`&lt;?&gt;` is a shorthand for `&lt;? extends Object&gt;`, it&#39;s also known as an *unbounded wildcard*. So you can specify any type of object in your generic.

For example the `List` class is declared as `List&lt;?&gt;`, because it can be a list of anything you want.

---

**Resources:**

 - [Effective java - Generics.pdf][1]
 - [Java language specification - Type Arguments and Wildcards][2]
 - [oracle.com - Java tutorials, wildcards][3]


  [1]: http://java.sun.com/docs/books/effective/generics.pdf
  [2]: http://docs.oracle.com/javase/specs/jls/se7/html/jls-4.html#jls-4.5.1
  [3]: http://docs.oracle.com/javase/tutorial/extra/generics/wildcards.html

Its a `wildcard` type. It is short for `? extends Object`

If you get it, all you know is its an `Object`. If you try to set, you can&#39;t because it could be any sub class of Object.

The wildcard Generic is &quot;something&quot;. It will be handled as something that extends from Object.

From Java [documentation](http://docs.oracle.com/javase/tutorial/java/generics/wildcards.html):

&gt; In generic code, the question mark (?), called the wildcard,
&gt; represents an unknown type. The wildcard can be used in a variety of
&gt; situations: as the type of a parameter, field, or local variable;
&gt; sometimes as a return type (though it is better programming practice
&gt; to be more specific). The wildcard is never used as a type argument
&gt; for a generic method invocation, a generic class instance creation, or
&gt; a supertype.

So, for instance, a `List&lt;?&gt;` is a list containing objects from unknown type.

Its a wildcard. Suppose that you have a collection but the type of that collection is not known, thus you denote it by &quot;?&quot;. It simply specifies that the type is not known.
For more details refer http://java.sun.com/j2se/1.5/pdf/generics-tutorial.pdf
I&#39;m sure it will help.

I&#39;m trying to get a list of all keys in a list of dictionaries in order to fill out the fieldnames argument for csv.DictWriter.

previously, I had something like this: 

    [
    {&quot;name&quot;: &quot;Tom&quot;, &quot;age&quot;: 10},
    {&quot;name&quot;: &quot;Mark&quot;, &quot;age&quot;: 5},
    {&quot;name&quot;: &quot;Pam&quot;, &quot;age&quot;: 7}
    ]

and I was using `fieldnames = list[0].keys()` to take the first dictionary in the list and extract its keys.

Now I have something like this where one of the dictionaries has more key:value pairs than the others (could be any of the results).  The new keys are added dynamically based on information coming from an API so they may or may not occur in each dictionary and I don&#39;t know in advance how many new keys there will be.


    [
    {&quot;name&quot;: &quot;Tom&quot;, &quot;age&quot;: 10},
    {&quot;name&quot;: &quot;Mark&quot;, &quot;age&quot;: 5, &quot;height&quot;:4},
    {&quot;name&quot;: &quot;Pam&quot;, &quot;age&quot;: 7}
    ]

I can&#39;t just use `fieldnames = list[1].keys()` since it isn&#39;t necessarily the second element that will have extra keys.

A simple solution would be to find the dictionary with the greatest number of keys and use it for the fieldnames, but that won&#39;t work if  you have an example like this: 

    [
    {&quot;name&quot;: &quot;Tom&quot;, &quot;age&quot;: 10},
    {&quot;name&quot;: &quot;Mark&quot;, &quot;age&quot;: 5, &quot;height&quot;:4},
    {&quot;name&quot;: &quot;Pam&quot;, &quot;age&quot;: 7, &quot;weight&quot;:90}
    ]
where both the second and third dictionary have 3 keys but the end result should really be the list `[&quot;name&quot;, &quot;age&quot;, &quot;height&quot;, &quot;weight&quot;]`




Extract all keys from a list of dictionaries

I want to create 3 AppSettings config files:

- Database.config
- Messages.config
- Global.config

And after add in my App.config:

    &lt;appSettings file=&quot;Database.config&quot; /&gt;
    &lt;appSettings file=&quot;Messages.config&quot; /&gt;
    &lt;appSettings file=&quot;Global.config&quot; /&gt;

But when I try to access a key that there is in one of three files with the `ConfigurationManager`, I got the following error: 

&gt;Configuration system failed to initialize. Sections must only appear once per config file.

I cannot have more than one AppSettings config file? 

Multiple AppSettings files, is it possible?

&gt; **Possible Duplicate:**  
&gt; [What does List&amp;lt;?&amp;gt; mean in java generics?](https://stackoverflow.com/questions/1844770/what-does-list-mean-in-java-generics)  
&gt; [What does the question mark in Java generics&amp;#39; type parameter mean?](https://stackoverflow.com/questions/3009745/what-does-the-question-mark-in-java-generics-type-parameter-mean)  

&lt;!-- End of automatically inserted text --&gt;

Apologies but it was difficult trying to search for `&lt;?&gt;`.

What does &lt;?&gt; mean in regards to Java generics? I understand `&lt;A extends B&gt;` and `&lt;A super B&gt;`, but I have never seen this question mark on its own before.

Generics what does &lt;?&gt; actually mean?

> Possible Duplicate: 
> <a href="https://stackoverflow.com/questions/1844770/what-does-list-mean-in-java-generics" target="_blank" rel="noopener noreferrer">What does List&#x3C;?> mean in java generics?</a> 
> <a href="https://stackoverflow.com/questions/3009745/what-does-the-question-mark-in-java-generics-type-parameter-mean" target="_blank" rel="noopener noreferrer">What does the question mark in Java generics' type parameter mean?</a>

Apologies but it was difficult trying to search for <code>&#x3C;?></code>.
What does  mean in regards to Java generics? I understand <code>&#x3C;A extends B></code> and <code>&#x3C;A super B></code>, but I have never seen this question mark on its own before.

I am reviewing the concept of inner classes in java. so far from what I&#39;ve understood and applied java inner classes has a link or access to the methods and fields of its outer/ enclosing class.

My Question: 
&lt;ol&gt;
&lt;li&gt;When should create or define an inner class? &lt;/li&gt;
&lt;li&gt;are inner classes considered to be called as &quot;Helper classes&quot; ?&lt;/li&gt;
&lt;li&gt;What are the indicators for you to make an inner class and what&#39;s their other purpose?&lt;/li&gt;&lt;/ol&gt;

What are the purposes of inner classes

I need to save the phone&#39;s timezone in the format [+/-]hh:mm

I am using TimeZone class to deal with this, but the only format I can get is the following:

    PST -05:00
    GMT +02:00

I would rather not substring the result, is there any key or option flag I can set to only get the value and not the name of that timezone (GMT/CET/PST...)?

Get TimeZone offset value from TimeZone without TimeZone name

Why in Java you&#39;re able to add Strings with the + operator, when String is a class?  In the`String.java` code I did not find any implementation for this operator. Does this concept violate object orientation?

How does the String class override the + operator?

I&#39;ve generated a private key with:

    openssl genrsa [-out file] –des3

After this I&#39;ve generated a public key with:

    openssl rsa –pubout -in private.key [-out file]

I want to sign some messages with my private key, and verify some other messages with my public key, using code like this:

    public String sign(String message) throws SignatureException{
        try {
            Signature sign = Signature.getInstance(&quot;SHA1withRSA&quot;);
            sign.initSign(privateKey);
            sign.update(message.getBytes(&quot;UTF-8&quot;));
            return new String(Base64.encodeBase64(sign.sign()),&quot;UTF-8&quot;);
        } catch (Exception ex) {
            throw new SignatureException(ex);
        }
    }

    public boolean verify(String message, String signature) throws SignatureException{
        try {
            Signature sign = Signature.getInstance(&quot;SHA1withRSA&quot;);
            sign.initVerify(publicKey);
            sign.update(message.getBytes(&quot;UTF-8&quot;));
            return sign.verify(Base64.decodeBase64(signature.getBytes(&quot;UTF-8&quot;)));
        } catch (Exception ex) {
            throw new SignatureException(ex);
        }
    }

