Java Tips

I'm not a frequent Java programmer, so things presented here may not be 100% correct.

Getting started

Install Java
Write some Java code, say Hello.java
Compile Hello.java to Hello.class, by running command line:
javac Hello.java
Write a HTML web page Hello.html that uses the applet Hello.class. At minimum, put one line
<applet code="Hello.class"></applet>
in the web page. More about HTML.
Open Hello.html with your web browser to view the applet. Please be aware of a cache problem if you modified the applet.
Alternatively you can type
appletviewer Hello.html
Note, the argument is the HTML file instead of the applet itself.

Several weird rules:

One public class in a file, File name = class name + ".java", i.e., Hello.java in the above case.
When running a java program:
java Hello is good;
java Hello.class is bad.
A cache problem: applets do not appear to be updated in Firefox after you change the code. This is because Java caches .class files, and is not clever enough to update them timingly.
Solution:
1. Open Java console: in my system Ubuntu Linux, it's in System → Preference → Sun Java 6 Plugin Control Panel.
2. In Advanced tab, expand Java console, choose Show console
3. Restart Firefox, now every time you open a web page an applet, a Java console window pops up, press the letter x on your keyboard while in that windows to clear the cache.
4. Now changes should appear properly, after pressing F5 in Firefox.
5. Note, you still need to compile .java files to .class files.

To use math functions import static java.lang.Math.*;, then use sin(x); Note, without static, you'll have to write Math.sin(x).
To convert string to integer: Integer.parseInt(s) (equivalent to C's atoi(s), or Python's int(s)); to Double: Double.parseDouble(s).
To format a real number to three decimal places import java.text.*; and NumberFormat df = new DecimalFormat("#0.00"); then String s = df.format(x); (equivalent to C's sprintf(s, "%.2f", x);, or Python's s = "%.2f" % x)
To allocate an array, e.g. int arr[] = new int[cnt]; allocates an integer array of cnt elements. For a two dimensional array: int arr[][] = new int[3][4];

Pros:

Cons:

Tutorials