Java Basics9-2-2004Opening DiscussionWhat did we talk about last class?What are the basic constructs in the programming languages you are familiar with? From reading or prior knowledge, how are those constructs different in Java?CS Majors mailing list.Minute essay commentsThe designs in general are just good javadoc comments. For Assn1 a longer one will go above the main class.Code formattingProblems to PresentDoes anyone have any problems to present?The ideal way to do this is for you to have the code in Eclipse so we can see it and you can run it.You can only present one each day yourself.CVSA problem you might face is getting code from your room to up on these machines. CVS can help you here.One of the great features of Eclipse is that it integrates nicely with CVS (concurrent versioning system). This gives a lot more power than just moving code around, but the moving code around works well also.Let's look at how you can do this.Java as a LanguageIn Java everything is in classes. There are no free standing functions or global variables. Also, the code for methods always goes inside the class declaration.All methods and members are individually given public or private specification. They can also be declared static or final. Final implies they can’t be extended by inheritance. Final variables can’t be changed after initialization.Java LibrariesOne of the real powers of Java is it’s libraries. These are grouped into packages. (You could specify packages for your code but we won’t discuss that this semester.)For example, the class System is in the java.lang package. The full name of the class is java.lang.System.No Preprocessor DirectivesYou import so you don’t have to type in full package names. This looks similar to #include in use, but it is quite different.No #define in Java. For constants use static final variables. For macros just use functions.There is also no conditional compilation in Java so #ifdef, #ifndef, etc. don’t exit. Assert has been added in 1.4.Primitive Types in JavaJava is not purely object-oriented because it does have primitive types. These types are boolean, char, byte, short, int, long, float, and double.Note that booleans and chars are NOT ints in Java (though you can cast chars to ints). This is significant because the statement if(v=3)does not compile. This helps cut down on bugs but might seem restrictive in some cases.Primitives as ClassesWhen you need to represent a primitive type as a class there are some classes in java.lang that can help.They are classes like Integer and Double that are basically wrapper classes.They do have some nice functionality in static methods as well like Integer.parseInt(String s).These classes are immutable.Java References vs. PointersIn Java when you declare an object you are really declaring a reference to an object. This is like a pointer but you can’t do pointer arithmetic. To get a real object you use the new operator. New is like malloc and returns a heap object.All objects are gotten with new so all objects exist on the heap.null is a universal symbol for references that don’t point to anything.More on ObjectsThere is no operator overloading in Java. You write and call normal methods instead.Doing = or == with object references assigns or compares references. Think of them as pointers without the *.Use a copy constructor to copy and equals() for value comparison.No -> or * (deref) operator because things are implicitly dereferenced.Garbage CollectionJava has automatic garbage collection. There is no delete operator (no free).When you are done using an object and you have no more reachable references to it, the system can determine this and free up the memory for it.As a result, you can allocate objects much more freely because you don’t have to worry about freeing them.ArraysArrays in Java are actually objects. An array type is denoted by placing [] after the normal type. When you do a new you are allocating the array object. If the items in it are objects you still have to allocate the individual objects.This is really nice for inclusion polymorphism.You use them like you would in C/C++, though they do have a length member that you can access to find out how many elements are in an array.StringsLike arrays, strings are a class in Java, called String. Literal strings in Java are objects of that class too.This class is in the java.lang package. Note that java.lang is the one package that is implicitly imported so you don’t have to use the import statement to refer to the classes in it directly.ExceptionsJava tries to prevent sloppy error handling by providing exceptions instead of requiring programmers to check return codes.try blocks surround code that might throw exceptions.catch blocks follow a try block and specify what type it is waiting for.finally catches everything and always happens.If an exception type can be thrown but isn’t caught, it must be in the throws clause of that method. Not true for RuntimeExceptions.Inheritance in JavaJava allows inheritance for subtyping and reuse like C++ does.Java has single inheritance of classes, but it also has things called interfaces. You can have multiple inheritance of interfaces.An interface is like a class except that it only has method signatures. Classes that implement it have to define those methods or be abstract.Java and the JVMJava code does not typically compile to executable format directly. Instead it compiles to bytecode that is stored in .class files. The bytecode was designed to be very small so that it can be easily moved across the net or put on small devices. It also allows code to run on many platforms without recompiles.In addition, this allows classes to be dynamically loaded.Let’s CodeNow we will sit down and write a bit of code just to show you what it looks like. We’ll do this until time comes for you to start minute essays.Minute EssayBoth the String class and the primitive wrapper classes in Java are immutable. This means that once created they can’t be changed. How does using classes like this impact your programming? It can have both good and bad effects. What are
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