome+"/lib/ant-launcher.jar:"
						+ antHome+"/lib/ant-nodeps.jar:"
						);
		try{
			
			Process process = pb.start();
			InputStream is = process.getInputStream();
			InputStreamReader isr = new InputStreamReader(is);
			BufferedReader br = new BufferedReader(isr);
			String line;
			while ((line = br.readLine()) != null) {
				System.out.println(line);
			}
			pb.wait();
		}
		catch(Exception e)
		
			System.out.println(e.getMessage());
		 	

		}
</code></pre>
If I set some properties using System.setProperties() before this method , is it available to this process started by ProcessBuilder?

I have a following method...which actually takes the list of sentences and splits each sentence into words. Here is it:

    public List&lt;String&gt; getWords(List&lt;String&gt; strSentences){
   	allWords = new ArrayList&lt;String&gt;();
    	Iterator&lt;String&gt; itrTemp = strSentences.iterator();
    	while(itrTemp.hasNext()){
    		String strTemp = itrTemp.next();
    		allWords = Arrays.asList(strTemp.toLowerCase().split(&quot;\\s+&quot;));    		
    	}
    	return allWords;
    }

I have to pass this list into a hashmap in the following format     
    
    HashMap&lt;String, ArrayList&lt;String&gt;&gt;

so this method returns List and I need an ArrayList? If I try to cast it doesn&#39;t work out... any suggestions?

Also, if I change the ArrayList to List in a HashMap, I get       

    java.lang.UnsupportedOperationException

because of this line in my code 

    sentenceList.add(((Element)sentenceNodeList.item(sentenceIndex)).getTextContent());

Any better suggestions?



List&lt;String&gt; to ArrayList&lt;String&gt; conversion issue

I&#39;m in the process of learning Java and I cannot find any good explanation on the `implements Closeable` and the `implements AutoCloseable` interfaces.

When I implemented an `interface Closeable`, my Eclipse IDE created a method `public void close() throws IOException`.

I can close the stream using `pw.close();` without the interface. But, I cannot understand how I can implement the`close()` method using the interface. And, what is the purpose of this interface?

Also I would like to know: how can I check if `IOstream` was really closed?

I was using the basic code below 

    import java.io.*;
    
    public class IOtest implements AutoCloseable {

	public static void main(String[] args) throws IOException  {
		
		File file = new File(&quot;C:\\test.txt&quot;);
		PrintWriter pw = new PrintWriter(file);
		
		System.out.println(&quot;file has been created&quot;);
		
		pw.println(&quot;file has been created&quot;);

	}

	@Override
	public void close() throws IOException {
		
		
	}

implements Closeable or implements AutoCloseable

I would like to make a Join query using Jpa repository with annotation @Query.

I have two tables:

     table user 
     with iduser,user_name 

and: 

     table area 
     with idarea, area_name and iduser

The native query is:

     SELECT
     u.user_name 
     FROM
      user as u 
      INNER JOIN area as a ON a.iduser = u.iduser
      WHERE
      a.idarea = 4

 Now I have a Table Hibernate entity 
 User and Area

So I tried with UserRespository 

    @Query(SELECT  u.userName FROM  User u 
      INNER JOIN Area a ON a.idUser = u.idUser
      WHERE
      a.idArea = :idArea)
    List&lt;User&gt; findByIdarea(@Param(&quot;idArea&quot;) Long idArea);

The Log says:

&gt; unexpected token:

Any Idea, please?

My table Entity
    
    #User Table
    @Entity
    @Table(name=&quot;user&quot;)
    public class User implements Serializable {

	    private static final long serialVersionUID = 1L;
	
	    private Long idUser;
	    private String userName;
	
	    @Id @GeneratedValue(strategy=GenerationType.AUTO)
	    @Column(name=&quot;iduser&quot;)
	    public Long getIdUser() {
		    return idUser;
	    }

	    public void setIdUser(Long idUser) {
		    this.idUser = idUser;
	    }
	
	    @Column(name=&quot;user_name&quot;)
	    public String getUserName() {
		    return userName;
	    }

	    public void setUserName(String userName) {
		    this.userName = userName;
	    }
    }

