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 opt for reference semantics in C++, either a pointer or a reference can be used.

class Foo {          // Declares class Foo
    int x = 0;       //  Private Member variable. It will
                     // be initialized to 0, if the
                     // constructor would not set it.
                     // (from C++11)
    public:
      Foo() : x(0)     //  Constructor for Foo; initializes
      {}               //  x to 0. If the initializer were
                     //  omitted, the variable would
                     //  be initialized to the value that
                     // has been given at declaration of x.

      int bar(int i) { // Member function bar()
          return 3*i + x;
      }
};

class Foo {               // Defines class Foo
    private int x;        // Member variable, normally declared
                          // as private to enforce encapsulation
                          // initialized to 0 by default

    public Foo() {        // Constructor for Foo
    }                     // no-arg constructor supplied by default

    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 as
Foo a(args);
// or (C++11):
Foo a{args};

Foo a = new Foo();
// declares a to be a reference to a new Foo object
// initialized using the default constructor

// Another constructor can be used as
Foo 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;
// would declare b to be reference to the object pointed to by 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 Cloneable interface
//     for this code to compile

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

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

std::cout << b.x << std::endl;
// outputs 0, because b is
// some object other than a

System.out.println(b.x);
// outputs 0, because b points to
// some object other 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;
// c is set to the value of the address of the Foo object created by operator new

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

Foo &d = *c;
// binds d to reference the same object to which c points

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

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

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

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

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

std::cout << d.x << std::endl;
// outputs 5, because d references the
// same object to which c points

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

• In C++, it is possible to declare a pointer or reference to a const object in order to prevent client code from modifying it. 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, the final keyword is similar to the const keyword in C++, but its usage is more limited.^[9] For the most part, const-correctness must rely on the semantics of the class' interface, i.e., it is not strongly enforced, except for public data members that are 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's members can still be modified 
// unless explicitly declared final in the declaring class

Foo *const b = new Foo();
// a declaration of a "const" pointer
// it is possible to modify the object,
// but the pointer will constantly point
// to the object assigned to it here

