Question

以下代码中的getDistance()函数无法正常工作。不同的实例未正确传递距离值。正确的值如下：d1 = 0，d2 = 1，d3 = 2。我的代码有什么问题？

 package javaapplication28;

public class JavaApplication28 {
    public interface State {
        public double getDistance();
        public State getParent();
    }

    public static void main(String[] args) {
        abstract class AbstractState implements
                State {
            private State parent = null;
            protected double distance = 0;

            public AbstractState(State parent) {
                this.parent = parent;
                this.distance = parent.getDistance() + 1;
            }
        }

        class ThisState extends AbstractState {
            State parent;
            public ThisState(State parent) {
                super(parent);
            }


            public State getParent() {
                return parent;
            }

            public double getDistance() {
                return distance;
            }
        }
        ThisState TS1 = new ThisState(null);
        ThisState TS2 = new ThisState(TS1);
        ThisState TS3 = new ThisState(TS2);

        int d1 = TS1.getDistance();
        int d2 = TS2.getDistance();
        int d3 = TS3.getDistance();
    }
}

感谢您回答我的问题！

Answer 1

当你运行时：

ThisState TS1 = new ThisState(null);

如果将parent设置为null：

，您将转到此构造函数

public AbstractState(State parent) {
  this.parent = parent;
  this.distance = parent.getDistance() + 1;
}

将导致构造函数的第二行中出现空指针异常。

您需要做的是仅在父级不为空时将距离设置为parent.getDistance() + 1

Answer 2

问题在于：

if (parent != null) { // Hello !?
    this.distance = parent.getDistance() + 1;
}

完整代码：

public class Application4995 {

    public interface State {
        public double getDistance();
        public State getParent();
    }

    public static void main(String[] args) {
       abstract class AbstractState implements State {
            private State parent = null;
            protected double distance = 0;

            public AbstractState(State parent) {
                this.parent = parent;
                if (parent != null) { // Hello !?
                    this.distance = parent.getDistance() + 1;
                }
            }
        }

        class ThisState extends AbstractState {
            State parent;
            public ThisState(State parent) {
                super(parent);
            }


            public State getParent() {
                return parent;
            }

            public double getDistance() {
                return distance;
            }
        }
        ThisState TS1 = new ThisState(null);
        ThisState TS2 = new ThisState(TS1);
        ThisState TS3 = new ThisState(TS2);

        double d1 = TS1.getDistance(); // or int d1 = (int) TS1.getDistance();
        double d2 = TS2.getDistance(); // or int d2 = (int) TS2.getDistance();
        double d3 = TS3.getDistance(); // or int d3 = (int) TS3.getDistance();
   }
}

在不同实例之间传递变量

2 个答案: