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General Java Questions II
ordered path? Vector does this but i need the key/value functionality. hashmaps do
not guarantee the order of the objects.
Answer: Take a look at java.util.TreeMap.
Red-Black tree based implementation of the SortedMap interface. This class
guarantees that the map will be in ascending key order, sorted according to the
natural order for the key's class (see Comparable), or by the comparator provided at
creation time, depending on which constructor is used.
Note that this implementation is not synchronized. If multiple threads access a map
concurrently, and at least one of the threads modifies the map structurally, it must be
synchronized externally.
Most people asked why there is an error, but my question is why this is NOT an
error
Please take a look:
r is a number and s is a character, why can I put them together to make a comparison
without compilation error? Could somebody tell me... thank you
double r = 34.5;
char s = 'c';
if (r > s) {
System.out.println("r > s");
} else {
System.out.println("r < s");
}
Answer: Yes, char is indeed a 16-bit value. However, the actual answer is in the Java
Language Specification, section 5.6.2, which is at the following URL:
http://java.sun.com/docs/books/jls/second_edition/html/conversions.doc.html#170983
In summary, the char is automagically promoted to a double. No explicit cast is
necessary since the language rules say that it gets "promoted" to a double
by John O'Conner
Q: == and equals ()... These two still make me confuse a lot of time.
Can somebody give me some thumb rule or explain it to me?
Answer: When you use == with a primitive -int, double, char, ... you are checking that
the values are identical. But if you use == with an object, you are checking that the 2
objects are stored at the same address. In other words the references pointing to the
same object...
Method equals () is different.
It is the same as ==, if it isn't overriden by the object class.
Many classes override the method equals (). In this case this method will check that
content of the object is the same or not, not addresses.
Q: Why do I get message like “wrong magic number” when I am trying to run
applet? What is magic number?
Answer: The first thing a JVM does when it loads a class is check that the first four
bytes are (in hex) CA FE BA BE. This is the "magic number" and thats why you are
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