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 code programming. With the exception of the goto statement (which is very rarely seen in real code and highly discouraged), 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 functions. Some commenters point out that these labelled flow control statements break the single point-of-exit property of structured programming.^[10]
• C++ provides low-level features which Java mostly lacks (one notable exception being the sun.misc.Unsafe API for direct memory access and manipulation). 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. Assembly language code can be imported to a C program and vice versa. This makes C language even faster. In Java, such code must reside in external libraries, and can only be accessed via 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 needed to compile for C++ is a function, for Java is a class.
• C++ allows a range of implicit conversions between native types (including some narrowing conversions), and also allows defining implicit conversions involving user-defined types. In Java, only widening conversions between native types are implicit; other conversions require explicit cast syntax.
  • A result 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, but will compile in C++. This is handy if the code was a typo and if(a == 5) was intended. However, current C++ compilers will usually generate a warning when such an assignment is performed within a conditional expression. Similarly, standalone comparison statements, e.g. a==5;, without a side effect usually lead to 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 are also always passed by value.^[11][12][13]
• 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 preferred 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.
  • This also implies that C++ compilers can automatically select the most efficient representation for the target platform (i.e., 64-bit integers for a 64-bit platform), while the representation is fixed in Java, meaning the values can either be stored in the less-efficient size, or must pad the remaining bits and add code to emulate the reduced-width behavior.
• The rounding and precision of floating point values and operations in C++ is implementation-defined (although only very exotic or old platforms depart from the IEEE 754 standard). Java provides an optional strict floating-point model (strictfp) that guarantees more consistent results across platforms, though at the cost of possibly slower run-time performance. However, Java does not comply strictly with the IEEE 754 standard. Most C++ compilers will, by default, comply partly with IEEE 754 (usually excluding strict rounding rules and raise exceptions on NaN results), but provide compliance options of varied strictness, to allow for some optimizing.^[14][15] If we label those options from least compliant to most compliant as fast, consistent (Java's strictfp), near-IEEE, and strict-IEEE, we can say that most C++ implementations default to near-IEEE, with options to switch to fast or strict-IEEE, while Java defaults to fast with an option to switch to consistent.
• In C++, pointers can be manipulated directly as memory address values. Java references are pointers to objects.^[16] 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, never primitives, other references, or arbitrary memory locations. In Java, memory can be read and written by arbitrary values using the sun.misc.Unsafe API, however it is deprecated and not recommended.
• In C++, pointers can point to functions or member functions (function pointers). The equivalent mechanism in Java uses object or interface references.
• Via 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 an effective means of resource management. Shared resources can be managed using shared_ptr, along with weak_ptr to break cyclic references. Java supports automatic memory management using garbage collection^[7] which can free unreachable objects even in the presence of cyclic references, but other system resources (files,^[5] streams, windows, communication ports, threads, etc.) must be explicitly released because garbage collection is not guaranteed to occur immediately after the last object reference is abandoned.
• C++ features user-defined operator overloading. Operator overloading allows for user-defined types to support operators (arithmetic, comparisons, etc.) like primitive types via user-defined implementations for these operators. It is generally recommended to preserve the semantics of the operators. Java supports no form of operator overloading (although its library uses the addition operator for string concatenation).
• Java features standard application programming interface (API) support for reflective programming (reflection) and dynamic loading of arbitrary new code.
• C++ supports static and dynamic linking of binaries.
• Java has generics, which 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 (also termed fundamental or built-in types) and user-defined types (also termed 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,^[1] but a class can implement multiple interfaces^[17] (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 mutex locks to support multi-threaded applications.^[18][19] Java also provides libraries for more advanced multi-threading synchronizing. C++11 has a defined memory model for multi-threading in C++, and library support for creating threads and for many synchronizing primitives. There are also many third-party libraries for this.
• C++ member functions can be declared as virtual functions, which means the method to be called is determined by the run-time type of the object (a.k.a. dynamic dispatching). By default, methods in C++ are not virtual (i.e., opt-in virtual). In Java, methods are virtual by default, but can be made non-virtual by using the final keyword (i.e., opt-out virtual).
• C++ enumerations are primitive types and support implicit conversion to integer types (but not from 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. As of C++11, C++ supports strongly-typed enumerations which provide more type-safety and explicit specification of the storage type.
• Unary operators '++' and '--': in C++ "The operand shall be a modifiable lvalue. [skipped] The result is the updated operand; it is an lvalue...",^[20] but in Java "the binary numeric promotion mentioned above may include unboxing conversion and value set conversion. If necessary, value set conversion {and/or [...] boxing conversion} is applied to the sum prior to its being stored in the variable.",^[21] i.e. in Java, after the initialization "Integer i=2;", "++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, which may be bypassed in specific circumstances via the Real time Java specification. Memory management in C++ is usually done via constructors, destructors, and smart pointers. The C++ standard permits garbage collection, but does not require it. Garbage collection is rarely used in practice.
• C++ can allocate arbitrary blocks of memory. Java only allocates memory via object instantiation. Arbitrary memory blocks may be allocated in Java as an array of bytes.
• Java and C++ use different idioms for resource management. Java relies mainly on garbage collection, which can reclaim memory,^[7] while C++ relies mainly on the Resource Acquisition Is Initialization (RAII) 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,^[7] while Java has finalizers.^[7] Both are invoked before an object's deallocation, but they differ significantly. A C++ object's destructor must be invoked implicitly (in the case of stack-bound variables) or explicitly to deallocate an object. The destructor executes synchronously just before the point in a program at which an object is deallocated. Synchronous, coordinated uninitializing and deallocating in C++ thus satisfy the RAII idiom. Destructors in C++ is the normal way of getting back the resources associated with an object, and is a needed counterpart to constructors.^[7] 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 deallocated. Very few objects need finalizers. A finalizer is needed by only objects that must guarantee some cleanup of the object state before deallocating, typically releasing resources external to the JVM.^[7] Direct usages of finalizers are usually not advised, as they are unpredictable, usually dangerous, and most of the time unneeded.^[7] One has to be cautious not to think of finalizers as C++ destructors.^[7] Rather, the try-with-resources or try-finally block achieves a more similar purpose as the destructor.^[7] One problem with finalizers or cleaners is that it is not guaranteed that they will run immediately.^[7] Hence, a finalizer should never be used for tasks that are time-critical.^[7] Additionally, finalizers come with severe performance penalties and significantly increase the time it takes for objects to be deallocated, so their use is discouraged and deprecated in Java 9.
  • With RAII in C++, one type of resource is typically wrapped inside a small class that allocates the resource upon construction and releases the resource upon destruction, and provide access to the resource in between those points. Any class that contain only such RAII objects do not need to define a destructor since the destructors of the RAII objects are called automatically as an object of this class is destroyed. In Java, safe synchronous deallocation of resources can be performed deterministically using the try/catch/finally construct. Alternatively, the try-with-resources construct, which was introduced in Java 7, should be used in preference to try-finally construct. ^[22] The try-with-resources construct is more concise and readable.^[22] It also provide more helpful diagnostic information, since suppressed exception are not discarded, and will be printed in the stack trace with information saying that they were suppressed.^[22]
  • 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). (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.^[23][24][25] The automatic garbage collector may give the false impression that in Java one does not need to think about memory management.^[5] However this is not quite true.^[5] Loosely speaking, this is because a program can have "memory leaks", more formally known as "unintentional object retentions".^[5] An example of a memory leak that may occur is for a program that has been written without any logical errors, except that it did not eliminate obsolete references.^[5] This results in higher use of garbage collector activity, higher memory footprint.^[5] In extreme circumstances, this problem can lead to an OutOfMemoryError, but this rarely happens.

