Showing 44-49 out of 50
2Inheritance.pdf-CPS209: Computer Science II Inher...
2Inheritance.pdf-CPS209: Computer Science II Inheritance Inheritance
2Inheritance.pdf-CPS209: Computer S...
2Inheritance.pdf-CPS209: Computer Science II Inheritance Inheritance
Page 44
Object
: The superclass of all classes
All classes extend
Object
Most useful methods:
String toString()
boolean equals(Object otherObject)
Object clone()
Page 45
Object
: toString()
•
Returns a string representation of the object
•
Useful for debugging
•
Example: toString() in class Rectangle returns something like:
•
java.awt.Rectangle[x=5,y=10,width=20,height=30]
•
toString() used by concatenation operator:
BankAccount
b = new
BankAccount(9876543,300);
String test = “xyz” +
b; // means
String test = “xyz” + b.toString();
•
toString() in class Object returns class name and object address:
•
Employee@d2460bf
Page 46
Object
: The superclass of all classes
•
It is common to override toString():
public class Employee
{
public String
toString()
{
return "Employee[name=" + name +
“ "
+ "payRate=” +
payRate
+ "]";
}
. . .
}
Page 47
class Object: equals(Object other)
equals() tests for equal
contents
. Examples:
BankAccount => compare balance
Coin => compare value
Employee => compare name and/or payRate
== tests for equal memory locations of objects!!
Must cast the “Object other” parameter to subclass
public class Employee
{
public boolean equals(Object other)
{
Employee otherEmpl = (Employee) other;
return name.equals(otherEmpl.name) && payRate == otherEmpl.payRate;
}
}
NOTE:
Strings are objects so must use equals method in class String to compare two strings!!!
Cannot just write:
return name == otherEmpl.name
&& payRate == otherEmpl.payRate;
Page 48
class Object: equals(Object other)
public class ExecutiveTester
{
public static void main(String[] args)
{
Executive exec1 = new Executive();
exec1.setName(“bossman”);
exec1.setPayRate(150.0);
Executive exec2 = new Executive();
exec2.setName(“bigger bossman”);
exec2.setPayRate(250.0);
if (exec1 == exec2)
System.out.println(“two references to same object”);
if (exec1.equals(exec2))
System.out.println(“two objects with same name and payrate”);
}
}
Page 49
Abstract Methods and Classes
•
When you extend a class, you have choice whether to override a
method or not.
•
Sometimes want to force the subclass creator to override a method
(method might be common to all subclasses so should define it in
superclass but there is no good default implementation) Example:
area() method of superclass Shape and subclasses (Circle,Rectangle)
•
You can make the method abstract (just signature, no body):
•
abstract public double area()
;
•
Now subclass programmer
must
implement method area()
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