Java数组(java.lang.NullPointerException)

Question

我的java代码中出现了这个错误，我似乎找不出我做错了什么。

java.lang.NullPointerException
at Race.toString(Race.java:94)
at TestA2Classes.start(TestA2Classes.java:39)
at TestA2.main(TestA2.java:12)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at edu.rice.cs.drjava.model.compiler.JavacCompiler.runCommand(JavacCompiler.java:272)

这是其中一个java文件的代码。问题似乎围绕着我使用数组的部分。我做错了什么？

public class Race {
public static final String[] RACE_DESCRIPTIONS = {"Sprint", "Distance", "Eliminator", "Keirin"};
public static final int SPRINT = 0;
public static final int DISTANCE = 1;
public static final int ELIMINATOR = 2;
public static final int KEIRIN = 3;
public static final int MAX_COMPETITORS = 8;

private int number;
private int typeIndex;
private MyDate date;
private boolean hasFinished;
private Competitor[] competitors;
private int numberOfCompetitors;

public Race(int number, int typeIndex, MyDate date) { 
    // TODO
    this.number = number;
    this.typeIndex = typeIndex;
    this.date = date;
    this.hasFinished = false;
    this.numberOfCompetitors = 0;
    Competitor[] competitors = new Competitor[MAX_COMPETITORS];

}

public int getNumber() {
    // TODO
    return number;
}

public boolean getHasFinished() {
    // TODO
    return hasFinished;
}

public int getTypeIndex() {
    // TODO
    return typeIndex;
}

public MyDate getDate() {
    // TODO
    return date;
}

public Competitor getCompetitor(int number) {
    // TODO
    Competitor competitor = competitors[0];
    for(int i=0; i<competitors.length; i++){
        if(competitors[i].getNumber() == number){
            return competitor;
        }
    }
    return null;
}

public void finishRace(int first, int second, int third) { 
    // TODO
    this.hasFinished = true;
    for(int i=0; i<competitors.length; i++){
        if(competitors[i].getNumber() == first){
            competitors[i].setPosition(1);
        } else if(competitors[i].getNumber() == second){
            competitors[i].setPosition(2);
        } else if(competitors[i].getNumber() == third){
            competitors[i].setPosition(3);
        } else{
            competitors[i].setPosition(0);
        }
    }
}

public boolean addCompetitor(Competitor competitor) {
    // TODO
    if(numberOfCompetitors < MAX_COMPETITORS){
        numberOfCompetitors++;
        return true;
    }
    return false;
}

public String toString() {
    // TODO

    String details = number + ", " + RACE_DESCRIPTIONS[typeIndex] + " [" + date + "]";
    if(hasFinished = false){
        details += ": Race not finished";
    } else if(hasFinished = true){
        details += "\n     1st: " + competitors[1].getName();
        details += "\n     2nd: " + competitors[2].getName();
        details += "\n     3rd: " + competitors[3].getName();
    }
    return details;
}

}

Answer 1

下面是构造函数中的问题：

Competitor[] competitors = new Competitor[MAX_COMPETITORS];

您不是在初始化member variable competitors，而是要在构造函数中创建local competitors variable。因此，您的实例数组变量没有初始化，将为null。由于您的数组将保持统一，因此请更改这里提到的构造函数：

public Race(int number, int typeIndex, MyDate date) { 
    // TODO
    this.number = number;
    this.typeIndex = typeIndex;
    this.date = date;
    this.hasFinished = false;
    this.numberOfCompetitors = 0;
    this.competitors = new Competitor[MAX_COMPETITORS];

}

正如Rohit在他的评论中指出的那样，您的代码中还有另一个问题。您没有初始化竞争对手数组元素。如果要创建新元素，则需要对每个元素执行新的操作，否则数组将不为空，但每个竞争者元素实际上将为空。对这些元素调用任何方法都会导致NullPointerException。

Answer 2

问题1

在构造函数中删除并将其移到声明数组的位置。

 Competitor[] competitors = new Competitor[MAX_COMPETITORS];

当然，在使用它之前，您必须先填充它们。

问题2

toString()法

 if(hasFinished = false){
        details += ": Race not finished";
    } else if(hasFinished = true){
        details += "\n     1st: " + competitors[1].getName();
        details += "\n     2nd: " + competitors[2].getName();
        details += "\n     3rd: " + competitors[3].getName();
    }

你在if条件下初始化布尔变量.不是比较.

那应该是

public String toString() {
    String details = number + ", " +  
                             RACE_DESCRIPTIONS[typeIndex] + " [" + date + "]";
    if(!hasFinished){
        details += ": Race not finished";
    } else{
        details += "\n     1st: " + competitors[1].getName();
        details += "\n     2nd: " + competitors[2].getName();
        details += "\n     3rd: " + competitors[3].getName();
    }
    return details;
}