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Difference in Date approach

 
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Difference in approach:To find number of days between two specified dates
1-Before Java 8
2-Since Java 8

I found out earlier API's were not thread safe and faced concurrency problem,how these are removed in the new API

Any more difference?
 
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Hello,

The old APIs allowed you to define a Marsian or Alpha Centaurian calendar and allowed you to deal with leap seconds as you seem fit depending on whether your system is posix compliant or not regarding Unix time and leap seconds while the new API has no abstract classes nor interfaces. The new API is more human based canned or normalized and makes more assumptions about what time it really is. I have always admired the logic, design and flexibility behind the old Calendar approach but most people think differently than I do about this topic.

http://www.coderanch.com/t/656059/java/java/Date-time-range-Java#3037437
 
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Sachin Tripathi wrote:I found out earlier API's were not thread safe and faced concurrency problem,how these are removed in the new API


Which APIs exactly do you mean? Java 8 has a new date and time API but nothing was removed. Nothing gets removed from the standard library, because it has to stay backward compatible with older versions of Java. So, the old date and time API still exists in Java 8, alongside the new API.

The old date and time API: For example class SimpleDateFormat is not thread-safe, you shouldn't make for example a public static final SimpleDateFormat constant that might get called from different threads at the same time, because then you might get wrong output. Also, class java.util.Date is mutable, which makes it vulnerable to concurrency problems (if multiple threads try to modify the same Date object at the same time, it might get messed up). The new date and time classes are designed to be thread-safe and immutable.
 
Sachin Tripathi
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I understand about backward compatibility,the burden programming language has to carry.
I was asking how the new Api's are designed that they don't suffer the problem as being suffered by their predecessor

Actually wanted to learn about this topic
Any useful link(all about old and new API) ?
 
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Jesper has already given you a link: here is another.
 
A.J. Côté
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A.J. Côté wrote:Hello,

The old APIs allowed you to define a Marsian or Alpha Centaurian calendar and allowed you to deal with leap seconds as you seem fit depending on whether your system is posix compliant or not regarding Unix time and leap seconds while the new API has no abstract classes nor interfaces. The new API is more human based canned or normalized and makes more assumptions about what time it really is...

http://www.coderanch.com/t/656059/java/java/Date-time-range-Java#3037437



Oops! Reading java.time.chrono package docs right now and It seems interesting. I especially like the "Danger Will Robinson if you implement your own" warnings in the relevant classes. Overall, the whole api seems very interesting and it seems like you can indeed implement your own Stardate Chronology (as per java 1.8) ;-)
 
Campbell Ritchie
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I would sooner implement a Klingon calendar than stardates
I am pretty sure you can do that, too.
 
A.J. Côté
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Campbell Ritchie wrote:I would sooner implement a Klingon java.time.chrono.Chronology than stardates
I am pretty sure you can do that, too.



Hi Campbell,

I was waiting for your reply. I took the liberty to adjust your post to better fit the new java 1.8 terminology (in bold).

A brief test will follow:

---begin test

java.time.chrono.Chronology

java.util.Calendar

Nope, javaranch autolinking functionality doesn't seem to get that one yet...

---end of test

Take care,
 
A.J. Côté
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The "Danger Will Robinson" warnings main class:

java.time.chrono.ChronoLocalDate:




java.time.chrono
Interface ChronoLocalDate

All Superinterfaces:
Comparable<ChronoLocalDate>, Temporal, TemporalAccessor, TemporalAdjuster

All Known Implementing Classes:
HijrahDate, JapaneseDate, LocalDate, MinguoDate, ThaiBuddhistDate


public interface ChronoLocalDate
extends Temporal, TemporalAdjuster, Comparable<ChronoLocalDate>

A date without time-of-day or time-zone in an arbitrary chronology, intended for advanced globalization use cases.

Most applications should declare method signatures, fields and variables as LocalDate, not this interface.

A ChronoLocalDate is the abstract representation of a date where the Chronology chronology, or calendar system, is pluggable. The date is defined in terms of fields expressed by TemporalField, where most common implementations are defined in ChronoField. The chronology defines how the calendar system operates and the meaning of the standard fields.
When to use this interface
The design of the API encourages the use of LocalDate rather than this interface, even in the case where the application needs to deal with multiple calendar systems.

This concept can seem surprising at first, as the natural way to globalize an application might initially appear to be to abstract the calendar system. However, as explored below, abstracting the calendar system is usually the wrong approach, resulting in logic errors and hard to find bugs. As such, it should be considered an application-wide architectural decision to choose to use this interface as opposed to LocalDate.
Architectural issues to consider
These are some of the points that must be considered before using this interface throughout an application.

1) Applications using this interface, as opposed to using just LocalDate, face a significantly higher probability of bugs. This is because the calendar system in use is not known at development time. A key cause of bugs is where the developer applies assumptions from their day-to-day knowledge of the ISO calendar system to code that is intended to deal with any arbitrary calendar system. The section below outlines how those assumptions can cause problems The primary mechanism for reducing this increased risk of bugs is a strong code review process. This should also be considered a extra cost in maintenance for the lifetime of the code.

