Comparison of 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 mainly for systems 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 to support network computing. 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 a syntax similar to C, but is not compatible with it. It was designed from scratch, with the goal of being easy to use and accessible to a wider audience.

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 :

C++	Java
Compatible with C source code, except for a few corner cases.	No backward compatibility with any previous language. The syntax is however strongly influenced by C/C++.
Allows both procedural programming and object-oriented programming.	Strictly enforces an object oriented programming paradigm.
Allows direct calls to native system libraries.	Call through the Java Native Interface and recently Java Native Access
Exposes low-level system facilities.	Runs in a protected virtual machine.
Optional automated bounds checking.	Always performs bounds checking.
Supports native unsigned arithmetic.	No native support for unsigned arithmetic.
Standardized minimum limits for all numerical types, but the actual sizes are implementation-defined. Standardized types are available in 3rd-party libraries.	Standardized limits and sizes of all primitive types on all platforms.
Pointers, References, and pass by value are supported	Primitive data types always passed by value. Objects references are passed by value. (comparable to using pointers for all class or struct parameters in C++).
Explicit memory management, though third party frameworks exist to provide garbage collection. Supports destructors	Automatic garbage collection (can be triggered manually). Doesn't have the concept of Destructor and usage of finalize() is not recommended.
Supports class, struct, union and can allocate them on heap or stack	Only supports class and allocates them on the heap. Java SE 6 optimizes with escape analysis to allocate some objects on the stack.
Allows explicitly overriding types.	Rigid type safety except for widening conversions. Autoboxing/Unboxing added in Java 1.5
The C++ Standard Library has a much more limited scope and functionality than the Java standard library but includes: Language support, Diagnostics, General Utilities, Strings, Locales, Containers, Algorithms, Iterators, Numerics, Input/Output and Standard C Library. Users must choose from a plethora of (mostly mutually incompatible) third-party libraries for threads, network I/O, GUI, and more functionality than the barebone implementations provided by the C++ Standard Library.	The standard library has grown with each release. By version 1.6 the library included support for locales, logging, containers and iterators, algorithms, GUI programming, graphics, multi-threading, networking, platform security, introspection, dynamic class loading, blocking and non-blocking I/O, and provided interfaces or support classes for XML, XSLT, MIDI, database connectivity, naming services (e.g. LDAP), cryptography, security services (e.g. Kerberos), print services, and web services.
Operator overloading for most operators	Meaning of operators is immutable.
Full multiple inheritance, including virtual inheritance.	Single inheritance only from classes, multiple from interfaces.
Compile time Templates	Run time Generics
Function pointers	No function pointer mechanism. Instead idioms like Interfaces, Adapters and Listeners are extensively used. (C# as the Java successor introduces the concept of Delegates)
No standard inline documentation mechanism. 3rd party software exists like Doxygen	javadoc standard documentation
const keyword for defining immutable variables and member functions that does not change the object.	final provides a limited version of const, equivalent to type* const pointers for objects and plain const of primitive types only. No const member functions, nor any equivalent to const type* pointers.
Supports the goto statement	Doesn't support goto
Source code can be written to be platform independent (can be compiled for Windows, BSD, Linux, Mac OS X, Solaris etc. without needing modification) and written to take advantage of platform specific features. Is typically compiled into native machine code.	Is compiled into byte code for the JVM. Is dependent on the Java platform but the source code is typically written not to be dependent on operating system specific features.

C++ is a powerful language designed for system programming. The Java language was designed to be simple and easy to learn with a powerful cross-platform library. The Java standard library is considerably large for a standard library. However, Java does not always provide full access to the features and performance of the platform that the software runs on. The C++ standard libraries are simple and robust providing containers and associative arrays..

Language features

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.
• In C++ declarations, a class name declares an object of that class as a value (a.k.a. value semantics). There is no way to do this in Java. Objects are not values in Java. In Java declarations, a class name declares a reference to an object of that class (a.k.a. reference semantics). The equivalent way to do this in C++ is to use "*" to declare a pointer.
• In C++, the operator "." takes an object as the left operand and accesses a member of the object. Since objects cannot be values in Java, and all objects are accessed through references, this cannot be done in Java. In Java, the "." operator takes a reference to an object as the left operand and access a member of that object. The equivalent operator in C++ is "->".

