The null Value

In most programs, objects are created and objects are destroyed, depending on the data and on what is being computed. A reference variable sometimes does and sometimes does not refer to an object. You may need a way to erase the reference inside a variable without creating a new reference. You do this by assigning null to the variable.

The value null is a special value that means "no object." A reference variable is set to null when it is not referring to any object.

String a =                // 1. an object is created;
   new String("stringy"); // variable a refers to it
String b = null;          // 2. variable b refers to no
                          // object.
String c =                // 3. an object is created
   new String("");        // (containing no characters)
                          // variable c refers to it
if (a != null)            // 4. statement true, so
System.out.println(a);    // the println(a) executes.

if (b != null)            // 5. statement false, so the
System.out.println(b);    // println(b) is skipped.

if (c != null)            // 6. statement true, so the
System.out.println(c);    // println(c) executes (but
                          // it has no characters to
                          // print).

Run Output:

stringy

Variables a and c are initialized to object references. Variable b is initialized to null. Note that variable c is initialized to a reference to a String object containing no characters. Therefore, println(c) executes, but it has no characters to print. Having no characters is different from the value being null.