    #AREA table

    @Entity
    @Table(name=&quot;area&quot;)
    public class Area  implements Serializable {

	    private static final long serialVersionUID = 1L;
	
	    private Long idArea;
	    private String areaName;
	    private Long idUser;
	
	    @Id @GeneratedValue(strategy=GenerationType.AUTO)
	    @Column(name=&quot;idarea&quot;)
	    public Long getIdArea() {
		    return idArea;
	    }
	
        public void setIdArea(Long idArea) {
		    this.idArea = idArea;
	    }
	
	    @Column(name=&quot;area_name&quot;)
	    public String getAreaName() {
		    return areaName;
	    }
	
        public void setAreaName(String areaName) {
		    this.areaName = areaName;
	    }
	
	    @Column(name=&quot;iduser&quot;)
	    public Long getIdUser() {
		    return idUser;
	    }
	
        public void setIdUser(Long idUser) {
		    this.idUser = idUser;
	    }		
    }

How To Define a JPA Repository Query with a Join

I read the JDK&#39;s source code about ConcurrentHashMap.

But the following code confused me:

    public boolean isEmpty() {
        final Segment&lt;K,V&gt;[] segments = this.segments;
        ...
    }

My question is:

&quot;this.segments&quot; is declared:

    final Segment&lt;K,V&gt;[] segments;

So, here, in the beginning of the method, declared a same type reference, point to the same memory.

Why did the author write it like this? Why didn&#39;t they use this.segments directly? Is there some reason?

Why does JDK sourcecode take a `final` copy of `volatile` instances

I am newbie to Java world (7 years of low level plain C).

When I started reading Java related sites:

 * http://wiki.apidesign.org/wiki/Main_Page
 * http://static.springsource.org/spring/docs/3.1.x/spring-framework-reference/html/

I confused by existing terminology:

 * class
 * package
 * module
 * component
 * container
 * service
 * framework
 * platform

I found many resources about terms definition (and a lot more):

 * https://stackoverflow.com/questions/3057526/framework-vs-toolkit-vs-library
 * https://stackoverflow.com/questions/7310283/difference-between-framework-and-container
 * https://stackoverflow.com/questions/1288708/what-are-the-containers-in-java
 * https://stackoverflow.com/questions/7151206/definition-of-a-java-container
 * http://martinfowler.com/articles/injection.html (Inversion of Control Containers and the Dependency Injection pattern)
 * http://www.cafeaulait.org/course/week8/36.html (What is a AWT Container?)
 * http://en.wikipedia.org/wiki/Web_container
 * http://netbeans.org/features/platform/ (The NetBeans Platform)
 * https://stackoverflow.com/questions/4241919/what-is-meant-by-application-framework
 * https://stackoverflow.com/questions/2964140/what-is-a-framework-what-does-it-do-why-do-we-need-a-framework
 * https://stackoverflow.com/questions/148747/what-is-the-difference-between-a-framework-and-a-library
 * https://stackoverflow.com/questions/724438/what-is-the-difference-between-a-class-library-and-a-framework
 * https://stackoverflow.com/questions/11576018/what-is-the-difference-between-a-javascript-framework-and-a-library
 * https://stackoverflow.com/questions/1415592/what-is-the-major-difference-between-a-framework-and-a-toolkit
 * http://blog.orderharmony.com/2011/07/libraries-vs-frameworks.html
 * http://www.javacodegeeks.com/2012/04/frameworks-vs-libraries-as-inheritance.html
 * https://stackoverflow.com/questions/48239046/difference-between-library-platform-and-framework
 * https://stackoverflow.com/questions/31503886/what-is-the-difference-between-a-platform-and-a-framework
 * https://stackoverflow.com/questions/25028243/what-is-the-difference-between-a-framework-and-a-platform
 * https://stackoverflow.com/questions/4099975/difference-between-a-module-library-and-a-framework

but each of these resources define them on its own purpose and I still can&#39;t distinct for example module from component in general case.

Please explain what means of these terms in big picture (can be single class a platform, what amount of modules is required to make a container, etc).