C++	Java
class Foo { // Declares class Foo public: int x; // Member variable Foo(): x(0) { // Constructor for Foo, } // initializes x int bar(int i) { // Member function bar() return 3*i + x; } };	class Foo { // Defines class Foo public int x = 0; // Member variable, // with initializer 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 // If you wanted to use another constructor, // you would declare it as "Foo a(args);"	Foo a; // declares a to be a reference/pointer to a Foo object a = new Foo(); // initializes using the default constructor // If you wanted to use another constructor, // you would declare it as "Foo a = new Foo(args);"
Foo b = a; // copies the contents of a to a new Foo object b // OR // Foo b(a)	Foo b = a.clone(); // copies the values of all members // of this instance if, and only if, // Foo implements a public method called // clone() which returns a new copy of the object
a.x = 5; // modifies the object a	a.x = 5; // modifies the object 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; you are required 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 a c->bar(5); // invokes Foo::bar() for *c	a.bar(5); // invokes Foo.bar() for a c.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, that is, you cannot modify the object pointed to by the pointer using 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. This is not possible in Java. You can declare a reference "final" in Java (like declaring a pointer "const" in C++), but this just prevents you from re-binding that reference; you can still modify the object referenced by the reference.

C++	Java
const Foo *a; // you cannot modify the object //pointed to by a through a	private final Foo a;//Use getA() instead of a public Foo getA(){ return a.clone() }
a = new Foo();	a=new Foo();//Only in constructor
a->x = 5; // ILLEGAL	getA().x=5; //Only modifies a clone of a's x, not a.x
Foo *const b = new Foo(); // you can declare a "const" pointer	final Foo b = new Foo(); // you can declare a "final" reference
b = new Foo(); //ILLEGAL, you can't re-bind it	b = new Foo(); // ILLEGAL, you can't re-bind it
b->x = 5; // LEGAL, you can still modify the object	b.x = 5; // LEGAL, you can still modify the object

• C++ supports goto statements; Java enforces structured control flow, and relies on labelled break and labelled 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.
• 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 inline assembler. In Java, such code has to 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.
• The minimal compilation unit in C++ is a function; the compilation unit in Java is a class. In C++, functions can be compiled separately. In Java, to compile and maintain methods separately requires moving them into super and extended classes or using some other code refactoring technique.
• 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.
• For passing parameters to functions, C++ supports both pass-by-reference and pass-by-value. In Java, parameters are always passed by value . However, in Java all non-primitive values are references to objects (in C++ terms, they are (smart)-pointers). Objects are not values in Java and only their references can be manipulated; C++ developers who are used to having objects as values may confuse this with pass-by-reference.
• Java built-in types are of a specified size and range defined by the virtual machine; 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.
• 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.
• 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. Yet, scoped resource management in C++ cannot be guaranteed; it is only a design pattern, and hence relies on programmers' adherence. Java supports automatic memory management using garbage collection, but other system resources (windows, communication ports, threads) often have to be explicitly released if the garbage collector can not determine they are no longer used.
• 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 binary to manage the space required for binary and performance.
• Java has generics, whose main purpose is to provide type-safe containers. C++ has templates, which provide more extensive support for generic programming.
• 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, though synchronized sections have to be left in LIFO order. Java also provides robust and complex libraries for more advanced multi-threading synchronization. In C++ there is currently no defined memory model for multi-threading; however, third party libraries provide support roughly equivalent to that of Java; obvious difference being the non-uniformity of these C++ libraries.
• 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 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 are actually instances of a class (they extend java.lang.Enum<E>) and may therefore define constructors, fields, and methods as any other class.

