Java generics is a feature in Java programming language that allows you to write more generic and reusable code. Generics provide a way to parameterize types, so that a class or method can operate on objects of various types without the need for explicit casting. This helps to improve type safety and makes the code more readable and maintainable.
The syntax for defining a generic type is to specify the type parameter in angle brackets following the class or method name. For example, the following code defines a generic class named Box that can hold any type of object:
public class Box<T> {
private T item;
public T getItem() {
return item;
}
public void setItem(T item) {
this.item = item;
}
}The Box class has a single type parameter T,
which can be any type. The setItem method
takes an argument of type T, and the getItem method
returns an object of type T. When you create an
instance of the Box class, you specify the actual
type to use for T.
Wildcard is another feature of Java generics, which allows you to represent an unknown type. Wildcards are denoted by the ? symbol, and they can be used in place of a type parameter when the exact type is unknown or unimportant.
For example, the following code defines a method that accepts a List of objects of an unknown type:
public void printList(List<?> list) {
for (Object elem : list) {
System.out.print(elem + " ");
}
System.out.println();
}The printList method takes a List of objects of
an unknown type using the wildcard ?.
This means that the method can accept a List of
any type. The method then iterates over the elements
in the list and prints each one to the console.
Wildcards can also be bounded by specifying upper or lower bounds using the extends and super keywords. For example, the following code defines a method that takes a List of objects that extend the Number class:
public void printNumbers(List<? extends Number> list) {
for (Number num : list) {
System.out.print(num + " ");
}
System.out.println();
}In this example, the ? wildcard is bounded by extends Number, which means that the method can accept a List of objects that extend the Number class, such as Integer, Double, or BigDecimal. This allows you to write more generic code that works with a wider range of types.
Java type erasure refers to the process by which the Java compiler removes type information from generic code at compile time. This is done to maintain compatibility with legacy code that was written before the introduction of generics in Java 5.
When you write generic code in Java,
you use type parameters to define
the type of the objects that will
be stored in a collection or used
as a method parameter. For example,
you might write a method that takes
a List of Strings as a parameter:
public void printList(List<String> list) {
for (String s : list) {
System.out.println(s);
}
}At compile time, the Java compiler replaces
the type parameter List<String> with the raw type List.
This means that the type information is erased,
and the method signature becomes:
public void printList(List list) {
for (Object o : list) {
String s = (String) o;
System.out.println(s);
}
}As you can see, the type information has been removed,
and the code now uses the raw type List instead of List<String>.
This can cause problems if you try to access
the generic type information at runtime,
because it is no longer available.
To work around this, Java uses a process
called type bounds to ensure that the generic code
is type-safe at runtime.
Type bounds specify the upper and lower limits
of the type that can be used with
a generic class or method.
For example, the type bound extends Number specifies
that the generic type must be a subclass
of the Number class.
In summary, Java erasure is the process by which the Java compiler removes type information from generic code at compile time, and type bounds are used to ensure type safety at runtime.
Generics allow you to define classes, interfaces, and methods that can work with any type. A generic type is a class or interface that is parameterized over types. The parameterized types are referred to as type parameters.
A wildcard in Java generics is a special type argument that can be used to match any type. The wildcard character "?" is used to represent an unknown type.
<? extends T> is a bounded wildcard that allows any type that is a subtype of T or T itself. It is useful when you want to read from a collection, but not write to it. <? super T> is also a bounded wildcard that allows any type that is a supertype of T or T itself. It is useful when you want to write to a collection, but not read from it.
Yes, for example, suppose we have a method that takes a list of numbers and returns the sum. We can use a wildcard to make the method more flexible, as follows:
public static double sum(List<? extends Number> numbers) {
double sum = 0.0;
for (Number number : numbers) {
sum += number.doubleValue();
}
return sum;
}This method can now accept a List of any type that is a subtype of Number, such as Integer, Double, or Float.
Type erasure is the process of removing all generic type information from a program when it is compiled. This is done to ensure compatibility with pre-existing code that was not designed to work with generics.
To create a generic class in Java, you can use the following syntax:
public class MyClass<T> {
// class definition
}The "T" in angle brackets is the type parameter, which can be replaced by any class or interface.
Yes, you can create a generic method in a non-generic class. To do so, you need to declare the type parameter before the return type, as follows:
public class MyClass {
public <T> void myGenericMethod(T t) {
// method body
}
}A raw type in Java generics is a class or interface that is used without any type parameter. Raw types are allowed for compatibility with pre-existing code that was not designed to work with generics.
Type inference in Java generics is the process by which the Java compiler infers the type arguments for a generic method invocation, based on the context in which the method is called.
A generic is a type that is parameterized over types. A parameterized type is a specific instance of a generic, with actual type arguments supplied for the type parameters.
Yes, you can use a wildcard in a method return type. For example, you can declare a method that returns a List of any type that is a subtype of Number, as follows:
public List<? extends Number> getNumbers() {
// method body
}This method can return a List of any type that is a subtype of Number.