1. Objects as Variables Featuring the Date Object
Copyright © Curt Hill

2. Introduction Recall the five basic types:
Number
String
Boolean
Undefined
Null
If it is not one of these, what is it?
One of the possibilities is an object
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3. Handles
Objects are always referred to by a handle
A handle is just a pointer
Although perhaps safer than pointers in C/C++
The handle is on the stack and refers to the value which is on the heap
An object must be initialized with the new operator
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4. The new operator Three functions performed by new:
Allocate memory on the heap
Invokes memory manager
Generate the address to this memory
This is usually received by a handle variable
Call the constructor for the object
Constructors initialize
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5. Form of New new object_name(parm) new is an operator
One of the few that is a word
object_name is the name of the kind of object
parm is the parameter to the constructor
May be left out in case of default constructor
Usually result is assigned to variable
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6. The Date object A Date represents a date and time Stored as a number
This number may represent any date within 100 million days of January 1, 1970
This is approximately 285,616 years
It handles this as an extrapolated Gregorian calendars
Thus it believes in October 10, 1582
October 5-14 were skipped in 1582 when calendar was started
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7. A constructor example
Suppose that we have the following code: var d; // Set to undefined d = new Date(); d2 = new Date; document.write("<P> ",d);
Both d and d2 use the default constructor
Result is: Wed Jan :51:01 GMT-0600 (Central Standard Time)
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8. What do I get?
The default constructor for Date initializes it to the current time from the system clock
Most objects convert to a string in an object specific fashion
With Date you get the string given above, which indicated date, time and timezone
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9. Date constructors
Date may be initialized with a succession of information:
Year, Month, Day, Hour, Minute, Second, Milliseconds
Month is 0-11, not 1-12
Provided in this order – as many or as few
A copy constructor also exists
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10. Examples
d1 = new Date; d2 = new Date(); d3 = new Date(2008,2,1); // March 1 d4 = new Date(d3); d5 = new Date(2008); // Jan 1 d3 = new Date(2008,2,1,10,30);
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11. Operators Generally operators work on the handles
Methods should be used for object access or manipulation
Assignment: d2 = d1; // copies handle d3 = new Date(d2); // copies // contents
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12. Date Methods
There are a number of get and sets that allow access and manipulation
Setting a value
setYear – 1 to 3 parameters
setMonth – 1 or 2 parameters
setDate
setHours – 1 to 4 parameters
setMinutes – 1 or 3 parameters
setSeconds– 1 or 2 parameters
setMilliseconds
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13. Getting a value The expected methods:
getYear
getMonth
getDate
etc
All take no parameters and return a number
A few less expected:
getDay
getTimeZoneOffset
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14. Examples Four days ago: d2.setDate(d2.getDate()-4)
Particular date d2.setYear(2007,11,15);
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15. What can be done to any object?
Execute any of the objects methods
Assign to a similar object
Direct comparison
However, assignment and comparison are on the handle not the actual object
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16. Assignment
Assignment of two object does not necessarily create new instances
It merely copies the handle
This can cause problems because of side effects
Consider d1 = new Date(2007,0,21); d2 = d1; // Handle assign d2.setMonth(0,2);
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17. Date Example d1 = new Date (2007,0,21); d1 January 21, 2007 d2
d2 = d1; d1
January 21, 2007 d2
d2.setMonth (0,2); d1
January 2, 2007 d2
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18. One more thought on assignment
Since direct assignment on objects only copies the handle it is better to use the copy constructor
Provided it exists
Thus avoid: d2 = d1; and use: d2 = new Date(d1); where possible
Also use this for value parameters
Copyright © Curt Hill

19. Comparisons Same problem with comparisons on handles
So with: d3 = new Date(d2);
d2 will not compare equal with d2 d2 == d3 is false because the handles are different, not the object contents
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20. valueOf method Returns the number that is the basis of date
Comparing this number will give reasonable results
Thus d3 = new Date(d2); d3.valueOf() == d2.valueOf() will be true
In most other cases the number is of dubious value
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21. Memory partitions The memory for programs is divided into three chunks
The code memory
Machine language instructions
The stack memory
Basics and handles
The heap memory
Objects
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22. Basic types are on the Stack
Allocated at declaration or first use
Deallocated at end of method that contains declaration
Quick allocation and deallocation
Only primitives and handles on stack
Stack also includes return addresses, function results and temporary results
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23. Objects are on the Heap Allocated by specific use of new keyword
Deallocated by the delete operator
Requires a handle (pointer)
All objects are on heap and nothing but objects on heap
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24. delete Disposes of a handle Form is: delete variable;
It returns a boolean value
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25. Memory Heap Stack s = 5; 5 January … t=null; null u= new Date()
c = ‘ABC’;
ABC
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26. Java and JavaScript
Java has primitives, but JavaScript has Basic types
The JavaScript basics are really objects
They just do not need a new or delete
They all have methods and properties
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