Perbandingan -- Java and C++

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

Design aims

The differences between the C++ and Java programming languages can be traced to their heritage, as they have different design goals.

• C++ was designed for systems and applications programming, extending the C programming language. To this procedural programming language designed for efficient execution, C++ has added support for statically typed object-oriented programming, exception handling, scoped resource management, and generic programming, in particular. It also added a standard library which includes generic containers and algorithms.
• Java was created initially as an interpreter for printing systems but grew to support network computing^{[disambiguation needed]}. It was once used as the base for the "HotJava" thin client system. It relies on a virtual machine to be secure and highly portable. It is bundled with an extensive library designed to provide a complete abstraction of the underlying platform. Java is a statically typed object-oriented language that uses similar (but incompatible) syntax to C++. It was designed from scratch with the goal of being easy to use and accessible to a wider audience. It includes an extensive documentation called Javadoc.

The different goals in the development of C++ and Java resulted in different principles and design trade-offs between the languages. The differences are as follows :

Language features

Syntax

• Java syntax has a context-free grammar that can be parsed by a simple LALR parser. Parsing C++ is more complicated. For example, Foo<1>(3); is a sequence of comparisons if Foo is a variable, but creates an object if Foo is the name of a class template.
• C++ allows namespace-level constants, variables, and functions. In Java, such entities must belong to some given type, and therefore must be defined inside a type definition, either a class or an interface.
• In C++, objects are values, while in Java they are not. C++ uses value semantics by default, while Java always uses reference semantics. To achieve reference semantics in C++, either a pointer or a reference can be used.

class Foo {   // Declares class Foopublic: int x;   // Member variable Foo(): x(0) {}   //  Constructor for Foo; initializes //  x to 0. If the initializer were //  omitted, the variable would not //  be initialized to a specific //  value. int bar(int i) { // Member function bar() return 3*i + x; }};

class Foo {   // Defines class Foo public int x; // Member variable,  //initialized to 0 by default public Foo() { // Constructor for Foo } public int bar(int i) {// Member method bar() return 3*i + x; }}

Foo a;// declares a to be a Foo object value,// initialized using the default constructor.// Another constructor can be used asFoo a(args);

Foo a;// declares a to be a reference to a Foo objecta = new Foo();// initializes using the default constructor// Another constructor can be used asFoo a = new Foo(args);

Foo b = a;// copies the contents of a to a new Foo object b;// alternative syntax is "Foo b(a)"

Foo b = a.clone();// copies the contents of the object pointed to by a to a new Foo object;// sets the reference b to point to this new object;// the Foo class must implement the Clonable interface for this code to compile

a.x = 5; // modifies the object a

a.x = 5; // modifies the object referenced by a

cout << b.x << endl;// outputs 0, because b is// a different object than a

System.out.println(b.x);// outputs 0, because b points to// a different object than a

Foo *c;// declares c to be a pointer to a// Foo object (initially// undefined; could point anywhere)

Foo c;// declares c to be a reference to a Foo// object (initially null if c is a class member;// it is necessary to initialize c before use// if it is a local variable)

c = new Foo;// binds c to reference a new Foo object

c = new Foo();// binds c to reference a new Foo object

Foo *d = c;// binds d to reference the same object as c

Foo d = c;// binds d to reference the same object as c

c->x = 5;// modifies the object referenced by c

c.x = 5;// modifies the object referenced by c

a.bar(5);  // invokes Foo::bar() for ac->bar(5); // invokes Foo::bar() for *c

a.bar(5); // invokes Foo.bar() for ac.bar(5); // invokes Foo.bar() for c

cout << d->x << endl;// outputs 5, because d references the// same object as c

System.out.println(d.x);// outputs 5, because d references the// same object as c

• In C++, it is possible to declare a pointer to a const type in order to prevent client code from modifying the object pointed to by that pointer. Functions and methods can also guarantee that they will not modify the object pointed to by a pointer by using the "const" keyword. This enforces const-correctness.
• In Java, const-correctness is enforced by the semantics of the class of which a given object is an instance. Client code is prevented from changing the internal state of an object if and only if every accessible member of the class is labeled final.

