使用多态Java

Question

我的目标是通过创建testInstrument.java文件打印出所有乐器正在演奏。由于某种原因，我收到了System.out.println(all[i].play());的错误

testInstrument.java

package my_instruments;

public class testInstrument {

    public static void main(String[] args) {
        // TODO Auto-generated method stub

        Guitar g = new Guitar();
        Flute f = new Flute();
        Piano p = new Piano();



        Instrument[] all = new Instrument[3];

        all[0] = g;
        all[1] = f;
        all[2] = p;

        for (int i=0; i<3; i++) {
            System.out.println(all[i].play());
        }


    }

}

Instrument.java

package my_instruments;

public class Instrument {

public Instrument() {

}

public void play() {
    System.out.println("Playing instrument");
}
}

Piano.java

package my_instruments;

public class Piano extends Instrument{
public Piano() {
    super();
}

public void play() {
    System.out.println("Playing piano");
}
}

Answer 1

试试这个：

for (int i=0; i<3; i++) {
       all[i].play();
    }

play方法已经在执行打印，并且不会返回任何要打印的内容。

Answer 2

您的play方法()正在执行打印操作，尝试从System.out.println();循环中删除打印语句

for (int i=0; i<3; i++) {
    all[i].play();
}

或

for (Instrument i : all) {
    i.play();
}