Erich's Java cheat sheet for C++ programmers

© Erich Kaltofen

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C++ Java
assignment operator= cannot be user-defined for a class and performs assignment of a reference to the instance of the class (see also reference types)
basic_string String and StringBuffer
bool boolean
char byte
const variables/data members final variables/fields
copy constructor no default; one implements the interface Cloneable by the method Object clone(), which can be an abstract (in C++ notion: virtual) method
data members fields, so-called instance variables (a term borrowed from Smalltalk)
delete does not exist; all unreferenced memory is garbage collected
derived classes subclasses; the keyword extends replaces C++'s colon.
destructors ~Class protected void finalize(); note, however, that these are used for freeing resources other than memory and are therefore rarely needed
exceptions, try, catch, throw, std:exception same concept; Java adds a keyword throws that is used to declare the exceptions a method throws; the hierarchy of exceptions is rooted in java.lang.Exception; a finally block is introduced to contain all common clean-up code.
extern "C" functions native methods
functions do not exist; static methods (``class methods'') are used
#include does not exist; the paths to the files are known and can be made know in the CLASSPATH environment variable
input/output: istream& operator», ostream& operator« and System.out are the streams; Java has number formatting tools in java. lang. Number and java. text. Format. NumberFormat
main(int argc, char* argv[]) public static void main(String [] args) within a public class
member functions methods
multiple inheritance does not exist; however, interfaces provide a weak form of multiple inheritance.
namespaces packages
namespace Namespace{...} packagePackage; which must appear as the first line in the file
nested (member, inner) classes Java 1.1 has static (``top-level'') and non-static (``member'') inner classes, as well as local classes and anonymous classes. Member classes can refer to the members of the outer class and to OuterClass.this; they cannot have the name of an outer class and cannot declare static members.
new Class(...) new Class(...), which returns a reference to the created object
NULL (the 0 pointer value) and the type void* null in Java is a keyword and represents an uninitialized reference
overloaded operators do not exist; however, methods can be overloaded. This may be a major shortcoming of Java, as one cannot revise old Java code by redefining the operators used (cf. MITMatlab)
passing arguments to base class constructor place the statement super(...); as the first statement in the subclass's constructor
public, private, protected modifiers similar as in C++; visibility of classes and nested classes can be also restricted; there are no friends, but within the same package protected members are visible
purely virtual member functions abstract methods; the enclosing class must also be declared abstract
reference types Type& all Java types except scalar primitive types are reference types; note that the method
void swap(T a, Tb) {T t; t = a; a = b; b = t;}
does nothing to its arguments.
scope resolution, operator :: does not exist; methods must be defined inside the class declaration. If a base class field is to be explicitly referred, one uses typecasting: ((Baseclass)Variable).Member; a direct base class member can be referred to by super.Member; typecasting has no effect on methods (see virtual member functions).
static data members static fields, so-called class variables; they are accessed by Class.Field rather than the C++ Variable.Member; they can be initialized by =...; within the class definition and need not be declared outside like C++ static data members.
static member functions static methods, so-called class methods; they are defined within the class declaration, unlike in C++.
this this, which is a reference to the object and has the type of the class, not a pointer; note that the call this(...); as the first statement in a constructor invokes a constructor call for the matching argument types.
traits marker interfaces
type_id instanceof; this is an operator returning a boolean, not a ``type_info'' as in C++.
using namespace Package; import Package.*;
virtual member functions in Java, all methods use dynamic method lookup and therefore are be default virtual. There is no way to explicity call an overridden base class method, but overwriting can be prevented by declaring a method final.
wchar_t char
wide character stream wostream PrintWriter replaces PrintStream that cannot hold unicode; the constructor of PrintStream has been deprecated in Java 1.1, but System.out is not.

Java concepts missing in C++
abstract windows toolkit AWT standard library for building a GUI
concatenation of strings by + operator  
documentation comments can be processed (e.g., by javadoc) for automatic online documentation
final methods those cannot be overridden by a subclass
interfaces are used to denote abstract classes without any method of their own. They can have static final fields. One class can implement several interfaces, but it must implement the abstract methods of each interface.
reflection allows the inspection of a class (which arguments does which member take? etc.); this is critical for plug-and-play design, such as a Java bean
right shift operator with zero extension >>>  
serialization C++ requires the programmer to implement object serialization member functions

C++ concepts missing in Java
const member functions do not exist; final methods cannot be overridden by subclasses
friend classes, functions do not exist; however, protected members are visible within the same package
goto is a reserved work in Java, but is not supported by the language; however break and continue statements can give a statement label
multiple inheritance virtual base classes seem unachievable by using interfaces
new(Pointer) Type(...); Pointer->~Type(); this is C++'s explicit memory allocation mechanism. In Java, all memory is managed by the VM and garbage collection is automatic. Thus, in C++, a garbage collector can be implemented, while in Java a memory manager cannot.¶
pointer types Type* do not exist; actually, since Java has only reference types, all variables are some kind of pointers and the = operator behaves like a pointer assignment
pointer to function, member not a serious restriction, as one may encapsulate a function in a function object
standard template library STL java.util.Vector provides an expandable vector. Java 1.2 provides Collections, which are essentially C++ STL containers, but many of the members are renamed. Note that List is a scrollable list in the AWT. There are third-party vendor container packages: See austern_mti/java/index.html, http://www.objectspace. com/developers/jgl/downloads/index.html§
templates there is a the GJ compiler http://www.cs.bell-labs. com/~wadler/pizza/gj/.§ C++'s template expansion mechanism is a full-fledged programming language and has been used for compiler optimization task (e.g., in the Blitz++ matrix library)
typedef asside as a shorthand, typedefs can be encapsulated in a class scope to provide a generic type; they function as assignments in template meta-programming.
¶Laurent Bernardin points out that this isn't exactly true: place all objects on arrays/lists for reuse
§These references were provided by Thierry Gautier

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Erich's Java cheat sheet for C++ programmers

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Erich Kaltofen 2002-11-04