设计模式（命令模式）

public class Trade {
    public void doSell(){
        System.out.println("doSell");
    }
    
    public void doBuy(){
        System.out.println("doBuy");
    }
}

public abstract class TradeCommand {
    protected Trade trade;
    
    public TradeCommand(Trade trade){
        this.trade = trade;
    }
    
    public abstract void execute();
}

public class BuyTradeCommand extends TradeCommand {

    public BuyTradeCommand(Trade trade) {
        super(trade);
    }

    @Override
    public void execute() {
        this.trade.doBuy();
    }
}

public class SellTradeCommand extends TradeCommand {

    public SellTradeCommand(Trade trade) {
        super(trade);
    }

    @Override
    public void execute() {
        this.trade.doSell();
    }
}

public class Invoke {
    public TradeCommand command;
    
    public void setCommand(TradeCommand command){
        this.command = command;
    }
    
    public void execute(){
        command.execute();
    }
}

public class App {

    public static void main(String[] args) {
        Trade trade = new Trade();
        Invoke invoke = new Invoke();
        invoke.setCommand(new SellTradeCommand(trade));
        invoke.execute();
        invoke.setCommand(new BuyTradeCommand(trade));
        invoke.execute();
    }
}