安全地使类型或方法能够对各种类型的对象进行操作

/* 
 来自 
*时代Java - N o w  J a v a . c o m*/

import java.util.ArrayList;

import java.util.HashMap;

import java.util.List;

import java.util.Map;


public class Main {

  public static void main(String[] args) {

    List<Player> team = new ArrayList<>();

    Player player1 = new Player("A", "AA", 5);

    Player player2 = new Player("B", "BB", 15);

    Player player3 = new Player("C", "CC", 1);

    Player player4 = new Player("D", "DD", 18);

    Player player5 = new Player("E", "EE", 7);


    team.add(player1);

    team.add(player2);

    team.add(player3);

    team.add(player4);

    team.add(player5);


    List objectList = new ArrayList();/* 来自 nowjava.com*/

    Object obj1 = "none";

    objectList.add(obj1);


    // Create a List that can be of type that is any superclass of Player

    List<? super Player> myTeam = objectList;

    for (Object p : myTeam) {

      System.out.println("Printing the objects...");

      System.out.println(p.toString());

    }


    Map<String, String> strMap = new HashMap<>();

    strMap.put("first", "Josh");

    strMap.put("last", "Juneau");

    System.out.println(strMap.values());

  }

}


class Player {


  private String firstName = null;

  private String lastName = null;

  private String position = null;

  private int status = -1;

  private int goals;


  public Player() {


  }


  public Player(String position, int status) {

    this.position = position;

    this.status = status;

  }


  public Player(String first, String last, int goals) {

    this.firstName = first;

    this.lastName = last;

    this.goals = goals;

  }


  public Player(String position, int status, String first, String last) {

    this.position = position;

    this.status = status;

    this.firstName = first;

    this.lastName = last;

  }


  public String playerString() {

    return getFirstName() + " " + getLastName() + " - " + getPosition();

  }


  /**

   * @return the firstName

   */

  public String getFirstName() {

    return firstName;

  }


  /**

   * @param firstName

   *          the firstName to set

   */

  public void setFirstName(String firstName) {

    this.firstName = firstName;

  }


  /**

   * @return the lastName

   */

  public String getLastName() {

    return lastName;

  }


  /**

   * @param lastName

   *          the lastName to set

   */

  public void setLastName(String lastName) {

    this.lastName = lastName;

  }


  /**

   * @return the position

   */

  public String getPosition() {

    return position;

  }


  /**

   * @param position

   *          the position to set

   */

  public void setPosition(String position) {

    this.position = position;

  }