Java Lejos 抛出 nullpointerException

Question

这个问题已经有答案了:

What is a NullPointerException, and how do I fix it? (12 个回答)

已关闭 7 年前。

StackOverflow 用户您好。这与Java有关，而不是与机器人本身有关。我试图做的是将传感器与运动方法分开，以使代码易于阅读，但我遇到了一个问题。

传感器.java

package sensors;

import lejos.hardware.sensor.EV3ColorSensor;
import lejos.hardware.sensor.EV3GyroSensor;
import lejos.hardware.sensor.EV3TouchSensor;
import lejos.hardware.sensor.EV3UltrasonicSensor;

public class Sensors {

    EV3TouchSensor touchSensor;
    EV3ColorSensor colorSensor;
    EV3GyroSensor gyroSensor;
    EV3UltrasonicSensor ultrasonicSensor;

    public Sensors(EV3TouchSensor t, EV3ColorSensor c, EV3GyroSensor g, EV3UltrasonicSensor u) {
        touchSensor = t;
        colorSensor = c;
        gyroSensor = g;
        ultrasonicSensor = u;
    }

    public int getColorSample(){
        int sample = colorSensor.getColorID();
        return sample;
    }

}

运动.java

public class Movement {

    RegulatedMotor left;
    RegulatedMotor right;
    EV3TouchSensor touchSensor;
    EV3ColorSensor colorSensor;
    EV3GyroSensor gyroSensor;
    EV3UltrasonicSensor ultrasonicSensor;

    public Movement(RegulatedMotor l, RegulatedMotor r, EV3TouchSensor t,  EV3ColorSensor c, EV3GyroSensor g,
            EV3UltrasonicSensor u) {
        left = l;
        right = r;
        touchSensor = t;
        colorSensor = c;
        gyroSensor = g;
        ultrasonicSensor = u;
    }

    //initialize sensors

    Sensors sensors = new Sensors(touchSensor, colorSensor, gyroSensor, ultrasonicSensor);

    public void moveForward() {
        // get the color sample of the ground
        //int sample = colorSensor.getColorID();
        int sample = sensors.getColorSample();

        // while machine is on the ground color
        while (sample != 7) {

            // get new sample
            //sample = colorSensor.getColorID();
            sample = sensors.getColorSample();

            // move forward
            syncForward();
        }
        // if on black, stop motors.
        syncStop();
    }

不要看其他方法，因为这些方法运行流畅，但错误出现在第 30 行，它试图从传感器类获取样本。我不知道，我也给出了注释掉的行，它们运行流畅。该错误来自访问传感器类，我无法想出解决方案。

我欠你的情!

Answer 1

啊，我花了一段时间才明白这一点。

问题出在这一行:传感器传感器=新传感器(touchSensor、colorSensor、gyroSensor、ultrasonicSensor);

您已经在构造函数之后编写了此初始化，但是，sensors 将在您初始化构造函数中的字段之前分配。这样，将创建仅包含空值的 Sensors 对象，并且在调用 sensors.getColorSample() 时，会出现 int Sample = colorSensor.getColorID( ); 在 Sensors 类中将抛出 NullPointerException。

要修复它，请尝试更改 Movement 类，如下所示:

public class Movement {

    RegulatedMotor left;
    RegulatedMotor right;
    EV3TouchSensor touchSensor;
    EV3ColorSensor colorSensor;
    EV3GyroSensor gyroSensor;
    EV3UltrasonicSensor ultrasonicSensor;

    // like in your code, sensors is still a field, but won't be initialized yet
    Sensors sensors;

    public Movement(RegulatedMotor l, RegulatedMotor r, EV3TouchSensor t,  EV3ColorSensor c, EV3GyroSensor g,
            EV3UltrasonicSensor u) {
        left = l;
        right = r;
        touchSensor = t;
        colorSensor = c;
        gyroSensor = g;
        ultrasonicSensor = u;

        // here is the initialization of sensors - at this point, the arguments shouldn't be null anymore (unless they are passed as null to the constructor)
        sensors = new Sensors(touchSensor, colorSensor, gyroSensor, ultrasonicSensor);
    }

    public void moveForward() {
        // this method should be okay, you don't need to change it

        // ...
    }

对 future 的一些提示:

• 将您的字段设置为私有(private) - 请参阅 Declaring Member Variables在 Java 教程中
• 如果字段不允许使用 null 值(例如，当它可能导致 NullPointerException 时)，请检查参数中是否存在 null 值，如果存在非法值，则引发异常。
• 尝试学习调试代码 - 您可能很快就会发现传感器中的字段全部为空，然后您可以再次调试并查看为什么会出现这种情况。要进一步阅读有关 NullPointerExceptions 的内容，我建议 What is a Null Pointer Exception, and how do I fix it? .