How to specify type parameters of generic methods?

Generic methods use type parameters to specify the data types that can be operated. The type parameter syntax is , which can be specified by explicitly specifying the type or using wildcards, such as: ? (unbounded), ? extends T (upper bound) and ? super T (lower bound). Wildcards specify the scope of a type parameter, for example, ? extends T means that the type parameter must be of type T or a subclass thereof.

Generic method type parameter specification

In Java, generic methods allow developers to create methods that can be used on multiple types Method of operation. When defining a generic method, we need to specify the type parameters. Type parameters determine the type of data a method can operate on.

Type parameter syntax

Type parameters are specified within angle brackets :

<T> void myMethod(T value) {
    // 代码主体
}

In this example, is a type parameter, which indicates that the method can operate on any type of object.

Specify type parameters

Type parameters can be specified by explicitly specifying the type:

<String> void myMethod(String value) {
    // 操作 String 类型的数据
}

You can also use wildcards to specify type parameters:

• ?: Unbounded wildcard, indicating that the type parameter can be of any type.
• ? extends T: upper bound wildcard, indicating that the type parameter must be of type T or its subclass.
• ? super T: lower bound wildcard, indicating that the type parameter must be of type T or its superclass.

Practical case

Suppose we have a List, which contains various types of objects. We want to write a method to print each element in the list:

import java.util.List;

public class Example {

    public static void main(String[] args) {
        List<Object> myList = List.of("Hello", 10, true);
        printElements(myList);
    }

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

In this example, the printElements method is generic and it takes the type parameter T . Therefore, it can print any type of data in the list.

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