I found a solution to convert my private key to PKCS8 format and load it. It works with some code like this:

    public PrivateKey getPrivateKey(String filename) throws Exception {

        File f = new File(filename);
        FileInputStream fis = new FileInputStream(f);
        DataInputStream dis = new DataInputStream(fis);
        byte[] keyBytes = new byte[(int) f.length()];
        dis.readFully(keyBytes);
        dis.close();
        PKCS8EncodedKeySpec spec = new PKCS8EncodedKeySpec(keyBytes);
        KeyFactory kf =
                KeyFactory.getInstance(&quot;RSA&quot;);
        return kf.generatePrivate(spec);
    }

And finally my question is: **How do I load my RSA Public Key from a file?**

I think maybe I need to convert my public key file to x509 format, and use `X509EncodedKeySpec`. But how can I do this?

Load RSA public key from file

I need to check if the thread running a certain piece of code is the main (UI) thread or not. How can I achieve this?

How to check if current thread is not main thread

I have a generic interface like this:

    interface A&lt;T&gt; {
        T getValue();
    }

This interface has limited instances, hence it would be best to implement them as enum values. The problem is those instances have different type of values, so I tried the following approach but it does not compile:

    public enum B implements A {
        A1&lt;String&gt; {
            @Override
            public String getValue() {
                return &quot;value&quot;;
            }
        },
        A2&lt;Integer&gt; {
            @Override
            public Integer getValue() {
                return 0;
            }
        };
    }

Any idea about this?

How to implement enum with generics?

What is the difference between these 2 functions?
 
    static void gPrint(List&lt;? extends Number&gt; l) {
	    for (Number n : l) {
			System.out.println(n);
		}
	}
	
	static &lt;T extends Number&gt; void gPrintA(List&lt;T&gt; l) {
		for (Number n : l) {
			System.out.println(n);
		}
	}

I see the same output.

Java Generics WildCard: &lt;? extends Number&gt; vs &lt;T extends Number&gt;

I have a json string, which I should deSerialize to the following class

    class Data &lt;T&gt; {
        int found;
        Class&lt;T&gt; hits
    }

How do I do it?
This is the usual way

    mapper.readValue(jsonString, Data.class);
But how do I mention what T stands for?


Jackson - Deserialize using generic class

I have an array of int:

    int[] a = {1, 2, 3};

I need a typed set from it:

    Set&lt;Integer&gt; s;

If I do the following:

    s = new HashSet(Arrays.asList(a));

it, of course, thinks I mean:

    List&lt;int[]&gt;

whereas I meant:

    List&lt;Integer&gt;

This is because int is a primitive. If I had used String, all would work:

    Set&lt;String&gt; s = new HashSet&lt;String&gt;(
        Arrays.asList(new String[] { &quot;1&quot;, &quot;2&quot;, &quot;3&quot; }));

How to easily, correctly and succinctly go from:
--

    A) int[] a...

to

    B) Integer[] a ...

Thanks!


Java int[] array to HashSet&lt;Integer&gt;

I understand this occurs with Java 7 when using varargs with a generic type;

But my question is..

What exactly does Eclipse mean when it says &quot;its use could potentially pollute the heap?&quot;

And 

How does the new `@SafeVarargs` annotation prevent this?

Content Type	Original Author	Original Content on Stackoverflow
Question	mezamorphic	View Question on Stackoverflow
Solution 1 - Java	Colin Hebert	View Answer on Stackoverflow
Solution 2 - Java	Peter Lawrey	View Answer on Stackoverflow
Solution 3 - Java	Francisco Spaeth	View Answer on Stackoverflow
Solution 4 - Java	praxmon	View Answer on Stackoverflow

Generics what does <?> actually mean?

Java Problem Overview

Java Solutions

Solution 1 - Java

Solution 2 - Java

Solution 3 - Java

Solution 4 - Java

Multiple AppSettings files, is it possible?

Extract all keys from a list of dictionaries

Attributions