Java question mark in generics

Question mark in Java generics

Method 2: Using String.replaceAll() Non-alphanumeric characters comprise of all the characters except alphabets and numbers . Thanks in Advance 🙂 Solution: For ordinary characters you should use the field of the : The class contains key code constants for things that are not characters.

Question mark in Java generics

I have read that is exactly the same as . Then what is the difference between:

I’ve tried adding String.class and MyClass.class into all these sets but in the second case it doesn’t compile.

Another example I don’t get is:

To me it seems they are identical but if I have a method which returns Set> , I can’t assign it’s return value to variable of type Set .

I am sorry if this is duplicate, but I have read all the other posts and there are very few examples and I still can’t understand it.

with Set , i can use any type i want like : Set s =new HashSet() or Set s =new HashSet() .Since, i can make ? refer to anything i’m not allowed to put anything other than null in the collection and i will get Object out of the collection.

Set says i can only assign type Object for ex : Set s =new HashSet() but not Set s =new HashSet() .In this case i can add any type because Object is the super base type but i will get Object out of the collection.

Set is the basic raw type , you can add anything and get an Object type out of it.

Set is different than Set> .The first one says «I am a Set of non specific Class types » while the second one says «I am a Set of some specific Class types but don’t know which».

1. Using generic classes without their generic parameters (like Set and Class , and, by extension, Set ) is a bad practice, to be avoided at all costs. If a class is generic, always use its generic form. If your IDE/compiler is not giving you big fat warnings when you try to use them, then you do not have sufficient warnings enabled.

1. Set is a set of virtually anything, but explicitly defined to be a set of nothing in particular, so it is practically unusable. Don’t do it.

So, you should never try to «assign [the] return value [of a method which returns Set> ] to variable of type Set «, because you should never have a variable of type Set , you should only have a variable of type Set> .

Better yet, try to have a variable of type Set> if applicable to your situation. The more specific, the better.

Also, notice the following:

 //invalid (gives errors) Set foo1.add( Generics2.class ); //error: cannot convert to capture //perfectly fine, though lame (don't use `Object`, use something specific) Set foo2 = new HashSet<>(); foo2.add( String.class ); foo2.add( Generics2.class ); //invalid (gives warnings, in my opinion it should give errors.) Set foo3 = new HashSet<>(); //warning "raw use of parameterized class 'foo3'" foo3.add( String.class ); //warning "unchecked call to member of raw type" foo3.add( Generics2.class ); //warning "unchecked call to member of raw type" 

What does the question mark character (‘?’) mean in C++?, It’s the conditional operator. It’s a shortcut for IF/THEN/ELSE. means: if a is true, return b, else return c. In this case, if f==r, return 1, else return 0. The question mark is the conditional operator. The code means that if f==r then 1 is returned, otherwise, return 0. The code could be rewritten as.

Java question mark operator

//ternary operator // takes in a conditional statement // a is returned when the condition is true and vice versa return (a == b) ? a : b; int n = (a < b) ? a + 10: b;

How the Question Mark (?) Operator Works in JavaScript, Three Main Uses for the Question Mark (?) in JavaScript: Ternary Operator Optional Chaining Nullish Coalescing We'll look at each of these in detail, starting with the most common way you'll see the ? operator being used – as a ternary operator. 1. Ternary Operator The term ternary means composed of three …

How to remove all non-alphanumeric characters from a string in Java

Given a string str , the task is to remove all non-alphanumeric characters from it and print the modified it.
Examples:

Input: @!Geeks-for’Geeks,123
Output: GeeksforGeeks123
Explanation: at symbol(@), exclamation point(!), dash(-), apostrophes(‘), and commas(, ) are removed.

Input: Geeks_for$ Geeks?<>[]
Output: GeeksforGeeks
Explanation: underscore(_), dollar sign($), white space, question mark(?), curly braces(<>), and square bracket([]) are removed.

Input: GeeksforGeeks123
Output: GeeksforGeeks123
Explanation: No need to remove any character, because the given string doesn’t have any non-alphanumeric character.

Method 1: Using ASCII values
Since the alphanumeric characters lie in the ASCII value range of [65, 90] for uppercase alphabets, [97, 122] for lowercase alphabets, and [48, 57] for digits. Hence traverse the string character by character and fetch the ASCII value of each character. If the ASCII value is not in the above three ranges, then the character is a non-alphanumeric character. Therefore skip such characters and add the rest in another string and print it.

Method 2: Using String.replaceAll()
Non-alphanumeric characters comprise of all the characters except alphabets and numbers . It can be punctuation characters like exclamation mark(!) , at symbol(@) , commas(, ) , question mark(?) , colon(:) , dash(-) etc and special characters like dollar sign($) , equal symbol(=) , plus sign(+) , apostrophes(‘) .