2) This interface does not enforce immutability of implementations. While the implementation notes indicate that all implementations must be immutable there is nothing in the code or type system to enforce this. Any method declared to accept a ChronoLocalDate could therefore be passed a poorly or maliciously written mutable implementation.

3) Applications using this interface must consider the impact of eras. LocalDate shields users from the concept of eras, by ensuring that getYear() returns the proleptic year. That decision ensures that developers can think of LocalDate instances as consisting of three fields - year, month-of-year and day-of-month. By contrast, users of this interface must think of dates as consisting of four fields - era, year-of-era, month-of-year and day-of-month. The extra era field is frequently forgotten, yet it is of vital importance to dates in an arbitrary calendar system. For example, in the Japanese calendar system, the era represents the reign of an Emperor. Whenever one reign ends and another starts, the year-of-era is reset to one.

4) The only agreed international standard for passing a date between two systems is the ISO-8601 standard which requires the ISO calendar system. Using this interface throughout the application will inevitably lead to the requirement to pass the date across a network or component boundary, requiring an application specific protocol or format.

5) Long term persistence, such as a database, will almost always only accept dates in the ISO-8601 calendar system (or the related Julian-Gregorian). Passing around dates in other calendar systems increases the complications of interacting with persistence.

6) Most of the time, passing a ChronoLocalDate throughout an application is unnecessary, as discussed in the last section below.
False assumptions causing bugs in multi-calendar system code
As indicated above, there are many issues to consider when try to use and manipulate a date in an arbitrary calendar system. These are some of the key issues.

Code that queries the day-of-month and assumes that the value will never be more than 31 is invalid. Some calendar systems have more than 31 days in some months.

Code that adds 12 months to a date and assumes that a year has been added is invalid. Some calendar systems have a different number of months, such as 13 in the Coptic or Ethiopic.

Code that adds one month to a date and assumes that the month-of-year value will increase by one or wrap to the next year is invalid. Some calendar systems have a variable number of months in a year, such as the Hebrew.

Code that adds one month, then adds a second one month and assumes that the day-of-month will remain close to its original value is invalid. Some calendar systems have a large difference between the length of the longest month and the length of the shortest month. For example, the Coptic or Ethiopic have 12 months of 30 days and 1 month of 5 days.

Code that adds seven days and assumes that a week has been added is invalid. Some calendar systems have weeks of other than seven days, such as the French Revolutionary.

Code that assumes that because the year of date1 is greater than the year of date2 then date1 is after date2 is invalid. This is invalid for all calendar systems when referring to the year-of-era, and especially untrue of the Japanese calendar system where the year-of-era restarts with the reign of every new Emperor.

Code that treats month-of-year one and day-of-month one as the start of the year is invalid. Not all calendar systems start the year when the month value is one.

In general, manipulating a date, and even querying a date, is wide open to bugs when the calendar system is unknown at development time. This is why it is essential that code using this interface is subjected to additional code reviews. It is also why an architectural decision to avoid this interface type is usually the correct one.
Using LocalDate instead
The primary alternative to using this interface throughout your application is as follows.
Declare all method signatures referring to dates in terms of LocalDate.
Either store the chronology (calendar system) in the user profile or lookup the chronology from the user locale
Convert the ISO LocalDate to and from the user's preferred calendar system during printing and parsing
This approach treats the problem of globalized calendar systems as a localization issue and confines it to the UI layer. This approach is in keeping with other localization issues in the java platform.

As discussed above, performing calculations on a date where the rules of the calendar system are pluggable requires skill and is not recommended. Fortunately, the need to perform calculations on a date in an arbitrary calendar system is extremely rare. For example, it is highly unlikely that the business rules of a library book rental scheme will allow rentals to be for one month, where meaning of the month is dependent on the user's preferred calendar system.

A key use case for calculations on a date in an arbitrary calendar system is producing a month-by-month calendar for display and user interaction. Again, this is a UI issue, and use of this interface solely within a few methods of the UI layer may be justified.

In any other part of the system, where a date must be manipulated in a calendar system other than ISO, the use case will generally specify the calendar system to use. For example, an application may need to calculate the next Islamic or Hebrew holiday which may require manipulating the date. This kind of use case can be handled as follows:
start from the ISO LocalDate being passed to the method
convert the date to the alternate calendar system, which for this use case is known rather than arbitrary
perform the calculation
convert back to LocalDate
Developers writing low-level frameworks or libraries should also avoid this interface. Instead, one of the two general purpose access interfaces should be used. Use TemporalAccessor if read-only access is required, or use Temporal if read-write access is required.

Implementation Requirements:
This interface must be implemented with care to ensure other classes operate correctly. All implementations that can be instantiated must be final, immutable and thread-safe. Subclasses should be Serializable wherever possible.

Additional calendar systems may be added to the system. See Chronology for more details.
Since:
1.8



 
Sachin Tripathi
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It produced following error


What does that mean?
 
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That you shouldn't use classes, constructors or methods that are marked as @Deprecated. This date constructor is one of those, because the official way to create a Date object for a specific date is to create a calendar (either using new GregorianCalendar or using Calendar.getInstance and then setting the values), then converting that to a Date (using the getTime method).
 
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