const Foo *a; // it is not possible to modify the object  // pointed to by a through a

final Foo a; // a declaration of a "final" reference: // it is possible to modify the object,  // but the reference will constantly point  // to the first object assigned to it

a = new Foo();

a = new Foo(); // Only in constructor

a->x = 5;// ILLEGAL

a.x = 5;// LEGAL, the object can still be modified

Foo *const b = new Foo();// a declaration of a "const" pointer

final Foo b = new Foo();// a declaration of a "final" reference

b = new Foo();//ILLEGAL, it is not allowed to re-bind it

b = new Foo();// ILLEGAL, it is not allowed to re-bind it

b->x = 5;// LEGAL, the object can still be modified

b.x = 5;// LEGAL, the object can still be modified

• C++ supports goto statements, which may lead to Spaghetti programming. With the exception of the goto statement, both Java and C++ have basically the same control flow structures, designed to enforce structured control flow, and relies on break and continue statements to provide some goto-like functionality. Some commenters point out that these labelled flow control statements break the single point-of-exit property of structured programming.^[3]
• C++ provides 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 an inline assembler.

In Java, such code must reside in external libraries, and can only be accessed through the Java Native Interface, with a significant overhead for each call.

Semantics

• C++ allows default values for arguments of a function/method. Java does not. However, method overloading can be used to obtain similar results in Java but generate redundant stub code.
• The minimum of code you need to compile for C++ is a function(method). The minimum for java is a class.
• C++ allows a range of implicit conversions between native types, and also allows the programmer to define implicit conversions involving user-defined types. In Java, only widening conversions between native types are implicit; 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 was a typo for if(a == 5). Yet current C++ compilers usually generate a warning when such an assignment is performed within a conditional expression. Similarly, standalone comparison statements, eg. a==5;, without a side effect generate a warning.
• For passing parameters to functions, C++ supports both pass-by-reference and pass-by-value. In Java, primitive parameters are always passed by value. Class types, interface types, and array types are collectively called reference types in Java and object references are passed by value.^[4][5][6]
• Java built-in types are of a specified size and range defined by the language specification. In C++, a minimal range of values is defined for built-in types, but the exact representation (number of bits) can be mapped to whatever native types are supported on a given platform.
  • For instance, 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 an optional strict floating-point model that guarantees consistent results across platforms, though possibly at the cost of slower run-time performance. Some C++ compilers also provide similar options for floating-point behavior.^[7]

• In C++, pointers can be manipulated directly as memory address values. Java references are pointers^[8] but these references can refer only to objects and arrays. Java references do not allow direct access to memory addresses or allow memory addresses to be manipulated with pointer arithmetic. In C++ one can construct pointers to pointers, pointers to ints and doubles, and pointers to arbitrary memory locations. Java references only access objects and arrays, never primitives, other references, or arbitrary memory locations.
• In C++, pointers can point to functions or methods (function pointers or functors). The equivalent mechanism in Java uses object or interface references.
• Through the use of stack-allocated objects, C++ supports scoped resource management, a technique used to automatically manage memory and other system resources that supports deterministic object destruction. While scoped resource management in C++ cannot be guaranteed (even objects with proper destructors can be allocated using new and left undeleted) it provides a good means of simple resource management. Shared resources can be managed using shared_ptr. In case of cyclic references, shared_ptr will fail to detect loss of accessibility and the resources will leak. Java supports automatic memory management using garbage collection which can free unaccessible objects even in presence of cyclic references, but other system resources (files, streams, windows, communication ports, threads(?)) must be explicitly released because garbage collection is not guaranteed to occur immediately after last object reference is abandoned.
• C++ features programmer-defined operator overloading which is not supported in Java. 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.
• C++ supports static and dynamic linking of binaries.
• Java has generics, whose main purpose is to provide type-safe containers. C++ has compile-time templates, which provide more extensive support for generic programming and metaprogramming. Java has annotations, which allow adding arbitrary custom metadata to classes and metaprogramming via an annotation processing tool.
• 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 type, which will allow the object to be manipulated via reference semantics.
• C++ supports multiple inheritance of arbitrary classes. In Java a class can derive from only one class, but a class can implement multiple interfaces (in other words, it supports multiple inheritance of types, but only single inheritance of implementation).
• Java explicitly distinguishes between interfaces and classes. In C++, multiple inheritance and pure virtual functions make it possible to define classes that function almost like Java interfaces do, with a few small differences.
• 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. Java also provides robust and complex libraries for more advanced multi-threading synchronization. Only as of C++11 is there a defined memory model for multi-threading in C++. There are also third-party libraries to provide support roughly equivalent to that of Java.
• C++ methods can be declared as virtual functions, which means the method to be called is determined by the run-time type of the object. By default, methods in C++ are not virtual. In Java, methods are virtual by default, but can be made non-virtual by using the final keyword.
• C++ enumerations are primitive types and support conversion to and comparison with other integer types. Java enumerations can be public static enum{enumName1,enumName2} and are used like classes. Another way is to make another class that extends java.lang.Enum<E>) and may therefore define constructors, fields, and methods as any other class.
• Unary operators '++' and '--': in C++ "The operand shall be a modifiable lvalue. [skipped] The result is the updated operand; it is an lvalue...",^[9] but in Java "A variable that is declared final cannot be {decremented,incremented} because when an access of such a final variable is used as an expression, the result is a value, not a variable.",^[10] i.e. in Java if "Integer i(2);" then "++i;" changes the reference i by assigning new object, while in C++ the object is still the same.

Resource management

• Java offers automatic garbage collection. Memory management in C++ is usually done through constructors, destructors, and smart pointers. The C++ standard permits garbage collection, but does not require it; garbage collection is rarely used in practice. The enforced use of automatic garbage collection means that writing real-time software can be difficult in Java.^[3]
• 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, which can reclaim memory, 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 destroyed 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 just prior to 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 can be performed explicitly using the try/catch/finally construct.
  • In C++, it is possible to have a dangling pointer – a stale reference to an object that has already been deallocated. Attempting to use a dangling pointer typically results in program failure. In Java, the garbage collector will not destroy a referenced object.
  • In C++, it is possible to have uninitialized primitive objects. Java enforces default initialization.
  • In C++, it is possible to have an allocated object to which there is no valid reference. Such an unreachable object cannot be destroyed (deallocated), and results in a memory leak. In 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.^[11]

Libraries

• C++ provides cross-platform access to many features typically available in platform-specific libraries. Direct access from Java to native operating system and hardware functions requires the use of the Java Native Interface.

Runtime

• Due to its unconstrained expressiveness, low level C++ language features (e.g. unchecked array access, raw pointers, type punning) cannot be reliably checked at compile-time or without overhead at run-time. Related programming errors can lead to low-level buffer overflows and segmentation faults. The Standard Template Library provides higher-level abstractions (like vector, list and map) to help avoid such errors. In Java, low level errors either 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, especially since Java 7, compiler analysis 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 "usually safe; slightly constrained; often have overhead" while C++ native arrays "have optional overhead; are slightly unconstrained; are possibly unsafe."

Templates vs. generics

Both C++ and Java provide facilities for generic programming, templates and generics, respectively. Although they were created to solve similar kinds of problems, and have similar syntax, they are actually quite different.

C++ Templates	Java Generics
Classes and functions can be templated.	Classes and methods can be genericized.
Parameters can be any type, integral value, or character literal.	Parameters can be reference types or boxed primitive types (i.e. Integer, Boolean...).
Separate copies of the class or function are likely to be generated for each type parameter when compiled.	One version of the class or function is compiled, works for all type parameters.
Objects of a class with different type parameters are different types at run time.	Type parameters are erased when compiled; objects of a class with different type parameters are the same type at run time. It just causes a different constructor. Because of this type erasure, it is not possible to overload methods using different instantiations of the generic class.
Implementation source code of the templated class or function must be included in order to use it (declaration insufficient).	Signature of the class or function from a compiled class file is sufficient to use it.
Templates can be specialized—a separate implementation could be provided for a particular template parameter.	Generics cannot be specialized.
Template parameters can have default arguments. (Prior to C++11, this was allowed only for template classes, not functions.)	Generic type parameters cannot have default arguments.
Does not support wildcards. Instead, return types are often available as nested typedefs. (Also, C++11 added keyword auto, which acts as a wildcard for any type that can be determined at compile time.)	Supports wildcard as type parameter.
Does not directly support bounding of type parameters, but metaprogramming provides this[12]	Supports bounding of type parameters with "extends" and "super" for upper and lower bounds, respectively; allows enforcement of relationships between type parameters.
Allows instantiation of an object with the type of the parameter type.	Does not allow instantiation of an object with the type of the parameter type (except through reflection).
Type parameter of templated class can be used for static methods and variables.	Type parameter of templated class cannot be used for static methods and variables.
Static variables are not shared between classes of different type parameters.	Static variables are shared between instances of classes of different type parameters.
Templated classes and functions do not enforce type relations for type parameters in their declaration. Use of an incorrect type parameter results in compilation failure, often generating an error message within the template code rather than in the user's code that invokes it. Proper use of templated classes and functions is dependent on proper documentation. Metaprogramming provides these features at the cost of additional effort.	Generic classes and functions can enforce type relationships for type parameters in their declaration. Use of an incorrect type parameter results in a type error within the code that uses it. Operations on parametrized types in generic code are only allowed in ways that can be guaranteed to be safe by the declaration. This results in greater type safety at the cost of flexibility.
Templates are Turing-complete (see template metaprogramming).	Generics are probably not Turing-complete.

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.
• Compiled Java code files are generally smaller than code files in C++ as Java bytecode is usually more compact than native machine code^{[citation needed]} and Java programs are never statically linked.
• C++ compilation features an additional textual preprocessing phase, while Java does not. Thus some users add a preprocessing phase to their build process for better support of conditional compilation.
• Java's division and modulus operators are well defined to truncate to zero. C++ (prior to C++11) does not specify whether or not these operators truncate to zero or "truncate to -infinity". -3/2 will always be -1 in Java and C++11, but a C++03 compiler may return either -1 or -2, depending on the platform. C99 defines division in the same fashion as Java and C++11. Both languages guarantee (where a and b are integer types) that (a/b)*b + (a%b) == a for all a and b (b != 0). The C++03 version will 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 and ABI dependent within given constraints. Thus, carefully written C++ code can take advantage of the 64-bit processor's capabilities while still functioning properly on 32-bit processors. However, this requires care and effort to ensure that the C++ source code is written in a portable manner. In contrast, Java's fixed integer sizes mean that programmer error in this regard are not possible, although this may incur a performance penalty since Java code cannot run using an arbitrary processor's word size.

Performance

In addition to running a compiled Java program, computers running Java applications generally must also run the Java virtual machine (JVM), while compiled C++ programs can be run without external applications. 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 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 JVM. For example:

Certain inefficiencies are inherent to the Java language itself, primarily:

