Comparison of Java and C++

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This is a comparison of the Java programming language with the C++ programming language.

Contents

[edit] Design aims

The differences between the C++ and Java programming languages can be traced to their heritage.

The different goals in the development of C++ and Java resulted in different principles and design tradeoffs between the languages.

C++ Java
backwards compatible with C backwards compatibility with previous versions
execution efficiency developer productivity
trusts the programmer protects the programmer
arbitrary memory access possible memory access only through objects
concise expression explicit operation
can arbitrarily override types type safety
procedural or object-oriented object-oriented
operator overloading meaning of operators immutable
powerful capabilities of language feature-rich, easy to use standard library

These principles combined with C++'s C heritage are responsible for most concrete language differences between Java and C++.

[edit] Language features

[edit] Syntax

  • Java syntax has a context-free grammar which can be parsed by a simple LALR parser. Parsing C++ is somewhat more complicated; for example, Foo<1>(3); is a sequence of comparisons if Foo is a variable, but it creates an object if Foo is the name of a class template.
  • C++ allows namespace level constants, variables, and functions. All such Java declarations must be inside a class or interface.
  • const in C++ indicates data to be 'read-only,' and is applied to types. final in Java indicates that the variable is not to be reassigned. For basic types such as const int vs final int these are identical, but for complex classes, they are different
C++ Java
const Rectangle r; final Rectangle r = new Rectangle();
r= anotherRectangle; //ILLEGAL r = anotherRectangle; // ILLEGAL
r.x = 5; // ILLEGAL, r is a const Rectangle r.x = 5; // LEGAL, r still refers to the same rectangle
  • C++ supports goto statements; Java does not, but its labelled break and labelled continue statements provide some structured goto-like functionality. In fact, Java enforces structured control flow, with the goal of code being easier to understand.
  • C++ provides some low-level features which Java lacks. In C++, pointers can be used to manipulate specific memory locations, a task necessary for writing low-level operating system components. Similarly, many C++ compilers support inline assembler. In Java, assembly code can still be accessed as libraries, through the Java Native Interface. However, there is significant overhead for each call.

[edit] Semantics

  • C++ allows a range of implicit conversions between native types, and also allows the programmer to define implicit conversions involving compound types. However, Java only permits widening conversions between native types to be implicit; any other conversions require explicit cast syntax.
    • A consequence of this is that although loop conditions (if, while and the exit condition in for) in Java and C++ both expect a boolean expression, code such as if(a = 5) will cause a compile error in Java because there is no implicit narrowing conversion from int to boolean. This is handy if the code were a typo for if(a == 5), but the need for an explicit cast can add verbosity when statements such as if (x) are translated from Java to C++.
  • For passing parameters to functions, C++ supports both true pass-by-reference and pass-by-value. As in C, the programmer can simulate by-reference parameters with by-value parameters and indirection. In Java, all parameters are passed by value, but object (non-primitive) parameters are reference values, meaning indirection is built-in.
  • Generally, Java built-in types are of a specified size and range; whereas C++ types have a variety of possible sizes, ranges and representations, which may even change between different versions of the same compiler, or be configurable via compiler switches.
    • In particular, Java characters are 16-bit Unicode characters, and strings are composed of a sequence of such characters. C++ offers both narrow and wide characters, but the actual size of each is platform dependent, as is the character set used. Strings can be formed from either type.
  • The rounding and precision of floating point values and operations in C++ is platform dependent. Java provides a strict floating-point model that guarantees consistent results across platforms, though normally a more lenient mode of operation is used to allow optimal floating-point performance.
  • In C++, pointers can be manipulated directly as memory address values. Java does not have pointers—it only has object references and array references, neither of which allow direct access to memory addresses. In C++ one can construct pointers to pointers, while Java references only access objects.
  • In C++ pointers can point to functions or methods (function pointers or functors). The equivalent mechanism in Java uses object or interface references.
  • C++ features programmer-defined operator overloading. The only overloaded operators in Java are the "+" and "+=" operators, which concatenate strings as well as performing addition.
  • Java features standard API support for reflection and dynamic loading of arbitrary new code.
  • Java has generics. C++ has templates.
  • Both Java and C++ distinguish between native types (these are also known as "fundamental" or "built-in" types) and user-defined types (these are also known as "compound" types). In Java, native types have value semantics only, and compound types have reference semantics only. In C++ all types have value semantics, but a reference can be created to any object, which will allow the object to be manipulated via reference semantics.
  • C++ supports multiple inheritance of arbitrary classes. Java supports multiple inheritance of types, but only single inheritance of implementation. In Java a class can derive from only one class, but a class can implement multiple interfaces.
  • Java explicitly distinguishes between interfaces and classes. In C++ multiple inheritance and pure virtual functions makes it possible to define classes that function just as Java interfaces do.
  • Java has both language and standard library support for multi-threading. The synchronized keyword in Java provides simple and secure mutex locks to support multi-threaded applications. While mutex lock mechanisms are available through libraries in C++, the lack of language semantics makes writing thread safe code more difficult and error prone.

