First, two big things--the main function and how to compile it, followed by lots of little differences.

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" );

    }

}

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>

Same in both languages (// and /* */ both work)

Class declarations 类的声明

Almost the same, but Java does not require a semicolon

C++

    class Bar {};

Java

    class Bar {}

Method declarations  方法的声明

Same, except that in Java, must always be part of a class, and may prefix with public/private/protected

Constructors and destructors  构造函数与析构函数

Constructor has same syntax in both (name of the class), Java has no exact equivalent of the destructor

Static member functions and variables 静态函数和变量

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; }

}

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();

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();

C++

If you're using a stack-based object, you access its fields with a dot:

myClass x;

x.my_field; // ok

But 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

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

C++

    class Foo : public Bar

    { ... };

Java

    class Foo extends Bar

    { ... }

C++

    public:

        void foo();

        void bar();

Java

    public void foo();

    public void bar();

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();

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 */ }

    }

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;

Java is a bit more verbose(冗长的): you must write boolean instead of merely bool.

C++

bool foo;

Java

boolean foo;

C++

    const int x = 7;

Java

    final int x = 7;

First, Java enforce throw specs at compile time--you must document if your method can throw an exception

C++

int foo() throw (IOException)

Java

int foo() throws IOException

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

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 );

    }

By Alex Allain