java.lang.NullPointerException

If you set a string to null then try to compare it with another value, you will get a java.lang.NullPointerException if you do it like this:

Solaris > cat prog39.java

public class prog39

{

public static void main (String args[])

  {

  String fred = null;

  if (fred.equals("OK"))

    System.out.println("Fred is OK");

  else

    System.out.println("Fred is not OK");

  }

}

Solaris > javac prog39.java

Solaris > java prog39

Exception in thread "main" java.lang.NullPointerException

        at prog39.main(prog39.java:6)

Solaris >

 

If you just make the string empty, the code will work, as you can see below:

 

Solaris > cat prog40.java

public class prog40

{

public static void main (String args[])

  {

  String fred1 = "";

  System.out.println

  ("Length of Fred1 is " + fred1.length());

  if (fred1.equals("OK"))

    System.out.println("Fred1 is OK");

  else

    System.out.println("Fred1 is not OK");

  

  String fred2 = new String();

  System.out.println

  ("Length of Fred2 is " + fred2.length());

  if (fred2.equals("OK"))

    System.out.println("Fred2 is OK");

  else

    System.out.println("Fred2 is not OK");

  }

}

Solaris > javac prog40.java

Solaris > java prog40

Length of Fred1 is 0

Fred1 is not OK

Length of Fred2 is 0

Fred2 is not OK

Solaris >

... and if the string contains a value, it works too: 

Solaris > cat prog41.java

public class prog41

{

public static void main (String args[])

  {

  String fred = "OK";

  if (fred.equals("OK"))

    System.out.println("Fred is OK");

  else

    System.out.println("Fred is not OK");

  }

}

Solaris > javac prog41.java

Solaris > java prog41

Fred is OK

Solaris >

... but if you really need to cater for a null string, you could try the following:

Solaris > cat prog42.java

public class prog42

{

public static void main (String args[])

  {

  String fred1 = null;

  if (fred1 != null && fred1.equals("OK"))

    System.out.println("Fred1 is OK");

  else

    System.out.println("Fred1 is not OK");

 

  String fred2 = "OK";

  if (fred2 != null && fred2.equals("OK"))

    System.out.println("Fred2 is OK");

  else

    System.out.println("Fred2 is not OK");

 

  String fred3 = null;

  if ("OK".equals(fred3))

    System.out.println("Fred3 is OK");

  else

    System.out.println("Fred3 is not OK");

 

  String fred4 = "OK";

  if ("OK".equals(fred4))

    System.out.println("Fred4 is OK");

  else

    System.out.println("Fred4 is not OK");

  }

}

Solaris > javac prog42.java

Solaris > java prog42

Fred1 is not OK

Fred2 is OK

Fred3 is not OK

Fred4 is OK

Solaris >

How to Concatenate Strings in Java

This example shows how to concatenate strings i.e. how to join them together:

andrew@UBUNTU:~/Java$ cat prog38.java
public class prog38
{
public static void main (String args[])
  {
  String first_name = "Andrew";
  String surname = "Reid";
  String full_name = first_name + " " + surname;
  System.out.println ("full_name = " + full_name);
  }
}
andrew@UBUNTU:~/Java$ javac prog38.java
andrew@UBUNTU:~/Java$ java prog38
full_name = Andrew Reid
andrew@UBUNTU:~/Java$

How to Extract a Character from a String in Java

This example shows how to define a string then extract the character at a given position in that string. It does this using the String.charAt() method. Note that character positions are numbered from zero, not one. The first part uses a string variable whereas the second uses a string literal:

andrew@UBUNTU:~/Java$ cat prog37.java
public class prog37
{
public static void main (String args[])
  {
  String Andrews_name = "Andrew";
  char first_letter = Andrews_name.charAt(0);
  System.out.println
   ("1st letter of Andrew = " + first_letter);
  char fourth_letter = "Andrew".charAt(3);
  System.out.println
   ("4th letter of Andrew = " + fourth_letter);
  }
}
andrew@UBUNTU:~/Java$ javac prog37.java
andrew@UBUNTU:~/Java$ java prog37
1st letter of Andrew = A
4th letter of Andrew = r
andrew@UBUNTU:~/Java$