**UPDATE 2019** From https://www.artima.com/lejava/articles/reuse3.html (my highlighting)

&gt;  *Bill Venners*: What is the difference between a framework, a platform, and a toolkit, and what are the different flexibility needs? 
&gt;
&gt;  *Erich Gamma*: With a **platform** I associate **long term stability**. It is safe to build on top of a platform. A platform makes **compatibility** guarantees. Frameworks often do not have this quality and I have seen **many framework** failures with regard to **stability**. If you look at Eclipse, yes it includes frameworks, toolkits, and provides platform APIs. All of this is bundled as plug-ins. **Frameworks abstract and provide higher level default functionality**. To do so the **framework needs to be in control**. This loss of control can lead to what is sometimes called frameworkitis. 
&gt;
&gt;  *Bill Venners*: And toolkits don&#39;t because...
&gt;
&gt;  *Erich Gamma*: With **toolkits** you **create and call toolkit objects and register listeners to react to events**. You&#39;re **in control**. **Frameworks try to be in control and tell you when to do what**. A **toolkit** gives you the **building blocks** but leaves it up to you to be in control.

Another quotation answers my naive question how many classes makes something a framework:

https://www.artima.com/lejava/articles/reuse.html

&gt; *Erich Gamma*: ... JUnit is a small framework, for example. It is the &quot;Hello, world&quot; of frameworks. You have `Test`, `TestCase`, `TestSuite` and relationships defined. Also, you hook into frameworks by subclassing somewhere. They use the so-called Hollywood principle of &quot;don&#39;t call us, we&#39;ll call you.&quot; The framework allows you to define your custom behavior, and it will call you back when it&#39;s your turn to do something. Same with JUnit, right? It calls you back when it wants to execute a test for you, but the rest is done in the framework. 

Class vs package vs module vs component vs container vs service vs platform in Java world

Windows has the [`setx`][1] command:

    Description:
        Creates or modifies environment variables in the user or system
        environment.

So you can set a variable like this:

    setx FOOBAR 1

And you can clear the value like this:

    setx FOOBAR &quot;&quot;

However, the variable does not get removed. It stays in the registry:

[![foobar][2]][2]

So how would you actually remove the variable?

  [1]: https://ss64.com/nt/setx.html
  [2]: http://i.stack.imgur.com/Lnjns.png



Command line to remove an environment variable from the OS level configuration

I&#39;d like to change the value of `process.env.PORT`, how can I do this?

I&#39;m running Ubuntu 12.04.

How to change value of process.env.PORT in node.js?

I have first executed the command: `export LD_LIBRARY_PATH=/usr/local/lib`

Then I have opened `.bash_profile` file: `vi ~/.bash_profile`.
In this file, I put:

    LD_LIBRARY_PATH=/usr/local/lib
    export LD_LIBRARY_PATH

Then if the terminal is closed and restarted, typing `echo $LD_LIBRARY_PATH` displays no result.

How to set the path permanently?

How to set the environmental variable LD_LIBRARY_PATH in linux

I set up an environment variable (Under `IDE Settings -&gt; Path Variables`)    

     APP_HOME = /path/to/app_home  

One of my tests is failing however with      

     System.out.println(&quot;APP HOME: &quot; + APP_HOME); 

 With      

     APP HOME: null/ 

 It does not look like that env variable is being read. What am i missing?




Setting up and using environment variables in IntelliJ Idea

I happen to run some commands blindly, in order to get things done.

I started to work with [Jenkins][1] recently, and then I had to use this `export` command to run the [Jenkins WAR archive][2]. What does the `export` command do in general, and why do we need to run this command, while running Jenkins (after the Jenkins home is set)?

  [1]: https://en.wikipedia.org/wiki/Jenkins_%28software%29
  [2]: https://updates.jenkins-ci.org/download/war/



Content Type	Original Author	Original Content on Stackoverflow
Question	user1731553	View Question on Stackoverflow
Solution 1 - Java	T.J. Crowder	View Answer on Stackoverflow
Solution 2 - Java	Bhesh Gurung	View Answer on Stackoverflow

Difference between System.getenv() & System.getProperty()

Java Problem Overview

Java Solutions

Solution 1 - Java

Solution 2 - Java

Git sign off previous commits?

Why sizeof int is wrong, while sizeof(int) is right?

Attributions