Why does Java have transient fields?

In C#, what makes a field different from a property, and when should a field be used instead of a property?

What is the difference between a field and a property?

I have a poorly designed class in a 3rd-party `JAR` and I need to access one of its **private** fields. For example,
why should I need to choose private field is it necessary?

    class IWasDesignedPoorly {
        private Hashtable stuffIWant;
    }

    IWasDesignedPoorly obj = ...;

How can I use reflection to get the value of `stuffIWant`?

How to read the value of a private field from a different class in Java?

In a Django form, how do I make a field read-only (or disabled)?

When the form is being used to create a new entry, all fields should be enabled - but when the record is in update mode some fields need to be read-only.

For example, when creating a new `Item` model, all fields must be editable, but while updating the record, is there a way to disable the `sku` field so that it is visible, but cannot be edited?

    class Item(models.Model):
        sku = models.CharField(max_length=50)
        description = models.CharField(max_length=200)
        added_by = models.ForeignKey(User)


    class ItemForm(ModelForm):
        class Meta:
            model = Item
            exclude = (&#39;added_by&#39;)

    def new_item_view(request):
        if request.method == &#39;POST&#39;:
            form = ItemForm(request.POST)
            # Validate and save
        else:
                form = ItemForm()
        # Render the view

Can class `ItemForm` be reused? What changes would be required in the `ItemForm` or `Item` model class? Would I need to write another class, &quot;`ItemUpdateForm`&quot;, for updating the item?

    def update_item_view(request):
        if request.method == &#39;POST&#39;:
            form = ItemUpdateForm(request.POST)
            # Validate and save
        else:
            form = ItemUpdateForm()


In a Django form, how do I make a field readonly (or disabled) so that it cannot be edited?

How can I add a new field to every document in an existent collection?

I know how to update an existing document&#39;s field but not how to add a new field to every document in a collection. How can I do this in the `mongo` shell?

Add new field to every document in a MongoDB collection

We&#39;re often told we should protect encapsulation by making getter and setter methods (properties in C#) for class fields, instead of exposing the fields to the outside world.

But there are many times when a field is just there to hold a value and doesn&#39;t require any computation to get or set. For these we would all do this number:

    public class Book
    {
        private string _title;
    
        public string Title
        {
              get =&gt; _title; 
              set =&gt; _title = value;
        }
    }

Well, I have a confession, I couldn&#39;t bear writing all that (really, it wasn&#39;t having to write it, it was having to look at it), so I went rogue and used public fields.

Then along comes C# 3.0 and I see they added automatic properties:

    public class Book
    {
        public string Title { get; set; } 
    }

Which is tidier, and I&#39;m thankful for it, but really, what&#39;s so different than just making a public field?

    public class Book
    {
        public string Title;
    }

 


Public Fields versus Automatic Properties

I have a hidden text field whose value gets updated via an AJAX response.

    &lt;input type=&quot;hidden&quot; value=&quot;&quot; name=&quot;userid&quot; id=&quot;useid&quot; /&gt;

When this value changes, I would like to fire an AJAX request. Can anyone advise on how to detect the change?

I have the following code, but do not know how to look for the value:

    $(&#39;#userid&#39;).change( function() {  
       	alert(&#39;Change!&#39;); 
    }) 

jQuery - Detect value change on hidden input field

I am in a mobile app and I use an input field in order user submit a number.

When I go back and return to the page that input field present the latest number input displayed at the input field.

Is there any way to clear the field every time the page load?


	$(&#39;#shares&#39;).keyup(function(){
		payment = 0;
		calcTotal();
		gtotal = ($(&#39;#shares&#39;).val() * 1) + payment;
		gtotal = gtotal.toFixed(2);
		$(&quot;p.total&quot;).html(&quot;Total Payment: &lt;strong&gt;&quot; + gtotal + &quot;&lt;/strong&gt;&quot;);
	});

How to empty input field with jQuery

Here&#39;s the table

**Users**

    UserId
    UserName
    Password
    EmailAddress

and the code..

&lt;!-- language: lang-csharp --&gt;

    public void ChangePassword(int userId, string password){
    //code to update the password..
    }


How to update only one field using Entity Framework?