^[5] The solution to this is to null out object references. ^[5] A second common reason for memory leak is the use of cache that has become no longer relevant. The solution to memory leaks due to using old cache is to represent the cache using a WeakHashMap.

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 RAII abstractions (like vector, list and map) to help avoid such errors. In Java, low level errors either cannot occur or are detected by the Java virtual machine (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 execution. In some cases, especially since Java 7, compiler analysis can prove a bounds check unneeded 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 quite different.

Miscellaneous

• Java and C++ use different means to divide code into multiple source files. Java uses a package system that dictates the file name and path for all program definitions. Its compiler imports the executable class files. C++ uses a header file source code inclusion system to share 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 and Java programs are never statically linked.
• C++ compiling features an added textual preprocessing phase, while Java does not. Thus some users add a preprocessing phase to their build process for better support of conditional compiling.
• Java's division and modulus operators are well defined to truncate to zero. C++ (pre-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 are defined in Java (int is 32-bit, long is 64-bit), while in C++ the size of integers and pointers is compiler and application binary interface (ABI) dependent within given constraints. Thus a Java program will have consistent behavior across platforms, whereas a C++ program may require adapting for some platforms, but may run faster with more natural integer sizes for the local platform.

An example comparing C++ and Java exists in Wikibooks.

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 latest, post-standardization draft of C++17 is available as well.^[38]

The C++ language evolves via an open steering committee called the C++ Standards Committee. The committee is composed of the creator of C++ Bjarne Stroustrup, the convener Herb Sutter, and other prominent figures, including many representatives of industries and user-groups (i.e., the stake-holders). Being an open committee, anyone is free to join, participate, and contribute proposals for upcoming releases of the standard and technical specifications. The committee now aims to release a new standard every few years, although in the past strict review processes and discussions have meant longer delays between publication of new standards (1998, 2003, and 2011).

The Java language is defined by the Java Language Specification,^[39] a book which is published by Oracle.

The Java language continuously evolves via 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^[40]—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.

The lack of a firm standard for Java and the somewhat more volatile nature of its specifications have been a constant source of criticism by stake-holders wanting more stability and conservatism in the addition of new language and library features. In contrast, the C++ committee also receives constant criticism, for the opposite reason, i.e., being too strict and conservative, and taking too long to release new versions.

Trademarks

"C++" is not a trademark of any company or organization and is not owned by any individual.^[41] "Java" is a trademark of Oracle Corporation.^[42]