Core Java Interview Questions -Language Fundamentals

Question:What is the out of below program? 

package com.lac.faq;

class A {
 public int i;
}

public class ParamPassDemo {
 public static void main(String[] args) {
  A a =new A();//Create an object of A
  a.i=10;
  change(a);
  a=null;
  System.out.println(a.i); //Guess result of print statement
 }
 private static void change(A a){
  a=null; //assign null
 }
 
}

Answer: The out put of the above program is 10. Most of us think the out put can be "java.lang.NullPointerException" but it is not because Objects in java are passed by copy of reference. When 'a' is passed to function 'change' the copy of reference 'a' is passed, so making the copied reference 'null' doesn't have any impact.


Question: Determine the Output of below program?

package com.lac.faq;

public class MyString extends String {
 public static void main(String[] args) {
  MyString s1="abc",s2="xyz";
  System.out.println(s1+s2);
  
 }

}

Answer: Compilation Error "MyString.java:3: cannot inherit from final java.lang.String"
Explanation: String class is a final class and it is not possible to inherit from a final class.

Table creation time MySQL

Problem: To find table creation/modified time in a given database.

Solution: MySQL maintains table level audit information in data base 'information_schema' and in table 'TABLES' 

Below Query fetches information about table 'foo'

select * from information_schema.TABLES where TABLE_NAME='foo'

Disabling portlet icons in Liferay

Problem:  Want to remove portlet icons like minimize, maximize, delete ..etc for all the portlets in liferay.  In some cases we dont want end user to delete or resize the deployed portlets. These operations should be done by only super admin/Omni admin/portal admin. In such case modify the portlet.vm to restrict these operations to only portal admin.

 Default icons for the tag cloud portlet.

 
 

Solution:  Edit portlet.vm and provide access to only portal admin.
 

 

   #if ($portlet_display.isShowBackIcon())
    #language("return-to-full-page")
   #else
   #if ($permissionChecker.isOmniadmin())
    $theme.iconOptions()
    $theme.iconClose()
    $theme.iconMinimize()
    $theme.iconMaximize()
   #end
   #end
  

Reading environment variable in Java

Following example helps to read environment variable.
 

 
package com.lac.demo;

public class ReadEnv {
 public static void main(String[] args) {
  //Reads environment variable PATH
  String value=System.getenv("PATH");
  System.out.println(value);
 }
}

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