I have the following class:

    public class Test {
        public static int a = 0;
        public int b = 1;
    }

Is it possible to use reflection to get a list of the static fields only? I&#39;m aware I can get an array of all the fields with `Test.class.getDeclaredFields()`. But it seems there&#39;s no way to determine if a `Field` instance represents a static field or not.

Retrieve only static fields declared in Java class

is there a way to disable all fields (textarea/textfield/option/input/checkbox/submit etc) in a form by telling only the parent div name in jquery/javascript?

disable all form elements inside div

I have an abstract base class and I want to declare a field or a property that will have a different value in each class that inherits from this parent class. 

I want to define it in the baseclass so I can reference it in a base class method - for example overriding ToString to say &quot;This object is of type &lt;i&gt;property/field&lt;/i&gt;&quot;.
I have got three ways that I can see of doing this, but I was wondering - what is the best or accepted way of doing this? Newbie question, sorry.

**Option 1:**  
Use an abstract Property and override it on the inherited classes. This benefits from being enforced (you have to override it) and it is clean. But, it feels slightly wrong to return a hard-code value rather than encapsulate a field and it is a few lines of code instead of just. I also have to declare a body for &quot;set&quot; but that is less important (and there is probably a way to avoid that which I am not aware of). 


    abstract class Father
    {
        abstract public int MyInt { get; set;}
    }

    class Son : Father
    {
        public override int MyInt
        {
            get { return 1; }
            set { }
        }
    }

**Option 2**  
I can declare a public field (or a protected field) and explicitly override it in the inherited class. The example below will give me a warning to use &quot;new&quot; and I can probably do that, but it feels wrong and it breaks the polymorphism, which was the whole point. Doesn&#39;t seem like a good idea...

    abstract class Mother
    {
        public int MyInt = 0;
    }

    class Daughter : Mother
    {
        public int MyInt = 1;
    }

**Option 3**  
I can use a protected field and set the value in the constructor. This seems pretty tidy but relies on me ensuring the constructor always sets this and with multiple overloaded constructors there is always a chance some code path won&#39;t set the value.

    abstract class Aunt
    {
        protected int MyInt;
    }

    class Niece : Aunt
    {
        public Niece()
        {
            MyInt = 1;
        }
    }


It&#39;s a bit of a theoretical question and I guess the answer has to be option 1 as it is the only &lt;i&gt;safe&lt;/i&gt; option but I am just getting to grips with C# and wanted to ask this of people with more experience.


Overriding fields or properties in subclasses

Is there a clean way to remove undefined fields from an object?

i.e.

    &gt; var obj = { a: 1, b: undefined, c: 3 }
    &gt; removeUndefined(obj)
    { a: 1, c: 3 }

I came across two solutions:

    _.each(query, function removeUndefined(value, key) {
      if (_.isUndefined(value)) {
        delete query[key];
      }
    });

or:

    _.omit(obj, _.filter(_.keys(obj), function(key) { return _.isUndefined(obj[key]) }))

Javascript - removing undefined fields from an object

I have a table with two fields (countries and ISO codes):

    Table1
    
       field1 - e.g. &#39;Afghanistan&#39; (without quotes)
       field2 - e.g. &#39;AF&#39;(without quotes)


In some rows the second field has whitespace at the start and/or end, which is affecting queries. 

    Table1
    
       field1 - e.g. &#39;Afghanistan&#39; (without quotes) 
       field2 - e.g. &#39; AF&#39; (without quotes but with that space in front)

Is there a way (in SQL) to go through the table and find/replace the whitespace in field2?


How to remove leading and trailing whitespace in a MySQL field?

I realize that it seems to be a duplicate of [https://stackoverflow.com/questions/295104/what-is-the-difference-between-a-field-and-a-property-in-c][1] but my question has a slight difference (from my point of view):

Once I know that

- I will not use my class with &quot;techniques that only works on properties&quot; and 
- I will not use validation code in the getter/setter.


Is there any difference (except the style/future development ones), like some type of control in setting the property?

