转：C++和Java的语法对比

因为要同时用到C++和Java，很多东西容易搞混，这里转了一个比较好的贴子：C++和Java的语法对比

地址：http://www.cprogramming.com/tutorial/java/syntax-differences-java-c++.html

以下是英文原帖，还有一个翻译过来的帖子供参考：http://www.cppprog.com/2009/0424/107.html

C++ and Java Syntax Differences Cheat Sheet

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 semicolon

C++

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/protected

Constructors and destructors

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

Static 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; }

}

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();

References vs. pointers

C++

// references are immutable, use pointers for more flexibility

int bar = 7, qux = 6;

int& foo = bar;

Java

// references are mutuable 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 exception

C++

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 );
}

Templates

This is still being to be added. See http://java.sun.com/j2se/1.5/pdf/generics-tutorial.pdf
for a good introduction.