Transcript Java Types Tutorial

Type Compatibility, Conversion,
Assignment and Casts
George Blank, Mike Meehan, Asra
Sultana
Variables and Types
• A variable is a location in memory where values can
be stored and referenced.
• Variables are associated with types, which have
particular data sizes and a restricted set of valid
operations that can be performed on them.
• Variable declarations assign an identifier, a type, and
may declare an initial value. Many types have a
default value. For example, an uninitialized integer
defaults to 0.
Variable (a memory location)
a value
(a default value)
address
a type
a name (identifier)
The address or value may be passed as a parameter
Identifiers
• Identifiers are used in Java to give a name to
classes, methods and variables that can be used to
access them. Java identifiers begin with a “letter,”
followed by letters or digits. A Java “letter” includes
the letters of any alphabet in Unicode plus the
underscore _ and dollar sign $.
• There is a Java standard naming convention for the
use of identifiers, which can be found at
http://java.sun.com/docs/codeconv/
Sample Code Standards
• Here are a few examples of code standards from the
section on declarations:
– 6.1 Use one declaration per line to encourage comments
– 6.2 Try to initialize local variables where they are declared
– 6.3 Put declarations only at the beginning of blocks
• There are a lot of details to learn, and it will take a while
to be proficient in Java coding.
Java Primitive Types
•
•
•
•
Boolean: boolean
Integer: byte, short, int, long
Character: char
Floating Point: float, double
Type Compatibility
• Two Types are compatible if values of one type can
appear wherever values of the other type are expected,
and vice versa.
• Java is a strongly typed language and from there comes
it’s safety and robustness.
• Every variable has a type which is strictly defined.
• All assignments ,whether explicit or via parameter passing
in method calls ,are checked for type compatibility and any
mismatches result in errors .
Type Conversion in Primitive Types
• Widening
– When one type of data is assigned to another type of variable,
an automatic(implicit) type conversion will take place if,the two
types are compatible and the destination type is larger than the
source type.
– Example: int i; byte b; i=b; //valid conversion
• Narrowing
– Converting a value of larger range to value of smaller range.
– Example: int i; byte b; b=i; //invalid
Common misunderstanding
• Implicit and explicit typecasting of primitive types is frequently
misunderstood.
• The next slide is a small test program, overloaded with
multiple constructors to show when and how type conversion
promotion is done with implicit casting.
• In this example, since the third value is an integer, but there
is no constructor that takes an integer, its value and the float
value automatically convert to double. (Case 5: double,
double, double)
public class typetest {
typetest(double a, double b, short c){
System.out.println("1 (dbl dbl short)");
}
typetest(float a, byte b, long c) {
System.out.println("2 (float byte long)");
}
typetest(long a, long b, long c) {
System.out.println("3 (long long long)");
}
typetest(float a, long b, short c) {
System.out.println("4 (float long short)");
}
typetest(double a, double b, double c) {
System.out.println("5 (dbl dbl dbl)");
}
public static void main(String[] args) {
typetest t = new typetest();
t.typetest(3.4, 3L, 3);
}
}
Type Casting in Primitive Types
• A cast is an explicit type conversion of incompatible types.
• General form : (target-type)value
Target-type is the desired type to convert the value to.
• A different type of conversion called truncation will occur
when a floating point value is assigned to an integer type.
• If the size of the whole number is too large to fit into the
target int type, then that value will be reduced modulo the
target type’s range.
Assignment
• For type conversion and casting the Assignment Operator is
used between the two values.
• General form: var = exp
• Example: int i=10;
long m=10000L;
double d=Math.PI;//PI=3.1415….
i=(int)m;//cast
m=i;//widening
m=(long)d;//cast
d=m;widening
Type Conversion in Expressions
• In addition to assignments, type conversions may occur in
expressions.
• Promotion rules that apply to expressions:
–
–
–
–
all byte and short values are promoted to int
If one operand is long,the whole expression is promoted to long
If one operand is a float ,the whole expression is promoted to float
If one operand is double ,the whole expression is promoted to
double
Promotion rules example
class Promote {
Public static void main(String args[]){
byte b=12;
char c=‘a’;
short s=1004;
int i=20000;
float f=5.67f;
double d=.1234;
double res=(f*b)+(i/c)-(d*s);
System.out.println(“result :” +res);}
}
Java Reference Types
• In addition to the primitive types, Java allows
types that are objects, and which can be user
defined with a class definition.
• Since objects can have a hierarchy, with
supertypes and subtypes, some operations
can be inherited by subtypes. Guidelines for
this must be understood to avoid problems.
The Object Type
• All reference types are subclasses of the
java.lang.Object type.
• Any object may be referred to by a reference
variable of type Object.
• However, Objects have limited functionality,
and it is advantageous to convert the generic
Object to a more specific class, if possible.
Type Casting Returns
• Type casting comes in two flavors: narrowing
and widening.
• Narrowing casts force a variable of one type
into a more restrictive structure (e.g. a parent
class casted to a child class, or an interface
casted to an implementing class).
