When an application loads classes using multiple ClassLoaders, unexpected behaviours can arise. For example, consider a class loaded by two different classloaders: Result is: All…
If you’re invoking external processes with Runtime.getRuntime().exec() or ProcessBuilder.start(), it is vitally important to handle the standard out and standard error streams. Otherwise, the parent process may block – it gets stuck without throwing an Exception to tell you why.
How do you delete a directory in Java? java.io.File defines a method delete() that deletes a file or directory:
public void delete(File dir) {
dir.delete();
}
If only it were that simple.
Here’s a silly gotcha in the Java language. I’ve got a method that parses a String to an int and returns a default value if the String cannot be parsed:
int parseOrDefault(String val, int def) {
try {
return Integer.parseInt(val);
} catch (NumberFormatException e) {
return def;
}
}
Java’s ScheduledExecutorService allows you to schedule Runnable tasks without having to worry too much about creating Threads. At its simplest, you can schedule a task like this:
Runnable task = () -> System.out.println("Hello world!");
executor.schedule(task, 10, TimeUnit.SECONDS);
System.out.println("Done!");
The output is:
Done!
Hello world!
That is we schedule the task, then print “Done!”. 10 seconds later the scheduled task executes and prints “Hello world!”.
But what happens if the Runnable throws an Exception?
The Execute Around idiom is a pattern that allows you to wrap an action with some standard setup / tear down steps. Examples might include:
The pattern allows you to pass in arbitrary actions and have them run with the same setup / tear down steps.
How do you reverse order a Stream in Java? There’s no obvious Stream.reverse() method. Is this an oversight in the API? Is there a workaround?
I’m studying for SCJP – currently tackling a rather dull section on regular expressions. I’ve just noticed that the exam does not cover BigDecimal. I really don’t understand this. BigDecimal is essential for any fixed precision decimal representation and calculation. Put another way: if you’re not aware of BigDecimal, you can’t effectively represent money in your program. Surely more useful than awareness of the \w, \s and \d regex metacharacters, no?
Spotted this nasty little quirk today – not for the first time. Every time I see it I want to get angry at someone but I’m not quite sure who.
What’s wrong with this list of properties?
private String name; private String address; private String eMail;
Nothing immediately obvious perhaps. Here are the standard generated getters and setters (signatures only):
public String getName(); public void setName(String name); public String getAddress(); public void setAddress(String address); public String getEMail(); public void setEMail(String eMail);
All perfectly valid. Now, given these auto-generated getters and setters, what names would standard bean introspection derive?
name address EMail
Hang on, what happened there? How did my eMail property turn into EMail?