Implementation mechanism of generics in Java

Java generics use type erasure technology and eliminate type information at runtime, retaining structure information. It uses wildcards to represent any type and limits the type scope through bounding generics. Through type erasure, generics increase code reusability and flexibility, reducing overhead and error potential.

The implementation mechanism of generics in Java

Generics are a powerful feature in the Java programming language, which allows you to create programs that can operate on different types of data. classes and methods. This improves code reusability and flexibility.

Type erasure

The implementation of generics in Java relies on Type erasure technology. This means that when compiling Java code, the compiler erases generic type information. Only the structural information of the generic type is retained, such as the name and number of type parameters.

This approach allows access to generic type information at runtime without creating additional overhead or performance issues.

Wild Card

Wildcard in Java is a special type of generic type that allows you to specify a placeholder that can match any type . Wildcard characters are represented by question marks (?).

For example, the following method will accept a list of any type:

public static <T> void printList(List<T> list) {
    for (T element : list) {
        System.out.println(element);
    }
}

List<String> strList = List.of("a", "b", "c");
printList(strList); // 可打印字符串类型列表

Bounded Generics

Bounded GenericsAllows you to limit the scope of a generic type . You can specify a boundary type and all accepted types must be that type or its subclass or interface.

For example, the following method will accept any type that implements the Comparable interface:

public static <T extends Comparable<T>> T max(T a, T b) {
    return a.compareTo(b) > 0 ? a : b;
}

Integer maxInt = max(10, 20); // 可在整数类型上使用该方法

Practical case

Suppose you want to create a type that can store any type Hash table of data. You can use the HashMap<K, V> class where K is the key type and V is the value type:

Map<String, Integer> studentAges = new HashMap<>();
studentAges.put("John", 20);
studentAges.put("Mary", 22);

System.out.println(studentAges.get("John")); // 输出：20

Generic Allows you to easily create programs that can handle different types of data. It increases code reusability, flexibility and reduces the likelihood of errors.

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