Decisions - Java Concepts - page 265

5.2.5 Testing for Null

An object reference can have the special value null if it refers to no object at all.
It is common to use the null value to indicate that a value has never been set. For
example,

The null reference refers to no object.

String middleInitial = null; // Not set
if (. . .)
middleInitial = middleName.substring(0, 1);

You use the == operator (and not equals ) to test whether an object reference is a
null reference:

if (middleInitial == null)
System.out.println(firstName + " " + lastName);
else
System.out.println(firstName + " " +
middleInitial + "." + lastName);

Note that the null reference is not the same as the empty string Ȓȓ. The empty
string is a valid string of length 0, whereas a null indicates that a string variable
refers to no string at all.

S ELF C HECK

3. What is the value of s.length() if s is

a. the empty string Ȓȓ?

b. the string Ȓ ȓ containing a space?

c. null ?

4. Which of the following comparisons are syntactically incorrect?

Which of them are syntactically correct, but logically questionable?

String a = "1";
String b = "one";
double x = 1;