java - 泛型方法上奇怪的 Java 泛型编译器行为

Question

我正在尝试使用泛型实现一个非常简单的命令模式实现

public abstract class Command { 
}


public interface CommandHandler<H extends Command> {

    boolean isActive();     
    void execute( H command );

}


public class CommandExecutionServiceImpl implements CommandExecutionService {

    private Map< Class<Command>,CommandHandler<Command>> commandMap; 


    public CommandExecutionServiceImpl(){       
        commandMap = new HashMap<Class<Command>, CommandHandler<Command>>();        
    }

    @Override
    public void executeCommand(Command command) {

        CommandHandler<Command> handler = commandMap.get(command.getClass());
        handler.execute(command);       

    }

    @Override
    public boolean isActive(Command command) {
        return false;
    }

    @Override
    public <H extends Command> void addCommandHandler(Class<H> commandClass, CommandHandler<H> handler) {       

        commandMap.put( commandClass, handler );

    }

编译失败

编译失败CommandExecutionServiceImpl.java:[36,12] put(java.lang.Class,CommandHandler) in java.util.Map,CommandHandler>不能应用于(java.lang.Class,CommandHandler)

我无法理解为什么编译器无法在 commandMap.put( commandClass, handler ); 处推断出类型。

如有任何帮助，我们将不胜感激。

Answer 1

看起来您想要 map 的键和值之间的通用关系。如果支持它，它可能看起来像这样:

private <H extends Command> Map<Class<H>, CommandHandler<H>> commandMap;

但这显然是不允许的。解决方法是使用 Josh Bloch 的 Typesafe Heterogeneous Container图案:

private Map<Class<? extends Command>, CommandHandler<? extends Command>> commandMap; 

@Override
public <H extends Command> void executeCommand(H command) {

    // addCommandHandler guarantees the safety of this unchecked cast
    @SuppressWarnings("unchecked")
    CommandHandler<H> handler = (CommandHandler<H>)commandMap.get(command.getClass());
    handler.execute(command);
}