What is casting in java and c plus plus?

Answer 1

Implicit casting is done automatically by the compiler and virtual machine.

Explicit casting is needed to convert types of data when Java is not sure if the result will be valid. There are two times when you will need to perform explicit casts: casting between primitives and casting between objects.

1. Between primitives: Primitive types come in different sizes. For example, an int is 32 bit, whereas a long is 64. If you are sticking a smaller type into a larger type (eg. an int into a long), there is no need to cast since there is no risk that the value will be larger than its container. However, when you go in the opposite direction (eg. a long into an int), a compilation error will occur since Java is afraid the value will be too large for its type. To "silence" the compiler, you can cast the value to the intended type. In doing so, you risk the value overflowing its container. (Example below)
2. Between objects: There are two types of object to object casts: upcasts and downcasts. Upcasting occurs when you take a subclass and stick it in a more general type (Object o = new String();). Upcasting is always implicit. Downcasting is going in the opposite direction. You always need to explicitly cast when downcasting, and Java will only allow you to do so if the conversion is possible. (Example below).

Examples between primitives:

byte b; short s;

s = b; //valid

b = s; //invalid

b = (byte)s; //valid

b = (byte)Short.MAX_VALUE; //valid, but overflow will occur.

Examples between objects:

Object o = s; //upcast - no explicit casting required

String s = (String)new Integer(); //always a compile error

String s = (String)getObject(); //assume that getObject() returns type Object. This will only work if the value stored in getObject() is a String or a subclass of String. Otherwise a runtime error will occur.

Answer 2

Casting is essentially converting one data type to another, such as:

double d = 4.0;

int c = d; //will give an error

Whereas code like this:

double d = 4.0;

int c = (int)d; //no error, 4 assigned to c

because you told Java you know you may lose precision (if d was equal to 4.9, c would still equal 4) but that you wish to simply truncate the other value, Java is happy to cast the double to an int.

Answer 3

Casting is the conversion of one type to another type, better known as a type cast. A class constructor that accepts one argument is a conversion constructor because it converts its argument to an object of the class. However, if the argument is of the same type as the class then it is known as a copy constructor.