First, two big things--the main function and how to compile it, followed by lots of little differences.
main function
C++
// free-floating function int main( int argc, char* argv[]) { printf( "Hello, world" ); }
Java
// every function must be part of a class; the main function for a particular // class file is invoked when java <class> is run (so you can have one // main function per class--useful for writing unit tests for a class) class HelloWorld { public static void main(String args[]) { System.out.println( "Hello, World" ); } }
Compiling
C++
// compile as g++ foo.cc -o outfile // run with ./outfile
Java
// compile classes in foo.java to <classname>.class javac foo.java // run by invoking static main method in <classname> java <classname>
Comments
Same in both languages (// and /* */ both work)Class declarations
Almost the same, but Java does not require a semicolonC++
class Bar {};
Java
class Bar {}
Method declarations
Same, except that in Java, must always be part of a class, and may prefix with public/private/protectedConstructors and destructors
Constructor has same syntax in both (name of the class), Java has no exact equivalent of the destructorStatic member functions and variables
Same as method declarations, but Java provides static initialization blocks to initialize static variables (instead of putting a definition in a source code file):class Foo { static private int x; // static initialization block { x = 5; } }
Scoping static methods and namespaces
C++
If you have a class and wish to refer to a static method, you use the form Class::method.class MyClass { public: static doStuff(); }; // now it's used like this MyClass::doStuff();
Java
All scoping in Java uses the . again, just like accessing fields of a class, so it's a bit more regular:class MyClass { public static doStuff() { // do stuff } } // now it's used like this MyClass.doStuff();
Object declarations
C++
// on the stack myClass x; // or on the heap myClass *x = new myClass;
Java
// always allocated on the heap (also, always need parens for constructor) myClass x = new myClass();
Accessing fields of objects
C++
If you're using a stack-based object, you access its fields with a dot:myClass x; x.my_field; // okBut you use the arrow operator (->) to access fields of a class when working with a pointer:
myClass x = new MyClass(); x->my_field; // ok
Java
You always work with references (which are similar to pointers--see the next section), so you always use a dot:myClass x = new MyClass(); x.my_field; // ok
References vs. pointers
C++
// references are immutable, use pointers for more flexibility int bar = 7, qux = 6; int& foo = bar;
Java
// references are mutable and store addresses only to objects; there are // no raw pointers myClass x; x.foo(); // error, x is a null ``pointer'' // note that you always use . to access a field
Inheritance
C++
class Foo : public Bar { ... };
Java
class Foo extends Bar { ... }
Protection levels (abstraction barriers)
C++
public: void foo(); void bar();
Java
public void foo(); public void bar();
Virtual functions
C++
virtual int foo(); // or, non-virtually as simply int foo();
Java
// functions are virtual by default; use final to prevent overriding int foo(); // or, final int foo();
Abstract classes
C++
// just need to include a pure virtual function class Bar { public: virtual void foo() = 0; };
Java
// syntax allows you to be explicit! abstract class Bar { public abstract void foo(); } // or you might even want to specify an interface interface Bar { public void foo(); } // and later, have a class implement the interface: class Chocolate implements Bar { public void foo() { /* do something */ } }
Memory management
Roughly the same--new allocates, but no delete in Java since it has garbage collection.NULL vs. null
C++
// initialize pointer to NULL int *x = NULL;
Java
// the compiler will catch the use of uninitialized references, but if you // need to initialize a reference so it's known to be invalid, assign null myClass x = null;
Booleans
Java is a bit more verbose: you must write boolean instead of merely bool.C++
bool foo;
Java
boolean foo;
Const-ness
C++
const int x = 7;
Java
final int x = 7;
Throw Spec
First, Java enforce throw specs at compile time--you must document if your method can throw an exceptionC++
int foo() throw (IOException)
Java
int foo() throws IOException
Arrays
C++
int x[10]; // or int *x = new x[10]; // use x, then reclaim memory delete[] x;
Java
int[] x = new int[10]; // use x, memory reclaimed by the garbage collector or returned to the // system at the end of the program's lifetime
Collections and Iteration
C++
Iterators are members of classes. The start of a range is <container>.begin(), and the end is <container>.end(). Advance using ++ operator, and access using *. vector myVec; for ( vector<int>::iterator itr = myVec.begin(); itr != myVec.end(); ++itr ) { cout << *itr; }
Java
Iterator is just an interface. The start of the range is <collection>.iterator, and you check to see if you're at the end with itr.hasNext(). You get the next element using itr.next() (a combination of using ++ and * in C++). ArrayList myArrayList = new ArrayList(); Iterator itr = myArrayList.iterator(); while ( itr.hasNext() ) { System.out.println( itr.next() ); } // or, in Java 5 ArrayList myArrayList = new ArrayList(); for( Object o : myArrayList ) { System.out.println( o ); }