• Widening casts do the opposite, moving from
a child class to a parent class.
Son of Type Casting
• It is generally safer to perform wider casts, and
the JVM will handle them implicitly.
• Narrowing requires explicit type casting, and
may result in a ClassCastException.
• Object thing = new String();
• String stringThing = (String) thing;
• Thread bogus = (Thread) thing; //
raises exception!
Type Conversion
• It most cases it makes sense to consider an
Object in the context of several different types.
• An integer may be represented as a long, a float,
a double or a String.
• Objects may be represented as integers when
using Container classes.
Conversion of Reference types
• Widening
– The conversion of a subtype to one of its supertypes is called
widening.
– The widening of reference types is always allowed and is carried
out implicitly.
• Narrowing
– Conversion of supertype to one of its subtypes is called
narrowing. (downcasting)
– It requires explicit casting,it is allowed at compile time,it is not
always safe and may result in runtime Exceptions.
Sample code fragment
class Student {..} //super class
class Undergraduate extends student{..}//subclass
class Graduate extends Student{..}//subclass
// instances
Student student1, student2;
student1 =new UnderGraduate();//widening,valid
student2 =new Graduate();//widening,valid
Graduate student3;
student3 = student2;//compilation error
student3 = (Graduate)student2;//explicit cast //done,valid
Polymorphic Assignment
• Polymorphic Assignment Rule:
– The type of the expression at the right hand side of an assignment
must be a subtype of the type of the variable at the left hand side
of the assignment.
• In the previous example,
• student3=(Graduate)student2; //compilation OK,
//runtime exception
• Downcasting results in exceptions being thrown,so,it should be
done with proper care
• There are two proper ways of downcasting.
Proper ways of Downcasting
• The pessimistic approach
exp instanceof Type
•
instanceof operator is used to check the class of an object
before downcasting
• The expression returns true if exp is an instanceof a class or
an interface type named Type.
if(student1 instanceof Graduate){
Graduate gradStudent=(Graduate)student1;
}else {
//student1 is not a graduate student
}
Continue..
• The optimistic approach
• ClassCastException is caught as follows,
try{
//…
Graduate gradStudent=(Graduate)student1;
//…
} catch(ClassCastException e){
//student1 is not a graduate student
}
Conversion Functions
• Primitives have Object wrapper classes that
facilitate type conversions.
• The Integer class, for example is a wrapper for
int.
• An Integer may be used anywhere an Object is
required (it extends java.lang.Object).
Integer Class: Usage
•
•
•
•
•
•
int i = 24;
Integer aye = new Integer(7);
Integer jay = new Integer(j);
Integer kay = new Integer(“144”);
String ess = aye.toString();
double dee = aye.floatValue() /
jay.intValue();
• Distinguish between Integer and int!
Integer Class:
Static Methods
• Static methods exist for on-the-spot type conversions:
• System.out.println(Integer.toString(42))
;
• int bug = Integer.valueOf(“0xF00F”);
• See also: java.lang.Number
Type Assignment
• Type conversion by assignment is a more restrictive
form of type conversion.
• Essentially, conversion by assignment may only occur
in cases where the assigned type is wider than the
assigning type.
• A notable exception is in the conversion from int to
byte, short, or char. Assignment is permissible if the
int may be losslessly represented by the narrower
class.
Type Compatibility
• After an iteration of development, it is possible that a
class becomes incompatible with previous versions.
• It is the responsibility of the new class to maintain
compatibility with older versions.
• Indicate incompatible versions of a class using the
serialVersionUID.
Managing Class Versions
• Avoid incompatible changes as much as possible.
• Serialize data in a format that is implementationagnostic (e.g. HashMap stores data in key-value
pairs, rather than using an internal hash structure).
• If an incompatible change in necessary, change the
serialVersionUID and prepare to handle the
invalidClassException
References
• Integer (Java 2 Platform SE v1.4.2). Sun
Microsystems. 2003. 4/29/06
<http://java.sun.com/j2se/1.4.2/docs/api/java/lang/Int
eger.html>.
• Number (Java 2 Platform SE v1.4.2). Sun
Microsystems. 2003. 4/29/06
<http://java.sun.com/j2se/1.4.2/docs/api/java/lang/Nu
mber.html>.
References, continued
• Versioning of Serializable Objects. Sun Microsystems.
1997. 4/29/06
<http://java.sun.com/products/jdk/rmi/doc/serialspec/version.doc.html>.
• Java Practices: Implementing Serializable. John
O'Hanley. 2006. 4/29/06
<http://www.javapractices.com/Topic45.cjp>.
References, continued
• VM Spec Java Programming Language Concepts.
Sun Microsystems. 1999. 4/29/06
<http://java.sun.com/docs/books/vmspec/2ndedition/html/Concepts.doc.html>.
• Xiaoping Jia, Object Oriented Software Development
Using Java. Addison Wesley, 2003
• Patrick Naughton and Herbert Schildt,
Java 2: The Complete Reference, Third edition