Question

我正在尝试打印Car数组中的所有元素。但是控制台显示它存在空指针问题。

假设我有一个名为Car的类，并且Car类中没有错。 CarLeadership类可操纵Car对象组。我将它们放在名为carArr（CarLeadership类中的一个属性）的数组中。

public class CarLeadership {
private Car[] carArr;
private int position; //point to the current element of carDatabase

//pass length of array
public CarLeadership(int maxCar) {  
    Car[] carArr = new Car[maxCar];
    position=0;                                                   
}

//parameterized constructor
public CarLeadership(Car[] carDatabase,int position) {
    carArr = carDatabase;
    this.position = position;
}

//copy constructor
public CarLeadership(CarLeadership c) {
    this.carArr = c.getCarArr();
    this.position = c.position;
} 

public void setCarArr(Car[] carArr) {
    this.carArr = carArr;
}

public void setCarArrElements(int index,Car carObj) {
    if(index >= carArr.length) {
        System.out.println("Index out of bounds");
        System.exit(0);
    }
    carArr[index] = carObj;
}

public Car[] getCarArr() {
    //deep copy
    Car[] anotherCarArr = new Car[this.carArr.length];
    for(int i=0; i<anotherCarArr.length;i++)
        anotherCarArr[i] = this.carArr[i];
    return anotherCarArr;
}

public void setPosition(int position) {
    this.position = position;
}

public int getPosition() {
    return position;
}

public String toString() { //instance has carArr
    String s="";
    for(int i=0;i<carArr.length;i++) 
        if(carArr[i]!= null) 
            s += ("Car: # "+ i + "\nMake: " +this.carArr[i].getMake()
                    +"\nModel: "+this.carArr[i].getModel()
                    +"\nYear: "+this.carArr[i].getYear()
                    +"\nPrice: "+this.carArr[i].getPrice()+"\n");
    return s;
}
}

//driver class
public class CarLeadershipDriver {

public static void main(String[] args) {
    // TODO Auto-generated method stub
    CarLeadership cc = new CarLeadership(5);
    cc.setCarArrElements(0,new Car("aa","cc",2018,242));
    cc.setCarArrElements(1,new Car("aa","aa",2018,242));
    cc.setCarArrElements(2,new Car("aa","cc",2018,242));    
    System.out.println(cc);
}
}

错误消息：

 Exception in thread "main" java.lang.NullPointerException
    at CarLeadership.setCarArrElements(CarLeadership.java:31)
    at CarLeadershipDriver.main(CarLeadershipDriver.java:7)

Answer 1

更改构造函数

public CarLeadership(int maxCar) {
    Car[] carArr = new Car[maxCar];
    position = 0;
}

到

public CarLeadership(int maxCar) {
    carArr = new Car[maxCar];
    position = 0;
}

仅此而已。

现在您的程序以输出运行：

Car: # 0
Make: aa
Model: cc
Year: 2018
Price: 242
Car: # 1
Make: aa
Model: aa
Year: 2018
Price: 242
Car: # 2
Make: aa
Model: cc
Year: 2018
Price: 242

Answer 2

问题在于您正在构造函数中初始化carArr的本地副本。

替换

//pass length of array
public CarLeadership(int maxCar) {  
    Car[] carArr = new Car[maxCar];
    position=0;                                                   
}

与

//pass length of array
public CarLeadership(int maxCar) {  
    carArr = new Car[maxCar];
    position=0;                                                   
}

或者如果您想更明确

//pass length of array
public CarLeadership(int maxCar) {  
    this.carArr = new Car[maxCar];
    position=0;                                                   
}

Answer 3

您在构造函数中创建了新数组，同时应将声明的大小分配给现有数组。

this.carArr =新车[maxCar];

Java数组返回NullPointerException

3 个答案: