I've been programming in C++ for the last two years, but now I have to learn Java. The problem is that I placed out of a CS course with AP credit, but I barely learned any Java in APCS.

What would the best way to go about this be? What are the most important differences that would trip up a C++ programmer? What do you think that a person who has taken an introductory college course in Java would be expected to know?

I made the same transition: 2 years of C++ in college, transferred to a different school for my BS and had to switch to Java.

What tripped me up was the transition to "reference by default". Where C++ passes by value, Java will always pass by reference because all objects are treated like references. (and if I just horribly mis-described that, someone more knowledgeable feel free to smite me, this is just how it was explained to me).

I miss operator overloading and copy constructors as well, but you can hack around that with .copy() and .add()/.subtract() methods (I did this while putting together a vector class for a game programming course).

As well: no header/implementation file woes, it's all in one file (one file per class). No free-floating methods either, they have to belong to a class. Static classes will let you call methods without a class instance (so if I create a static Math class, I can use its methods without creating an instance of that class, etc)

Your data structures and programming paradigms should mostly be unchanged past that...the Java API is a great reference for little differences, and I found the programming tests at CodingBat (formerly JavaBat) to be great hands-on practice in Java.

Quote:Original post by Storyyeller
What are the most important differences that would trip up a C++ programmer?

No pointers and the likes - in Java all class types are reference types while primitive types are value types and all references and values are passed-by-value.

Boxing - primitive types like int turning into reference types like Integer and vice versa.

Anonymous inner classes - Pretty straightforward and self-descriptive, you just don't get them in C++, that's all.

No default parameters on methods - Sorry. At least you can chain constructors, unlike C++.

No copy constructors - There is an element of 'duh' about this when you wrap your head around the lack of value types for classes. You can make things that look and work similarly to copy constructors and that's fine but obviously, unlike C++, Java isn't treating it as anything special.

Garbage Collection - Memory is managed for you, almost...

Non-deterministic destruction - You can't guarantee when or if an object will be finalised by the garbage collector so for objects that wrap unmanaged resources, like a file stream, you need to remember to call close() else you leak resources. Which brings me swiftly to:

Almost no RAII - One of the best features of C++ is RAII and Java almost totally lacks it although you can glean it back with complex arrangements of try-catch-finally blocks and perhaps clever use of exceptions, but it's barely a substitute IMHO. I gather the next version of Java may be adding RAII, I hope so.

Checked and unchecked exceptions - Some exceptions you absolutely have to catch and handle; if you don't then you have have to explicitly declare them on the method signature so that the caller has to handle them. All of C++'s exceptions behave like unchecked exceptions; Java has both kinds.

Off the top of my head that's all I can think of. Of course there are other differences but they shouldn't be too surprising or pitfall-like.

Quote:What do you think that a person who has taken an introductory college course in Java would be expected to know?

Only the basics, conditional logic, loops, methods, classes etc. If you know all these things already then an introductory Java class will be a doddle.

[Edited by - dmatter on August 23, 2010 11:47:54 AM]

Hilarious Antheus. You should be a stand-up. I don't think that kind of post belongs in For Beginners.

Upon the first lines of reading, I thought that would be all serious. Then the entire CS knowledge thingy made me thinky for some seconds, and then I roffled all over the place. As often as I disagree with Antheus (I think), this one made me rate him up without any further ado.

If you know singleton you don't need to know any other patterns because it replaces all of them cause you only need one, doesn't it?