How to create a read-only collection in java?

Collections is one of the most used feature in Java/J2EE applications . Read only List means a List where you can not perform modification operations like add, remove or set. You can only read from the List by using get method or by using Iterator of List, This kind of List is good for certain requirement where parameters are final and can not be changed. In Java you can use Collections.unModifiableList() method  to create read only List , Collections.unmodifiableSet() for creating read-only Set like read only HashSet and similarly creating a read only Map in Java.

 import java.util.ArrayList;  
 import java.util.Collections;  
 import java.util.List;  
 public class ReadOnlyCollections {  
           public static void main(String[] args) {  
                List<String> myList = new ArrayList<>();  
                myList.add("1");  
                myList.add("2");  
                myList.add("3");  
                System.out.println(myList);  
                // Convert to unmodifiable .  
                myList = Collections.unmodifiableList(myList);  
                myList.add("4");  
                System.out.println(myList);  
           }  
 }  
 Output -   
 [1, 2, 3]  
 Exception in thread "main" java.lang.UnsupportedOperationException  
      at java.util.Collections$UnmodifiableCollection.add(Unknown Source)  
      at ReadOnlyCollections.main(ReadOnlyCollections.java:15)  

How to find disk space utilization using Java?

 import java.io.File;  
 import java.nio.file.FileSystem;  
 import java.nio.file.FileSystems;  
 import java.nio.file.Path;  
 import java.util.Iterator;  
 public class DiskSpace {  
  public static void main(String[] args) {  
            FileSystem fs = FileSystems.getDefault();  
            Iterable drives = fs.getRootDirectories();  
            Iterator path = drives.iterator();  
            while(path.hasNext()){  
                  Path p = path.next();    
                  String drive = p.toString();  
                  System.out.println(drive);  
                  File f = new File(drive);  
                  float freeSpace = f.getFreeSpace();  
                  float totalSpace = f.getTotalSpace();  
                  if(totalSpace > 0){  
                          float percentFree = (freeSpace/totalSpace)*100;  
                          System.out.println(drive + "---- " +freeSpace + "------ " + totalSpace + "---------- " + percentFree);  
                  }  
            }  
      }  
 }  

Difference between JSP include directive and JSP include action

@include 
<%@ include file=”filename” %> is the JSP include directive. At JSP page translation time, the content of the file given in the include directive is ‘pasted’ as it is, in the place where the JSP include directive is used. Then the source JSP page is converted into a java servlet class. The included file can be a static resource or a JSP page. Generally JSP include directive is used to include header banners and footers.

The JSP compilation procedure is that, the source JSP page gets compiled only if that page has changed. If there is a change in the included JSP file, the source JSP file will not be compiled and therefore the modification will not get reflected in the output. is the JSP include action element. 


<jsp:include>
The jsp:include action element is like a function call. At runtime, the included file will be ‘executed’ and the result content will be included with the soure JSP page.
 When the included JSP page is called, both the request and response objects are passed as parameters. If there is a need to pass additional parameters, then jsp:param element can be used. If the resource is static, its content is inserted into the calling JSP file, since there is no processing needed.