Fundamentals - SCJP Sun Certified Programmer for Java Platform SE6 - page 57

The following ReferenceDemo program demonstrates comparing references. Examine the
code and try to determine its output.
1. import java.io.File;
2. import java.util.Date;
3.
4. public class ReferenceDemo {
5. public static void main(String [] args) {
6. File f1 = new File(“mydata.txt”);
7. File f2 = new File(“mydata.txt”);
8. if(f1 != f2) {
9. System.out.println(“f1 != f210.
11. }
12. Date today = new Date();
13. Date now = today;
14. if(today == now) {
15. System.out.println(“today == now”);
16. }
17.
18. String s1 = “Hello”;
19. String s2 = “Hello”;
20. if(s1 == s2) {
21. System.out.println(“s1 == s2”);
22. }
23.
24. String x1 = new String(“Goodbye”);
25. String x2 = new String(“Goodbye”);
26. if(x1 == x2) {
27. System.out.println(“x1 == x2”);
28. }
29. }
30. }
Let's study this program. The references f1 and f2 point to two different File objects,
so the two references cannot be equal. It is irrelevant that the two File objects look the
same in memory; they are clearly two different objects so their references are not equal. On
the other hand, there is only one Date object in memory and today and now both point to
it, so today == now is true .
Comparing String references in Java tends to be confusing because of how the JVM
treats string literals. Because String objects are immutable, the JVM can reuse string
literals for effi ciency and to save memory. Because “Hello” is a String literal known at
compile time, the JVM only creates one “Hello” object in memory, and s1 and s2 both