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The null Value | page 9 of 10 |
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 need a way to say that a variable does not now refer to an object. 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.
public class nullDemo
{
public static void main (String[] args)
{
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 chars)
// 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 .
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