2.1 Jb0105: java oop: similarities and differences between java  (Page 3/4)

Java does not support pointers (at least it does not allow you to modify the address contained in a pointer or to perform pointer arithmetic) . Much of the need for pointers was eliminated by providing types for arrays and strings. For example, the oft-used C++ declaration char* ptr needed to point to the first character in a C++ null-terminated "string" is not required in Java, because a string is a true object in Java.

A class definition in Java looks similar to a class definition in C++, but there is no closing semicolon. Also forward reference declarations that are sometimes required in C++ are not required in Java.

The scope resolution operator (::) required in C++ is not used in Java. The dot is used to construct all fully-qualified references. Also, since there are no pointers, the pointer operator (->) used in C++ is not required in Java.

In C++, static data members and functions are called using the name of the class and the name of the static member connected by the scope resolution operator. In Java, the dot is used for this purpose.

Like C++, Java has primitive types such as int , float , etc. Unlike C++, the size of each primitive type is the same regardless of the platform. There is no unsigned integer type in Java. Type checking and type requirements are much tighter in Java than in C++.

Unlike C++, Java provides a true boolean type. (Note, the C++ equivalent of the Java boolean type may have been introduced into C++subsequent to the original publication of this document.)

Conditional expressions in Java must evaluate to boolean rather than to integer, as is the case in C++. Statements such as

if(x+y)...

are not allowed in Java because the conditional expression doesn't evaluate to a boolean .

The char type in C++ is an 8-bit type that maps to the ASCII (or extended ASCII) character set. The char type in Java is a 16-bit type and uses the Unicode character set (the Unicode values from 0 through 127 match the ASCII character set) . For information on the Unicode character set see (External Link) .

Unlike C++, the>>operator in Java is a "signed" right bit shift, inserting the sign bit into the vacated bit position. Java adds an operator that inserts zeros into the vacated bit positions.

C++ allows the instantiation of variables or objects of all types either at compile time in static memory or at run time using dynamic memory. However, Javarequires all variables of primitive types to be instantiated at compile time, and requires all objects to be instantiated in dynamic memory at runtime.Wrapper classes are provided for all primitive types to allow them to be instantiated as objects in dynamic memory at runtime if needed.

C++ requires that classes and functions be declared before they are used. This is not necessary in Java.

The "namespace" issues prevalent in C++ are handled in Java by including everything in a class, and collecting classes into packages.

C++ requires that you re-declare static data members outside the class. This is not required in Java.

In C++, unless you specifically initialize variables of primitive types, they will contain garbage. Although local variables of primitive types can be initialized in the declaration, primitive data members of a class cannot be initialized in the class definition in C++.