[edit] Memory management

  • Java requires automatic garbage collection. Memory management in C++ is usually done by hand, or through smart pointers. The C++ standard permits garbage collection, but does not require it; garbage collection is rarely used in practice. When permitted to relocate objects, modern garbage collectors can improve overall application space and time efficiency over using explicit deallocation.
  • C++ can allocate arbitrary blocks of memory. Java only allocates memory through object instantiation. (Note that in Java, the programmer can simulate allocation of arbitrary memory blocks by creating an array of bytes. Still, Java arrays are objects.)
  • Java and C++ use different idioms for resource management. Java relies mainly on garbage collection, while C++ relies mainly on the RAII (Resource Acquisition Is Initialization) idiom. This is reflected in several differences between the two languages:
    • In C++ it is common to allocate objects of compound types as local stack-bound variables which are destructed when they go out of scope. In Java compound types are always allocated on the heap and collected by the garbage collector (except in virtual machines that use escape analysis to convert heap allocations to stack allocations).
    • C++ has destructors, while Java has finalizers. Both are invoked prior to an object's deallocation, but they differ significantly. A C++ object's destructor must be implicitly (in the case of stack-bound variables) or explicitly invoked to deallocate the object. The destructor executes synchronously at the point in the program at which the object is deallocated. Synchronous, coordinated uninitialization and deallocation in C++ thus satisfy the RAII idiom. In Java, object deallocation is implicitly handled by the garbage collector. A Java object's finalizer is invoked asynchronously some time after it has been accessed for the last time and before it is actually deallocated, which may never happen. Very few objects require finalizers; a finalizer is only required by objects that must guarantee some clean up of the object state prior to deallocation—typically releasing resources external to the JVM. In Java safe synchronous deallocation of resources is performed using the try/finally construct.
    • In C++ it is possible to have a dangling pointer – a reference to an object that has been destructed; attempting to use a dangling pointer typically results in program failure. In Java, the garbage collector won't destruct a referenced object.
    • In C++ it is possible to have an object that is allocated, but unreachable. An unreachable object is one that has no reachable references to it. An unreachable object cannot be destructed (deallocated), and results in a memory leak. By contrast, in Java an object will not be deallocated by the garbage collector until it becomes unreachable (by the user program). (Note: weak references are supported, which work with the Java garbage collector to allow for different strengths of reachability.) Garbage collection in Java prevents many memory leaks, but leaks are still possible under some circumstances.[citation needed]

[edit] Libraries

[edit] Runtime

  • C++ is normally compiled directly to machine code which is then executed directly by the operating system. Java is normally compiled to byte-code which the Java virtual machine (JVM) then either interprets or JIT compiles to machine code and then executes. In theory, dynamic recompilation can be used for either language, particularly Java, but at present, both languages are rarely, if ever dynamically recompiled.
  • Due to the lack of constraints in the use of some C++ language features (e.g. unchecked array access, raw pointers), programming errors can lead to low-level buffer overflows, page faults, and segmentation faults. The Standard Template Library, however, provides higher-level abstractions (like vector, list and map) to help avoid such errors. In Java, such errors either simply cannot occur or are detected by the JVM and reported to the application in the form of an exception.
  • The Java language requires specific behavior in the case of an out-of-bounds array access, which generally requires bounds checking of array accesses. This eliminates a possible source of instability but usually at the cost of slowing down execution. In some cases, compiler analyses can prove a bounds check unnecessary and eliminate it. C++ has no required behavior for out-of-bounds access of native arrays, thus requiring no bounds checking for native arrays. C++ standard library collections like std::vector, however, offer optional bounds checking. In summary, Java arrays are "always safe; possibly fast" while C++ native arrays are "always fast; potentially unsafe."