Is there any additional difference between:

    public string MyString { get; set; }

and 

    public string myString;

(I am aware that, that the first version requires C# 3.0 or above and that the compiler does create the private fields.)

  [1]: https://stackoverflow.com/questions/295104/what-is-the-difference-between-a-field-and-a-property-in-c

Difference between Property and Field in C# 3.0+

Is there a way to use javascript and JQuery to add some additional fields to be sent from a HTTP form using POST?

I mean:

    &lt;form action=&quot;somewhere&quot; method=&quot;POST&quot; id=&quot;form&quot;&gt;
      &lt;input type=&quot;submit&quot; name=&quot;submit&quot; value=&quot;Send&quot; /&gt;
    &lt;/form&gt;

    &lt;script type=&quot;text/javascript&quot;&gt;
      $(&quot;#form&quot;).submit( function(eventObj) {
        // I want to add a field &quot;field&quot; with value &quot;value&quot; here
        // to the POST data

        return true;
      });
    &lt;/script&gt;

How to add additional fields to form before submit?

If I specify *@AllArgsConstructor* using **Lombok**, it will generate a constructor for setting all the declared (not final, not static) fields. 
Is it possible to omit some field and this leave generated constructor for all other fields?



Java Lombok: Omitting one field in @AllArgsConstructor?

I would like to have a text input field containing the &quot;$&quot; sign in the very beginning, and no matter what editing occurs to the field, for the sign to be persistent.

I would be good if only numbers were accepted for input, but that&#39;s just a fancy addition.


HTML text input field with currency symbol

I am wondering, since a lot of things can be done using reflection, can I change a private readonly field after the constructor completed its execution?  
(note: just curiosity)

    public class Foo
    {
     private readonly int bar;
     
     public Foo(int num)
     {
      bar = num;
     }
     
     public int GetBar()
     {
      return bar;
     }
    }
    
    Foo foo = new Foo(123);
    Console.WriteLine(foo.GetBar()); // display 123
    // reflection code here...
    Console.WriteLine(foo.GetBar()); // display 456



Can I change a private readonly field in C# using reflection?

Why can&#39;t Java classes have abstract fields like they can with abstract methods?

For example: I have two classes that extend the same abstract base class.  These two classes each have a method that is identical except for a String constant, which happens to be an error message, within them. If fields could be abstract, I could make this constant abstract and pull the method up into the base class. Instead, I have to create an abstract method, called `getErrMsg()` in this case, that returns the String, override this method in the two derived classes, and then I can pull up the method (which now calls the abstract method).

Why couldn&#39;t I just make the field abstract to begin with? Could Java have been designed to allow this?

Why not abstract fields?

The data type of the field is String. I would like to fetch the data where character length of field name is greater than 40.

I tried these queries but returning error.
1. 

    db.usercollection.find(
    {$where: &quot;(this.name.length &gt; 40)&quot;}
    ).limit(2);

    output :error: {
    	&quot;$err&quot; : &quot;TypeError: Cannot read property &#39;length&#39; of undefined near &#39;40)&#39; &quot;,
    	&quot;code&quot; : 16722
    }

this is working in 2.4.9 But my version is 2.6.5



String field value length in mongoDB

Imagine you have a form where you switch visibility of several fields. And if the field is not displayed you don&#39;t want its value to be in request.

How do you handle this situation?


How to avoid sending input fields which are hidden by display:none to a server?

I created a class called _`Kwadrat`_. The class has three `int` fields. My _Development Environment_ suggests that I access the fields from _`Kwadrat`_ created objects via the __`::`__ &amp; __`-&gt;`__ operators. I tried both operators, and found that the __`-&gt;`__ operator is able to successfully access the data in the objects fields, although, the same cannot be said for the __`-&gt;`__ operator.
I have also found that the __`.`__ operator will access class members as well. I am confused, and don&#39;t understand why there are three members for accessing object members &amp;/or methods. Can someone please explain to me what the difference is between the three operators?

