Java Generics
Generics provide a compile-time type safety detection mechanism, which allows programmers to detect illegal type. The essence of generics is a parameterized type, which means that the data type being operated on is specified as a parameter.
For example, if we want to write a sorting method that can sort an integer array, a string array, or even any other type of array, we can use Java generics.
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1. Generic method
You can write a generic Method, which can receive different types of parameters when called. Depending on the parameter types passed to the generic method, the compiler handles each method call appropriately.
For example:
public static < E > void printArray( E[] inputArray ){
for ( E element : inputArray ){
System.out.printf( "%s ", element );
}
}Generic boundary
(1) indicates that the type represented by the wildcard is of type T Subclass.
(2) indicates that the type represented by the wildcard is the parent class of the T type.
2. Generic class
The declaration of a generic class is similar to the declaration of a non-generic class, except that a type parameter declaration part is added after the class name. Like generic methods, the type parameter declaration part of a generic class also contains one or more type parameters, separated by commas.
A generic parameter, also known as a type variable, is an identifier used to specify the name of a generic type. Because they accept one or more parameters, these classes are called parameterized classes or parameterized types.
For example:
public class Box<T> {
private T t;
public void add(T t) {
this.t = t;
}
public T get() {
return t;
}
}3. Type wildcard?
Type wildcard generally uses ? instead of specific type parameters. For example, List is logically the parent class of List, List and all List
4. Type erasure
Generics in Java are basically implemented at the compiler level. The type information in generics is not included in the generated Java byte code. Type parameters added when using generics will be removed by the compiler during compilation. This process is called type erasure.
Types such as List and List defined in the code will become List after compilation. All the JVM sees is the List, and the type information attached by generics is invisible to the JVM.
The basic process of type erasure is also relatively simple. The first step is to find the specific class used to replace the type parameters. This concrete class is usually Object. If an upper bound for a type parameter is specified, this upper bound is used. Replace all type parameters in the code with specific classes.
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