Resource management

• Java offers automatic garbage collection. Memory management in C++ is usually done through constructors, destructors, and smart pointers - notably those in Common Language Runtime - CLR. 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.
• 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 only reclaim memory and may be a last shot at other resources, 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 has to be performed explicitly using the try/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 won't 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 object that is allocated, but has no reachable reference to it. 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.
  • Java is more prone to leaking non-memory resources, while idiomatic C++ makes that much harder.

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

• 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.
• 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, 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 "always safe; severely constrained; always have overhead" while C++ native arrays "have optional overhead; are completely unconstrained; are potentially 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 or integral value.	Parameters can only be reference types (not primitive types).
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.
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 (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.	Supports wildcard as type parameter if it is only used once.
Does not directly support bounding of type parameters, but metaprogramming provides this	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 class of type parameter type.	Does not allow instantiation of class of type parameter type.
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 a classes of different type parameters.
Templated classes and functions do not enforce type relations for type parameters in their declaration. Use of a wrong type parameter results in the template code "not working", usually generating an error message at a place in the template code where an operation is not allowed for that type and not in the user's code. 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 a wrong type parameter results in a type error at 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 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. (See Comparison of imports and includes.)
• 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.
• In both languages, arrays have a fixed size. In Java, arrays are first-class objects, while in C++ they are merely a continuous run of their base objects, often referred to using a pointer to their first element and an optional length. In Java, arrays are bounds-checked and know their length, while in C++ you can treat any subsequence as an array in its own right. Both C++ and Java provide container classes (std::vector and java.util.ArrayList 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 (where a and b are integer types) that (a/b)*b + (a%b) == a for all a and b (b != 0). The C++ 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, care must be taken to write the C++ program in a portable manner. In contrast, Java's fixed integer sizes mean that programmer error in this regard shouldn't be possible. 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 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:

Java/C++ statement	C++ generated code (x86)	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, it is frequently argued that advances in just in time (JIT) compiler technology for long-running server and desktop Java processes will close the performance gap.

Several studies of mostly numerical benchmarks argue that Java could potentially be faster than C++ in some circumstances, for a variety of reasons:

• Pointers make optimization difficult since they may point to arbitrary data, though many C++ compilers provide the C99 keyword restrict which corrects this problem.
• Compared to C++ implementations which make unrestrained use of standard implementations of malloc/new for memory allocation, implementations of Java garbage collection may have better cache coherence as its allocations are generally made sequentially.
• Run-time compilation can potentially use additional information available at run-time to optimise code more effectively, such as knowing what processor the code will be executed on.

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. Just-in-time compilation, used in recent Java virtual machines, allows optimization using current run-time measurements. This adds an additional runtime overhead as code is optimised, but is theoretically capable of increasing performance in processes that run for a very long time.

One comprehensive study of microbenchmarks shows quite a large variation in results but indicates that Java often outperforms C++ in operations such as heap allocation and file I/O while C++ often outperforms Java in arithmetic and trigonometric operations. For numerical processing, Java has shown significant gains with each new version but still lags behind C++ and Fortran. The requirement for reproducibility of floating-point results across all platforms can be enforced with the strictfp keyword in Java. However, other authors note that when benchmarks are tweaked to be algorithmically comparable, overall C++ runs about twice as fast as Java..

Because Java always passes references to objects instead of objects themselves, passing arguments to functions is always an efficient operation, similar to passing a pointer or passing an object by reference in C++. In contrast, C++ allows passing objects by value to functions, which can cause significant overhead in the case that much memory must be copied or inefficient copy constructors must be called.

Setting aside the compilation discussion, the same algorithm in C++ and Java can have a huge difference in performance due the number of object copies performed. On most cases, though, there are other solutions (like using StringStream rather than String for mutable strings), but that often makes the code more bloated, obscured and increases the creation of temporary objects, increasing memory footprint and impacting in speed.

Proprietary Control

C++ is not a trademark of any company or organization and is not owned by any individual. Java is a trademark of Sun Microsystems. On April 20, 2009, Oracle announced that it had agreed to acquire Sun Microsystems which will make Oracle the owner of Java and raises questions about the future of Java

External references

• How Java Differs from C — excerpt 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