---

# &amp;nbsp; &amp;nbsp; &amp;nbsp; &amp;nbsp; &amp;nbsp; 1. `-&gt;`
# &amp;nbsp; &amp;nbsp; &amp;nbsp; &amp;nbsp; &amp;nbsp; 2. `::` 
# &amp;nbsp; &amp;nbsp; &amp;nbsp; &amp;nbsp; &amp;nbsp; 3. `.`

---

```C++


    #include &lt;iostream&gt;

    using namespace std;

    class Kwadrat{

    public: 
        int val1,
            val2,
            val3;

        Kwadrat(int val1, int val2, int val3)
        {
            this-&gt;val1 = val1; // Working
            this.val2 = val2;  // Doesn&#39;t Work!
            this::val3 = val3; // Doesn&#39;t Work!
        }
    };


    int main()
    {
        Kwadrat* kwadrat = new Kwadrat(1,2,3);

        cout&lt;&lt;kwadrat-&gt;val1&lt;&lt;endl;
        cout&lt;&lt;kwadrat-&gt;val2&lt;&lt;endl;
        cout&lt;&lt;kwadrat-&gt;val3&lt;&lt;endl;

        return 0;
    }


```


What is the difference between &quot;::&quot; &quot;.&quot; and &quot;-&gt;&quot; in c++

I&#39;ve been wondering what the best (i.e. cleanest/safest/most efficient) way of handling multiple constructors in Java is? Especially when in one or more constructors not all fields are specified:

    public class Book
    {

        private String title;
        private String isbn;

        public Book()
        {
          //nothing specified!
        }

        public Book(String title)
        {
          //only title!
        }

        ...     
 
    }

What should I do when fields are not specified? I&#39;ve so far been using default values in the class so that a field is never null, but is that a &quot;good&quot; way of doing things?

Best way to handle multiple constructors in Java

I wonder is there a way to get all private fields of some class in java and their type.

For example lets suppose I have a class

    class SomeClass {
        private String aaa;
        private SomeOtherClass bbb;
        private double ccc;
    }
Now I would like to get all private fields (`aaa`, `bbb`, `ccc`) of class `SomeClass` (Without knowing name of all fields upfront) and check their type.

Java reflection get all private fields

I see a lot of example code for C# classes that does this:

    public class Point {
        public int x { get; set; }
        public int y { get; set; }
    }

Or, in older code, the same with an explicit private backing value and without the new auto-implemented properties:

    public class Point {
        private int _x;
        private int _y;
    
        public int x {
            get { return _x; }
            set { _x = value; }
        }
    
        public int y {
            get { return _y; }
            set { _y = value; }
        }
    }


My question is why. Is there any functional difference between doing the above and just making these members public fields, like below?

    public class Point {
        public int x;
        public int y;
    }

To be clear, I understand the value of getters and setters when you need to do some translation of the underlying data. But in cases where you&#39;re just passing the values through, it seems needlessly verbose.

Auto-implemented getters and setters vs. public fields

I looked through the MongoDB documentation and googled this question but couldn&#39;t really find a suitable answer. So, here is what I&#39;m looking for. 
Assume I have a collection with elements like this:

    {
       &quot;foo&quot; : &quot;bar&quot;,
       &quot;test&quot; : &quot;test&quot;,
       &quot;key&quot; : &quot;value&quot;,
    }

What I&#39;d like to achieve is find an element by searching in all (maybe except for finitely many ;-) ) fields. In other words: Given a query, I do NOT know in which field the query should be found.

In my thinking something like this 

    db.things.find({_ANY_ : &quot;bar&quot;}) 

would give me the example element.

Thank you for your help.

Searching for value of any field in MongoDB without explicitly naming it

When have you ever personally come upon the **&lt;a href=&quot;https://en.wikipedia.org/wiki/Halting_problem&quot;&gt;halting problem&lt;/a&gt;** in the field? This can be when a co-worker / boss suggested a solution which would violate the fundamental limits of computation, or when you realized yourself that a problem you were trying to solve was, in fact, impossible to solve.

