Tài liệu Initialization ppt

Initialization
2
Objectives
•
Present field initialization options for fields
–
default values
–
variable initializers
–
constructors
3
Instance field default values
•
Instance fields set to default values when object created
–
0 for numeric types
–
false for bool
–
'\x0000' for char
–
null for references
set to default values
Rational a = new Rational();
instance fields
class Rational
{
int numerator;
int denominator;

}
numerator 0
denominator 0
a
4
Variable initializer
•
Instance fields can be initialized at point of definition
–
called variable initializer
–
executed each time an object is created
–
convenient way to overwrite default values
initialize
class Rational
{
int numerator;
int denominator = 1;

}
5
Constructor
•
Class can supply constructor to do initialization
–
automatically invoked when object created
–
implemented using same name as class with no return type
constructor
class Rational
{
int numerator;
int denominator;
public Rational(int numerator, int denominator)
{
this.numerator = numerator;
this.denominator = denominator;
}

}
6
Invoking constructor
•
Constructor automatically invoked when object created
create object,
invoke constructor
Rational a = new Rational(2, 5);
numerator 2
denominator 5
a
7
Multiple constructors
•
Class can supply multiple constructors
–
parameter lists must be different
constructor
class Rational
{
public Rational(int numerator, int denominator)
{
this.numerator = numerator;
this.denominator = denominator;
}
public Rational(int numerator)
{
this.numerator = numerator;
this.denominator = 1;
}

}
constructor
8
Default constructor
•
Can supply constructor that takes no arguments
–
often called the default constructor
no argument
constructor
class Rational
{
public Rational()
{
this.denominator = 1;
}

}
9
Selecting constructor to invoke
•
Compiler selects constructor version automatically
–
based on arguments passed
two ints
Rational a = new Rational(2, 5);
Rational b = new Rational(6);
Rational c = new Rational();
one int
no arguments
10
Constructor initializer
•
One constructor can invoke another constructor
–
use :this( ) syntax before constructor body
–
called constructor initializer
–
can put common code in constructor that others call
class Rational
{
public Rational(int numerator, int denominator)
{
this.numerator = numerator;
this.denominator = denominator;
}
public Rational(int numerator)
:this(numerator, 1)
{

}

}
call 2 argument
constructor
11
Compiler generated constructor
•
Compiler creates default constructor
–
only if no constructors supplied by programmer
•
Compiler generated constructor
–
takes no arguments
–
has empty body
–
calls no argument constructor for base class
12
Initialization order
•
Initialization options executed in well defined order
1. fields set to default values
2. variable initializers run in textual order top to bottom
3. constructor executed

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