[edit] Miscellaneous

  • Java and C++ use different techniques for splitting up code in multiple source files. Java uses a package system that dictates the file name and path for all program definitions. In Java, the compiler imports the executable class files. C++ uses a header file source code inclusion system for sharing declarations between source files. (See Comparison of imports and includes.)
  • Compiled Java code files are generally smaller than code files in C++. First, Java bytecode is usually more compact than native machine code [citation needed]. Second, templates and macros in C++, including those in the standard library, can result in duplication of similar code after compilation. Third, dynamic linking with standard libraries eliminates binding the libraries at compile time.
  • C++ compilation features a textual preprocessing phase, while Java does not. Java supports many optimizations that mitigate the need for a preprocessor, but some users add a preprocessing phase to their build process for better support of conditional compilation.
  • In Java, arrays are container objects which you can inspect the length of at any time. In both languages, arrays have a fixed size. Further, C++ programmers often refer to an array only by a pointer to its first element, from which they cannot retrieve the array size. However, C++ and Java both provide container classes (std::vector and java.util.Vector respectively) which are resizable and store their size.
  • Java's division and modulus operators are well defined to truncate to zero. C++ does not specify whether or not these operators truncate to zero or "truncate to -infinity". -3/2 will always be -1 in Java, but a C++ compiler may return either -1 or -2, depending on the platform. C99 defines division in the same fashion as Java. Both languages guarantee that (a/b)*b + (a%b) == a for all a and b (b != 0). The C++ version will be sometimes be faster, as it is allowed to pick whichever truncation mode is native to the processor.
  • The sizes of integer types is defined in Java (int is 32-bit, long is 64-bit), while in C++ the size of integers and pointers is compiler-dependent. Thus, carefully-written C++ code can take advantage of the 64-bit processor's capabilities while still functioning properly on 32-bit processors. However, C++ programs written without concern for a processor's word size may fail to function properly with some compilers.[citation needed] In contrast, Java's fixed integer sizes mean that programmers need not concern themselves with varying integer sizes, and programs will run exactly the same [citation needed]. This may incur a performance penalty since Java code cannot run using an arbitrary processor's word size.

[edit] Performance

Main article: Computing performance

This section compares the relative computing performance of C++ and Java on common operating systems such as Windows and Linux.

Early versions of Java were significantly outperformed by statically compiled languages such as C++. This is because the program statements of these two closely related Level 6 languages may compile to a few machine instructions with C++, while compiling into several byte codes involving several machine instructions each when interpreted by a Java JVM. For example:

Java/C++ statement C++ generated code Java generated byte code
vector[i]++; mov edx,[ebp+4h]

mov eax,[ebp+1Ch]
inc dword ptr [edx+eax*4]

 aload_1

iload_2
dup2
iaload
iconst_1
iadd
iastore

While this may still be the case for embedded systems because of the requirement for a small footprint, advances in just in time (JIT) compiler technology for long-running server and desktop Java processes has closed the performance gap and in some cases given the performance advantage to Java. In effect, Java byte code is compiled into machine instructions at run time, in a similar manner to C++ static compilation, resulting in similar instruction sequences.

Several studies of mostly numerical benchmarks showing mixed results, argue that Java should be faster than C++ for a number of reasons:[1][2]

  • pointers make optimization difficult since they may point to arbitrary data or code
  • newly allocated memory is close together because garbage collection compacts memory and allocations are thus sequential in memory; and hence more likely to be 'in the cache'
  • run-time compilation can do a better job because it knows what processor it is running on and what code is running - the hot spots

Static compilers can be given hints, using command line switches or pragmas, regarding the processor and other optimization options, but these choices may be wrong for where and when the code is actually run. JIT technology has achieved the next level in compiler evolution; from hints and directives, through optimization based on historic measurement, to optimization using current run-time measurements.

One comprehensive study of microbenchmarks shows quite a large variation in results but indicates that, in general, Java outperforms C++ in operations such as memory allocation and file I/O while C++ outperforms Java in arithmetic and trigonometric operations.[3] For numerical processing, Java has shown significant gains with each new version but still lags C++ and Fortran, in part due to the requirement for reproducibility of floating-point results across all platforms.[4]

[edit] References

  1. ^ "Performance of Java versus C++" by J.P. Lewis and Ulrich Neuman, USC, Jan. 2003 (updated 2004)
  2. ^ "Java will be faster than C++" by Kirk Reinholtz, JPL, Apr 2001
  3. ^ "Microbenchmarking C++, C# and Java" by Thomas Bruckschlegel, Dr. Dobbs, June 17, 2005
  4. ^ "Java and Numerical Computing" by Ronald F. Boisvert, José Moreira, Michael Philippsen and Roldan Pozo, NIST, Dec 2000

[edit] See also

[edit] External references

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