The most recent time I came up with it was when studying type checkers. Our class realized that it would be impossible to write a perfect type checker (one that would accept all programs that would run without type errors, and reject all programs that would run with type errors) because this would, in fact, solve the halting problem. Another was when we realized, in the same class, that it would be impossible to determine whether a division would ever occur by zero, in the type-checking stage, because checking whether a number, at run-time, is zero, is also a version of the halting problem.

When have you come upon the halting problem in the field?

I&#39;ve got this model:

    class Visit(models.Model):
        timestamp  = models.DateTimeField(editable=False)
        ip_address = models.IPAddressField(editable=False)


If a user visits multiple times in one day, **_how can I filter for unique rows based on the ip field?_** (I want the unique visits for today)

    today = datetime.datetime.today()
    yesterday = datetime.datetime.today() - datetime.timedelta(days=1)

    visits = Visit.objects.filter(timestamp__range=(yesterday, today)) #.something?


**EDIT:**

I see that I can use:

    Visit.objects.filter(timestamp__range=(yesterday, today)).values(&#39;ip_address&#39;)

to get a ValuesQuerySet of just the ip fields. Now my QuerySet looks like this:

    [{&#39;ip_address&#39;: u&#39;127.0.0.1&#39;}, {&#39;ip_address&#39;: u&#39;127.0.0.1&#39;}, {&#39;ip_address&#39;:
     u&#39;127.0.0.1&#39;}, {&#39;ip_address&#39;: u&#39;127.0.0.1&#39;}, {&#39;ip_address&#39;: u&#39;127.0.0.1&#39;}]

**How do I filter this for uniqueness without evaluating the QuerySet and taking the db hit?** 

    # Hope it&#39;s something like this...
    values.distinct().count()

Get distinct values of Queryset by field

what is the right field type for IP address in mysql? and what is the right way of storing it using PHP?

IP address storing in MySQL database using PHP

According to [this documentation](http://pandas.pydata.org/pandas-docs/stable/comparison_with_sql.html#left-outer-join) I can only make a join between fields having the same name.

Do you know if it&#39;s possible to join two DataFrames on a field having different names?

The equivalent in SQL would be:

    SELECT *
    FROM df1
    LEFT OUTER JOIN df2
      ON df1.id_key = df2.fk_key

Pandas: join DataFrames on field with different names?

I&#39;d like to create a class where the client can store a lambda expression like `[]() -&gt; void {}` as a field of the class, but I can&#39;t figure out how to do so.  [One answer suggested using `decltype`](https://stackoverflow.com/a/5849247/34537), which I tried with no success.   Here is a [ideone source link](http://ideone.com/9fC2K). The below is the source and result:

    #include &lt;cstdio&gt;
    auto voidLambda = []()-&gt;void{};

    class MyClass {
    public:
         decltype(voidLambda) t;
         MyClass(decltype(voidLambda) t) { 
            this-&gt;t = t;
         }
    };
   
    int main() {
       MyClass([] {
          printf(&quot;hi&quot;);
       });
    }

Result:

    prog.cpp: In constructor &#39;MyClass::MyClass(&lt;lambda()&gt;)&#39;:
    prog.cpp:3:79: error: no matching function for call to &#39;&lt;lambda()&gt;::__lambda0()&#39;
    prog.cpp:2:20: note: candidates are: &lt;lambda()&gt;::&lt;lambda&gt;(const&lt;lambda()&gt;&amp;)
    prog.cpp:2:20: note:                 &lt;lambda()&gt;::&lt;lambda&gt;(&lt;lambda()&gt;&amp;&amp;)
    prog.cpp:3:88: error: no match for &#39;operator=&#39; in &#39;((MyClass*)this)-&gt;MyClass::t = t&#39;
    prog.cpp: In function &#39;int main()&#39;:
    prog.cpp:5:27: error: no matching function for call to &#39;MyClass::MyClass(main()::&lt;lambda()&gt;)&#39;
    prog.cpp:3:48: note: candidates are: MyClass::MyClass(&lt;lambda()&gt;)
    prog.cpp:3:14: note:                 MyClass::MyClass(const MyClass&amp;)

Does anyone know how to do this?

How can I store a lambda expression as a field of a class in C++11?

