java generics example

java generics example

Here is a Java generics example:

// Generics Example: Using Wildcards with Subtypes
public class GenericsExample {
public static void print(String str) {
System.out.println("Printing String: " + str);
}
public static void print(List<? extends String> list) {
for (String s : list) {
print(s);  // Can only use the methods of the superclass
}
}
public static void main(String[] args) {
List stringList = new ArrayList<>();
stringList.add("Hello");
stringList.add("World");
List intList = new ArrayList<>();
intList.add(1);
intList.add(2);
print(stringList);  // Allowed: String
print(intList);      // Not allowed: Integer
List<? extends Number> numberList = stringList;  // Error: Type mismatch: cannot convert from List to List<? extends Number>
}
}

In the above code, print(List<? extends String> list) is a generic method that accepts any subtype of String. This means it can accept a List<String> or any other subclass of String.

However, when we try to use intList with this method, the compiler throws an error. This is because Integer is not a subtype of String, so we cannot assign an instance of Integer to a variable that expects a subtype of String.

Now let's consider the line where we're trying to assign stringList to numberList. The compiler also throws an error here, and the reason is similar: even though stringList contains elements of type String, which are instances of Number, we cannot assign it directly to a variable that expects a subtype of Number.

Here's how you can use wildcards with subtypes:

// Generics Example: Using Wildcards with Subtypes
public class GenericsExample {
public static void print(List<? extends Number> list) {
for (Number n : list) {
System.out.println("Printing Number: " + n);
}
}
public static void main(String[] args) {
List stringList = new ArrayList<>();
stringList.add("Hello");
stringList.add("World");
print(stringList);  // Allowed: String
List intList = new ArrayList<>();
intList.add(1);
intList.add(2);
print(intList);      // Not allowed: Integer
}
}

In the above code, print(List<? extends Number> list) is a generic method that accepts any subtype of Number. This means it can accept a List<Integer> or any other subclass of Number.

When we try to use stringList with this method, the compiler does not throw an error. This is because String is a subtype of Number, so we can assign stringList to numberList.

Java generics question mark

I'm happy to help with your Java generics question! Please note that I'll respond in English as per your request.

Java generics are a feature that allows developers to define type-safe containers, such as classes and interfaces. Generics were introduced in Java 5 (Tiger) to address the limitations of raw types, which can lead to ClassCastExceptions at runtime.

The syntax for declaring a generic class is similar to declaring any other class:

public class GenericClass {
// ...
}

Here, GenericClass is a generic class with one type parameter T. The type parameter is specified when an instance of the class is created, like so:

GenericClass stringContainer = new GenericClass<>();

In this example, stringContainer is an instance of GenericClass with the type parameter set to String.

Now, let's talk about the question mark. In Java generics, the ? character is used for wildcard types and reifiable types.

Wildcards

A wildcard type is a placeholder that can be replaced by any reference type at runtime. Wildcard types are denoted using the following syntax:

public class GenericClass {
// ...
}

Here, ? is a wildcard that must be extended from T. This means that you can pass instances of classes that extend T to methods that take GenericClass<? extends T> as an argument. For example:

public class Dog {}
public class Cat extends Dog {}
GenericClass<? extends Animal> animalContainer = new GenericClass<>();
animalContainer.store(new Dog()); // OK
animalContainer.store(new Cat()); // OK

In this example, you can pass instances of Dog and Cat to the store method because they both extend Animal.

Reifiable Types

A reifiable type is a type that has a concrete representation at runtime. In other words, a reifiable type is a type that can be used as an argument to methods like instanceof, getClass, or clone. Reifiable types are denoted using the following syntax:

public class GenericClass {
// ...
}

Here, ? is a wildcard that must be a supertype of T. This means that you can pass instances of classes that are supertypes of T to methods that take GenericClass<? super T> as an argument. For example:

public class Animal {}
public class Mammal extends Animal {}
GenericClass<? super Animal> animalContainer = new GenericClass<>();
animalContainer.store(new Animal()); // OK
animalContainer.store(new Mammal()); // OK

In this example, you can pass instances of Animal and Mammal to the store method because they are both supertypes of Animal.

I hope this helps clarify how Java generics work with wildcards!