为何调用toString()方法打印后程序未添加空白行？

问题根源 #

你使用System.out.println(c.toString() + "\n\n")时，println方法会自动在输出内容末尾追加一个换行符，导致手动添加的\n\n和方法自带的换行叠加后，没有达到预期的空行分隔效果；再结合你提供的输出结果来看，当前写法的换行逻辑完全没有生效，核心问题出在println的自动换行特性与手动换行的冲突上。

修复方案 #

方案1：替换println为print

去掉println自带的自动换行，仅依赖手动添加的\n\n实现分隔：

for(Bicycle c : Bikes){
    if(c instanceof MountainBike){
        ((MountainBike) c).setHeight(99);
        System.out.println("New Mountain Bike Info: ");
        System.out.print(c.toString() + "\n\n");
    }
    
    if(c instanceof TandemBike){
        ((TandemBike) c).setBetweenSpace(88);
        System.out.println("New Tandem Bike Info: ");
        System.out.print(c.toString() + "\n\n");    
    }
   
    if(c instanceof RoadBike){
        ((RoadBike) c).setHandlebarType("NewType");
        System.out.println("New Road Bike Info: ");
        System.out.print(c.toString() + "\n\n");   
    }
}

方案2：单独输出空行

用无参的println()输出空行，逻辑更直观清晰：

for(Bicycle c : Bikes){
    if(c instanceof MountainBike){
        ((MountainBike) c).setHeight(99);
        System.out.println("New Mountain Bike Info: ");
        System.out.println(c.toString());
        // 输出两个空行实现分隔
        System.out.println();
        System.out.println();
    }
    
    if(c instanceof TandemBike){
        ((TandemBike) c).setBetweenSpace(88);
        System.out.println("New Tandem Bike Info: ");
        System.out.println(c.toString());
        System.out.println();
        System.out.println();    
    }
   
    if(c instanceof RoadBike){
        ((RoadBike) c).setHandlebarType("NewType");
        System.out.println("New Road Bike Info: ");
        System.out.println(c.toString());
        System.out.println();
        System.out.println();   
    }
}

额外优化 #

由于一个Bicycle实例只会是MountainBike/TandemBike/RoadBike中的一种，把三个独立if改成else if可以减少不必要的判断，提升代码效率：

for(Bicycle c : Bikes){
    if(c instanceof MountainBike){
        ((MountainBike) c).setHeight(99);
        System.out.println("New Mountain Bike Info: ");
        System.out.println(c.toString());
        System.out.println();
        System.out.println();
    } else if(c instanceof TandemBike){
        ((TandemBike) c).setBetweenSpace(88);
        System.out.println("New Tandem Bike Info: ");
        System.out.println(c.toString());
        System.out.println();
        System.out.println();
    } else if(c instanceof RoadBike){
        ((RoadBike) c).setHandlebarType("NewType");
        System.out.println("New Road Bike Info: ");
        System.out.println(c.toString());
        System.out.println();
        System.out.println();
    }
}

内容的提问来源于stack exchange，提问作者Aaron Hall