I want to compare two dates with time, I want all the results `from tbl where date1 &gt; date2`

    Select * From temp where mydate &gt; &#39;2009-06-29 04:00:44&#39;;

but it is just comparing dates not time. it is giving me all the result set of today&#39;s date

    &#39;2009-06-29 11:08:57&#39;
    &#39;2009-06-29 11:14:35&#39;
    &#39;2009-06-29 11:12:38&#39;
    &#39;2009-06-29 11:37:48&#39;
    &#39;2009-06-29 11:52:17&#39;
    &#39;2009-06-29 12:12:50&#39;
    &#39;2009-06-29 12:13:38&#39;
    &#39;2009-06-29 12:19:24&#39;
    &#39;2009-06-29 12:27:25&#39;
    &#39;2009-06-29 12:28:49&#39;
    &#39;2009-06-29 12:35:54&#39;
    &#39;2009-06-29 12:36:54&#39;
    &#39;2009-06-29 12:49:57&#39;
    &#39;2009-06-29 12:58:04&#39;
    &#39;2009-06-29 04:13:20&#39;
    &#39;2009-06-29 04:56:19&#39;
    &#39;2009-06-29 05:00:23&#39;
    &#39;2009-06-29 05:04:26&#39;
    &#39;2009-06-29 05:08:17&#39;
    &#39;2009-06-29 05:26:57&#39;
    &#39;2009-06-29 05:29:06&#39;
    &#39;2009-06-29 05:32:11&#39;
    &#39;2009-06-29 05:52:07&#39;

Thanks in advance!

Mysql Compare two datetime fields

How do I make certain fields in a ModelForm required=False?

If I have:

    class ThatForm(ModelForm):
      class Meta:
        widgets = {&quot;text&quot;: Textarea(required=False)}

Or if I have:

    class ThatForm(ModelForm):
      text = Textarea(required=False)

Django returns:

    __init__() got an unexpected keyword argument &#39;required&#39;

Django: Make certain fields in a ModelForm required=False

I just found out in Java you can declare a field &#39;static transient&#39; - the compiler doesn&#39;t complain. This doesn&#39;t seem to be useful in any way since static fields are not serialized, as we all know.

But I wonder, is there actually a case where &#39;static transient&#39; fields are useful?

Java: Static transient fields

I am not sure if this is possible but I want to iterate through a class and set a field member property without referring to the field object explicitly:

    public class Employee
    {
      public Person _person = new Person();
    	
      public void DynamicallySetPersonProperty()
      {
        MemberInfo[] members = this.GetType().GetMembers();
    
        foreach (MemberInfo member in members.Where(a =&gt; a.Name == &quot;_person&quot;))
        //get the _person field
        {
    
          Type type = member.GetType();
          PropertyInfo prop = type.GetProperty(&quot;Name&quot;); //good, this works, now to set a value for it
    
          //this line does not work - the error is &quot;property set method not found&quot;
          prop.SetValue(member, &quot;new name&quot;, null);
        }
      }
    }
    
    
    public class Person
    {
      public string Name { get; set; }
    }



In the answer that I marked as the answer you need to add:

      public static bool IsNullOrEmpty(this string source)
      {
        return (source == null || source.Length &gt; 0) ? true : false;
      }


c# - How to iterate through classes fields and set properties

I have one `UNIQUE` field in my table, and I need to search over it quickly. Do I need to index it?

Do searches over unique fields and indexed fields vary in speed or resource usage?

MySQL: unique field needs to be an index?

In [C# in depth][1] (an excellent book thus far), Skeet explains *events aren&#39;t fields*. I read this section many times and I don&#39;t understand why the distinction makes any difference.

I am one of those developers that confuse events and delegate instances. In my mind, they are the same. Aren&#39;t both just a form of indirection? We can multicast both. An event is setup as a field as shorthand...sure. But, we are adding or removing handlers. Stacking them up to be called when the event fires. Don&#39;t we do the same thing with delegates, stack them up and call invoke?


  [1]: http://www.amazon.com/Depth-Second-Edition-Jon-Skeet/dp/1935182471

All Field Solutions on Gang of Coders

Total of 38 Field Solutions