• All objects are allocated on the heap. For functions using small objects this can result in performance degradation as stack allocation, in contrast, costs essentially zero. However, this advantage is obsoleted by modern JIT compilers utilising escape analysis or escape detection to allocate objects on the stack. Escape analysis was introduced in Oracle JDK 6.
• Methods are virtual by default. This slightly increases memory usage by adding a single pointer to a virtual table per each object. It also induces a startup performance penalty, since a JIT compiler must perform additional optimization passes even for de-virtualization of small functions.
• A lot of casting required even using standard containers induces a performance penalty. However, most of these casts are statically eliminated by the JIT compiler, and the casts that remain in the code usually do not cost more than a single CPU cycle on modern processors, thanks to branch prediction.
• Array access must be safe. The compiler is required to put appropriate range checks in the code. The naive approach of guarding each array access with a range check is not efficient, so most JIT compilers generate range check instructions only if they cannot statically prove the array access is safe. Even if all runtime range checks cannot be statically elided, JIT compilers try to move them out of inner loops to make the performance degradation as low as possible.
• Lack of access to low-level details prevents the developer from improving the program where the compiler is unable to do so.^[13] Programmers can interface with the OS directly by providing code in C or C++ and calling that code from Java by means of JNI.

In contrast, various optimizations in C++ are either too difficult or impractical to implement:

• Pointers make optimization difficult since they may point to arbitrary data. However, in some cases this is obsoleted as new compilers introduced a strict-aliasing rule^[14] and because of support of the C99 keyword restrict.^[15]
• Java garbage collection may have better cache coherence than the usual usage of malloc/new for memory allocation, as its allocations are generally made sequentially. Nevertheless, arguments exist^{[weasel words]} that both allocators equally fragment the heap and neither exhibits better cache locality.
• Due to the lack of garbage collection in C++, programmers must supply their own memory management code, often in the form of reference-counted smart pointers.
• Since the code generated from various concretisations of the same templated class in C++ is not shared, excessive use of templates may lead to significant increase of the executable code size.
• Run-time compilation can potentially use additional information available at run-time to improve code more effectively, such as the processor on which the code will be executed. However, this claim is effectively made obsolete as most state-of-the-art C++ compilers generate multiple code paths to employ the full computational abilities of the given system^[16]
• Run-time compilation allows for more aggressive virtual function inlining than is possible for a static compiler, because the JIT compiler has complete information about all possible targets of the virtual call, even if they are in different dynamically loaded modules. Currently available JVM implementations have no problem in inlining most of the monomorphic, mostly monomorphic and dimorphic calls, and research is in progress to inline also megamorphic calls, thanks to the recent invoke dynamic enhancements added in Java 7.^[17] Inlining can allow for further optimisations like loop vectorisation or loop unrolling, resulting in a huge overall performance increase.
• Because dynamic linking is performed after code generation and optimisation in C++, function calls spanning different dynamic modules cannot be inlined.
• Because thread support is provided by libraries in C++, C++ compilers have no chance to perform thread-related optimisations. In Java, thread synchronisation is built into the language, so the JIT compiler can, with the help of escape analysis, easily elide or coarse locks,^[18] significantly improving performance of multithreaded code. This technique was introduced in Sun JDK 6 update 10 and is named biased locking.^[19]

Official standard and reference of the language

Language specification

The C++ language is defined by ISO/IEC 14882, an ISO standard, which is published by the ISO/IEC JTC1/SC22/WG21 committee. The Java language is defined by the Java Language Specification,^[20] a book which is published by Sun (now Oracle).

The Java language continuously evolves through a process called the Java Community Process, and the world's programming community is represented by a group of people and organizations - the Java Community members^[21] - which is actively engaged into the enhancement of the language, by sending public requests - the Java Specification Requests - which must pass formal and public reviews before they get integrated into the language.
In contrast, the C++ programming community does not have this power, because the C++ language specification was statically defined by ISO.

Trademarks

"C++" is not a trademark of any company or organization and is not owned by any individual.^[22] "Java" is a trademark of Sun Microsystems (now Oracle).^[23]

References

External links

• Object Oriented Memory Management: Java vs. C++
• Chapter 2:How Java Differs from C, chapter from Java in a Nutshell by David Flanagan
• Java vs. C++ resource management comparison - Comprehensive paper with examples
• Java vs C performance... again... - In-depth discussion of differences between Java and C / C++ with regard to performance