The approach is to use the String.replaceAll method to replace all the non-alphanumeric characters with an empty string.

Below is the implementation of the above approach:

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What Does the Question Mark in Java Generics' Type Parameter Mean

What does the question mark in Java generics' type parameter mean?

means "A class/interface that extends HasWord ." In other words, HasWord itself or any of its children. basically anything that would work with instanceof HasWord plus null .

In more technical terms, ? extends HasWord is a bounded wildcard, covered in Item 31 of Effective Java 3rd Edition, starting on page 139. The same chapter from the 2nd Edition is available online as a PDF; the part on bounded wildcards is Item 28 starting on page 134.

Update: PDF link was updated since Oracle removed it a while back. It now points to the copy hosted by the Queen Mary University of London's School of Electronic Engineering and Computer Science.

Update 2: Lets go into a bit more detail as to why you'd want to use wildcards.

If you declare a method whose signature expect you to pass in List , then the only thing you can pass in is a List .

However, if said signature was List then you could pass in a List instead.

Note that there is a subtle difference between List and List . As Joshua Bloch put it: PECS = producer-extends, consumer-super.

What this means is that if you are passing in a collection that your method pulls data out from (i.e. the collection is producing elements for your method to use), you should use extends . If you're passing in a collection that your method adds data to (i.e. the collection is consuming elements your method creates), it should use super .

This may sound confusing. However, you can see it in List 's sort command (which is just a shortcut to the two-arg version of Collections.sort). Instead of taking a Comparator , it actually takes a Comparator . In this case, the Comparator is consuming the elements of the List in order to reorder the List itself.

Significance of ? in generics

It is a wildcard which means any type that is extending Object(which also includes object).

So you can say that is a shorthand for

Check Oracle docs for Type Arguments and Wildcards

In generic code, the question mark (?), called the wildcard,
represents an unknown type. The wildcard can be used in a variety of
situations: as the type of a parameter, field, or local variable;
sometimes as a return type (though it is better programming practice
to be more specific). The wildcard is never used as a type argument
for a generic method invocation, a generic class instance creation, or
a supertype.

Parameter with question mark and super

? here means everything that is a superclass of T

super means what you can put into the class (at most this, perhaps a superclass).

Because super indicates the lower bounding class of a generic element. So, Action1 could represent Action1 or Action1 .

What does the symbol ? mean?

In generic code, the question mark (?), called the wildcard, represents an unknown type.

The wildcard can be used in a variety of situations: as the type of a parameter, field, or local variable; sometimes as a return type.

So in order to answer the question: it is a Wildcard-> Official Doc so you can handle classes event when you dont know the type.

What does the question mark in java mean?

In Java ? is known as Wildcard, you can use it to respresent an unknown type .

The upper bounded wildcard, , where Foo is any type, matches Foo and any subtype of Foo. The process method can access the list elements as type Foo:

public static void process(Map list) /* code */
>

In your case it is known as Upper bounded wildcard.

It means, that any object, that can extend the A class is applicable in this conditionn.

In Java Collections Map Key,? What does ? refer to?

The question mark (?) represents an unknown type.

In your example, Map , it means that it will match a map containing values of any type. It does not mean you can create a Map and insert values of any type in it.

Quoting from the documentation:

In generic code, the question mark (?), called the wildcard, represents an unknown type. The wildcard can be used in a variety of situations: as the type of a parameter, field, or local variable; sometimes as a return type (though it is better programming practice to be more specific). The wildcard is never used as a type argument for a generic method invocation, a generic class instance creation, or a supertype.

For example, say you want to create a function that will print the values of any map, regardless of the value types:

static void printMapValues(Map myMap) for (Object value : myMap.values()) System.out.print(value + " "); 
>
>

Then call this function passing a Map as argument:

Map myIntMap = new HashMap<>();
myIntMap.put("a", 1);
myIntMap.put("b", 2);
printMapValues(myIntMap);

The wildcard allows you to call the same function passing a Map , or any other value type, as argument:

Map myStrMap = new HashMap<>();
myStrMap.put("a", "one");
myStrMap.put("b", "two");
printMapValues(myStrMap);

This wildcard is called unbounded, since it gives no information about the type. There are a couple of scenarios where you may want to use the unbounded wildcard:

• If you're calling no methods except those defined in the Object class.
• When you're using methods that don't depend on the the type parameter, such as Map.size() or List.clear() .

A wildcard can be unbounded, upper bounded, or lower bounded:

• List is an example of an unbounded wildcard. It represents a list of elements of unknown type.
• List is an example of an upper bounded wildcard. It matches a List of type Number , as well as its subtypes, such as Integer or Double .
• List is an example of a lower bounded wildcard. It matches a List of type Integer , as well as its supertypes, Number and Object .

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