General Java Questions I

to import all the classes in a package.

Answer: Strictly speaking, "import java.util.*;" does not import the whole of java.util. It

is an "import on demand" which imports any class or interface in java.util that is

needed. If the first import statement would have done the job, then HashMap is the

only class the second one would import.

No measurable differences in compile time performance. You can do the test, but I

suspect you would get identical byte code, so no difference in run time performance.

There is a practical difference when two packages contain classes with the same

name. Suppose you also imported java.awt.* and tried to declare a List. It would be

ambiguous. If you use the second form you would either import java.util.List or

java.awt.List.

There is also a documentation difference. The first one makes it clear exactly what

imported classes are being used. 

by Patricia Shanahan

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

getting that error, you are trying to load a file that isnt a class and so the class loader

in the JVM is throwing out that exception.

Make sure you transfer the class files to site in binary mode, rather than text or ASCII

mode. 

An error from the browser saying "cannot start applet ... bad magic number" usually

means that one of the class files on the server is corrupted.  

Replace your class binary files on the web server; clean up the cache of your

browser, and reload your applet.

Q: In java, I found a lot of methods, which let you enter a line (read (), readLine ()

e.c.t). They all wait until return is pressed, and then start providing you the

information.

Does anyone know if there is a read method available whit the desired behaviour, i.e.

which doesn t wait for return being pressed?

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