Agenda
Homework review
Object identity and equality
Method overriding vs overloading
Tidy up
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Object equality
Object equality is
concerned with whether
the state of 2 distinct
objects is equal
Shape s1 = new Shape( 20, 20, 3, -5 );
Shape s2 = s1;
Shape s3 = new Shape( 20, 20, 3, -5 );
Shape s4 = new Shape( 20, 20, 3, -5 );
Shape instance
s1
x = 20
y = 20
deltaX = 3
deltaY = -5
s2
s1 and s2 identify the
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same instance
2
class Shape {
private int x, y;
private int deltaX, deltaY;
…
}
Identity vs equality
Object identity
Object identity is
determined by
reference (pointer)
equality
2
Shape instance
s3
x = 20
y = 20
deltaX = 3
deltaY = -5
s4
Shape instance
x = 20
y = 20
deltaX = 3
deltaY = -5
The objects referred to
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by s3 and s4 are equal
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Object
Object equality
equals( obj : Object ) : boolean
n OOPLs provide a means
Object obj1 = new Object( );
Object obj2 = obj1;
to test for object
equality
Object instance
obj1
n In Java the root
superclass, Object,
this
implements a method
obj2
named equals( ) which is
intended to be
obj1.equals( obj2 ); // => yields true
overridden by
subclasses
class Object {
…
n By default, Object’s
public boolean equals( Object other )
{
equals( ) method tests
return this == other;
for pointer equality
}
}
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Object equality
class Shape {
private int x, y, deltaX, deltaY;
Parameter type must match that of
the method being overridden
// Constructor as before.
…
Ensure a null argument
doesn’t cause a
NullPointerException
// Override method equals.
public boolean equals( Object other ) {
boolean result = false;
Check that the argument points
to some type of Shape object
if( ( other != null ) && other instanceof Shape ) {
Shape s = (Shape ) other;
result = x == s.x && y == s.y &&
s.deltaX == deltaX &&
s.deltaY == deltaY;
}
return result;
}
Cast to type Shape to access
Shape specific attributes
Compare values of this Shape
instance’s variables with those of
the argument’s
}
}
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Method overloading
Method overloading is where a class contains
more than one method of the same name
String
indexOf( str : String ) : int
indexOf( str : String, fromIndex : int ) : int
In a statically typed OOPL like Java, a call to
an overloaded method is resolved at compile
time String
String myString = new String( “Bounce.java” );
myString.indexOf( “.” ) ;
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indexOf( str : String ) : int
indexOf( str : String ) : Integer
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Constructor overloading
class Shape {
Constructors are
private int x, y, deltaX, deltaY;
commonly overloaded private static final int DELTA_X = 5;
private static final int DELTA_Y = 5;
Use of this is
Shape( int x, int y, int deltaX, int deltaY ) {
sometimes useful to public
this.x = x;
this.y = y;
distinguish between
this.deltaX = deltaX;
this.deltaY = deltaY;
constructors
}
public Shape( int x, int y ) {
this( x, y, DELTA_X, DELTA_Y );
}
…
}
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Key points
Object identity is determined by reference (pointer)
equality; do two pointers refer to the same object?
Object equality is concerned with object state; do
two objects have the same values for the instance
variables?
In Java, object equality is implemented by overriding
Object’s equals( ) method; by default Object
implements equals( ) to carry out an identity test
Method overloading enables multiple versions of a
particular method to be defined
Overloaded methods are disambiguated by the number,
order and types of their arguments
Calls to overloaded methods are resolved at compile time
based
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