Java Generics 简明教程

Type Parameter Naming Conventions

根据惯例，类型参数名称会被命名为单个大写字母，这样可以很容易地将类型参数与普通类或接口名称区分开来。以下是常用的类型参数名称列表：

By convention, type parameter names are named as single, uppercase letters so that a type parameter can be distinguished easily with an ordinary class or interface name. Following is the list of commonly used type parameter names −

  1. E − Element, and is mainly used by Java Collections framework.

  2. K − Key, and is mainly used to represent parameter type of key of a map.

  3. V − Value, and is mainly used to represent parameter type of value of a map.

  4. N − Number, and is mainly used to represent numbers.

  5. T − Type, and is mainly used to represent first generic type parameter.

  6. S − Type, and is mainly used to represent second generic type parameter.

  7. U − Type, and is mainly used to represent third generic type parameter.

  8. V − Type, and is mainly used to represent fourth generic type parameter.

以下示例将展示上述概念。

Following example will showcase above mentioned concept.

Example

使用任意你选择的编辑器创建以下 Java 程序。

Create the following java program using any editor of your choice.

GenericsTester.java

package com.tutorialspoint;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

public class GenericsTester {
   public static void main(String[] args) {
      Box<Integer, String> box = new Box<Integer, String>();
      box.add(Integer.valueOf(10),"Hello World");
      System.out.printf("Integer Value :%d\n", box.getFirst());
      System.out.printf("String Value :%s\n", box.getSecond());

      Pair<String, Integer> pair = new Pair<String, Integer>();
      pair.addKeyValue("1", Integer.valueOf(10));
      System.out.printf("(Pair)Integer Value :%d\n", pair.getValue("1"));

      CustomList<Box> list = new CustomList<Box>();
      list.addItem(box);
      System.out.printf("(CustomList)Integer Value :%d\n", list.getItem(0).getFirst());
   }
}

class Box<T, S> {
   private T t;
   private S s;

   public void add(T t, S s) {
      this.t = t;
      this.s = s;
   }

   public T getFirst() {
      return t;
   }

   public S getSecond() {
      return s;
   }
}

class Pair<K,V>{
   private Map<K,V> map = new HashMap<K,V>();

   public void addKeyValue(K key, V value) {
      map.put(key, value);
   }

   public V getValue(K key) {
      return map.get(key);
   }
}

class CustomList<E>{
   private List<E> list = new ArrayList<E>();

   public void addItem(E value) {
      list.add(value);
   }

   public E getItem(int index) {
      return list.get(index);
   }
}

这将产生以下结果。

This will produce the following result.

Output

Integer Value :10
String Value :Hello World
(Pair)Integer Value :10
(CustomList)Integer Value :10