Comparing Values:

Assignment Statement: a single equal sign.  For example, x = 5;
Comparing (Equal) Statement: a set of double equal signs.  For example, 10 == 10;
Comparing (NOT EQUAL): For example, 12 != 10;

Comparing Floating-Point Numbers

Consider this code
    double r = Math.sqrt(2);
    double d = r * r - 2;
    if (d == 0)      //don't use == to compare floating-point numbers

       System.out.println("sqrt(2)squared minus 2 is 0");

    else

       System.out.println("sqrt(2)squared minus 2 is not 0, but " + d);

It prints:
    sqrt(2)squared minus 2 is not 0 but 4.440892098500626E-16

• To avoid round off errors, don't use == to compare floating-point numbers

• To compare floating-point numbers test whether they are close enough
  |x - y| ≤ ε where ε is a small number such as 10-14

   final double EPSILON = 1E-14;

   if (Math.abs(x - y) <= EPSILON)

      // x is approximately equal to y

Comparing Strings

• Don't use == for strings!

  if (input == "Y") // WRONG!!!

• Use equals method:

  if (input.equals("Y"))

• == tests identity, equals tests equal contents

public class Tester { public static void main(String [] args) { String myname = "Fredrick"; String hisname = myname.substring(0,4); String nickname = "Fred"; System.out.println("Test by == "); if( hisname == "Fred" ) //This result is wrong!!! System.out.println("his name is Fred."); else System.out.println("his name is not Fred. \n"); if( nickname == "Fred" ) System.out.println("his nickname is Fred.\n"); else System.out.println("his nickname is not Fred. \n"); System.out.println("Test by equals method "); if(hisname.equals("Fred")) System.out.println("his name is Fred.\n"); else System.out.println("his name is not Fred.\n"); if(nickname.equals("Fred")) System.out.println("his nickname is Fred.\n"); else System.out.println("his nickname is not Fred. \n"); } }

• Case insensitive test ("Y" or "y")

  if (input.equalsIgnoreCase("Y"))

• s.compareTo(t) < 0 means:
   s comes before t in the dictionary

• "car" comes before "cargo"

• All uppercase letters come before lowercase:  "Hello" comes before "car"

Comparing Objects

• == tests for identity, equals for identical content

• Rectangle box1 = new Rectangle(5, 10, 20, 30);
   Rectangle box2 = box1;

• Rectangle box3 = new Rectangle(5, 10, 20, 30);
   box1 != box3,

• but box1.equals(box3)
   box1 == box2

• Use equals to test whether two rectangles have the same contents.

 

• If you have your own class, you have to implement your own equals method.  It does not automatically exist.

Testing for null

• null reference refers to no object
   String middleInitial = null; // Not set
   if ( . . . )
      middleInitial = middleName.substring(0, 1);

• Can be used in tests:
   if (middleInitial == null)
      System.out.println(firstName + " " + lastName);
   else
      System.out.println(firstName + " " + middleInitial + ". " + lastName);

• Use ==, not equals, to test for null

• null is not the same as the empty string ""

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

1. the empty string ""?

2. the string " " containing a space?

3. null?

   Answer: (a) 0; (b) 1; (c) an exception is thrown.

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;

double y = 3 * (1.0 / 3);

1. a == "1"     use equals method

2. a == null

3. a.equals("")

4. a == b       use equals method

5. a == x       type not match

6. x == y       round off error

7. x - y == null   type not match

8. x.equals(y)     no equals method for double type

Answer: Syntactically incorrect: e, g, h. Logically questionable:  a, d, f.