如何解决代码中的空指针异常？

Question

我正在从事一个涉及三个代码文件的项目：一个 class 用于乘客 - Passenger.java，一个 class 用于飞机本身 - Airplane.java，和 driver class - 运行Airplane.java。使用 main 中的 Passenger 实例化添加了 5 名乘客以及我计划稍后更新的乘客数量，因此它不限于 5 名。问题是每当我运行 IntelliJ 中的主文件，无法找出原因。错误代码表示它发生在 Airplane.java 文件的第 58 行，这让我假设这是因为 addPassenger 方法。

乘客Class（如果需要）

public class Passenger {

    private String name;
    private int  birthYear;
    private double weight;
    private char gender;
    private int numCarryOn;

    //Default constructor
    public Passenger(){
        name = "";
        birthYear = 1900;
        weight = 0.0;
        gender = 'u';
        numCarryOn = 0;
    }

    //Alt default constructor
    public Passenger(String newName, int newBirthYear, double newWeight, char newGend, int newNumCarryOn){

        this.setName(newName);
        this.setBirthYear(newBirthYear);
        this.setWeight(newWeight);
        this.setGender(newGend);
        this.setNumCarryOn(newNumCarryOn);
    }

    //Calculate age
    public int calculateAge(int currentYear){

        int age = 0;

        if (currentYear < birthYear){
            return -1;
        }
        else {
           age = currentYear - birthYear;
        }

        return age;
    }

    //Gain weight
    public void gainWeight(){
        weight += 1;
    }

    //Gain weight based on input
    public void gainWeight(double pounds){

        if (weight > 0){
            weight = weight + pounds;
        }

    }

    //Get birthYear
    public int getBirthYear(){
        return birthYear;
    }

    //Get gender
    public char getGender(){
        return gender;
    }

    //Get name
    public String getName(){
        return name;
    }

    //Get weight
    public double getWeight(){
        return weight;
    }

    //Get numCarryOn
    public int getNumCarryOn(){
        return numCarryOn;
    }

    //isFemale
    public boolean isFemale(){

        if (gender == 'f'){
            return true;
        }
        else {
            return false;
        }
    }

    //isMale
    public boolean isMale(){
        if (gender == 'm'){
            return true;
        }
        else {
            return false;

        }
    }

    //loseWeight
    public void loseWeight(){

        if (weight > 0){
            weight -= 1;
        }

    }

    //lose weight by value
    public void loseWeight(double weight2lose){

        if (weight - weight2lose >= 0){
            weight -= weight2lose;
        }

    }

    //print
    public void printDetails(){

        System.out.printf("Name: %20s | Year of Birth: %4d | Weight: %10.2f | Gender: %c | NumCarryOn: %2d\n", name, birthYear, weight, gender, numCarryOn);

    }

    //setGender
    public void setGender(char newGender){

        if (newGender == 'f' || newGender == 'm') {
            this.gender = newGender;
        } else {
            this.gender = 'u';
        }
    }

    //setName
    public void setName(String newName){
        name = newName;
    }

    //setBirthYear
    public void setBirthYear(int newBY){
        birthYear = newBY;
    }

    //setWeight
    public void setWeight(double newWeight){

        if (newWeight < 0){
            weight = -1;

        }
        else {
            weight = newWeight;
        }

    }

    //setNumCarryOn
    public void setNumCarryOn(int newNumCarryOn){

        if (newNumCarryOn < 0){
            numCarryOn = 0;

        }
        else if (newNumCarryOn > 2){
            numCarryOn = 2;
        }
        else {
            numCarryOn = newNumCarryOn;
        }
    }
}

飞机Class

import java.util.Arrays;

public class Airplane {

    private Passenger[] passengers;
    private String airplaneName;
    private int numPassengers;
    public int count = 0;

    //default constructor
    public Airplane(){

        airplaneName = "";
        passengers = new Passenger[100];
        numPassengers = 0;
    }

    //default constructor with String input
    public Airplane(String otherairplaneName){

        airplaneName = otherairplaneName;
        passengers = new Passenger[100];
        numPassengers = 0;

    }

    //default constructor with int input
    public Airplane(int otherArraySize){

        airplaneName = "";

        if (otherArraySize < 0){
            otherArraySize = 0;
            passengers = new Passenger[otherArraySize];
        }

        numPassengers = 0;

    }

    //default constructor with String and int input
    public Airplane(String otherAirplaneName, int otherArraySize){

        airplaneName = otherAirplaneName;

        if (otherArraySize < 0){
            otherArraySize = 0;
            passengers = new Passenger[otherArraySize];
        }

        numPassengers = 0;

    }

    //add a passenger
    public void addPassenger(Passenger a){

        for (int i = 0; i < passengers.length; i++){

            if (passengers.length - count >= 0){
                passengers[i] = a;
                break;
            }

        }
    }
}

Driver

public class RunAirplane {
    public static void main(String[] args) {
        Airplane a1 = new Airplane("Flight 1", 100);
        Passenger p1 = new Passenger("Albert", 1879, 198.5, 'm', 2);
        Passenger p2 = new Passenger("Grace", 1906, 105, 'f', 1);
        Passenger p3 = new Passenger("Tim", 1955, 216.3, 'm', 2);
        Passenger p4 = new Passenger("Steve", 1955, 160, 'm', 2);
        Passenger p5 = new Passenger("Sergey", 1973, 165.35, 'm', 1);

        a1.addPassenger(p1);
        a1.count += 1;
        a1.addPassenger(p2);
        a1.count += 1;
        a1.addPassenger(p3);
        a1.count += 1;
        a1.addPassenger(p4);
        a1.count += 1;
        a1.addPassenger(p5);
        a1.count += 1;
        a1.printAllDetails();

    }
}

Answer 1

您正在尝试访问全局变量 passengers，只有当参数 otherArraySize 小于您正在使用的构造函数中指定的 0 时，该变量才会被初始化。尝试反转该参数的条件。这样你的数组就会初始化。

public Airplane(String otherAirplaneName, int otherArraySize){

        airplaneName = otherAirplaneName;

        if (otherArraySize > 0){
            otherArraySize = 0;
            passengers = new Passenger[otherArraySize];
        }

        numPassengers = 0;

    }

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