Question

我想给一个名为State的类的每个子项赋值，而我有一个状态数组最初都是null，我在这里接收空指针引用：

//finding all the neighbor states of a given configuration

public State[] neighborStates(String config, int modeFlag){
    State[] neighborStates=new State[7];
    int i=0;
    for (Operation o : Operation.values()){
        neighborStates[i].config=move(config,o.name().charAt(0));
        neighborStates[i].realCost++;
        neighborStates[i].opSequence+=o.name();
        neighborStates[i].heuristicCost=getHeuristicCost(neighborStates[i].config, modeFlag);
        i++;
    }       

    return neighborStates;
}

我将代码更改为此但我还是获得了NPE：

public State[] neighborStates(String config, int modeFlag){
        State[] neighborStates=new State[8];
        int i=0;
        for (Operation o : Operation.values()){
            neighborStates[i] = new State(move(config,o.name().charAt(0)),neighborStates[i].realCost++,
                                getHeuristicCost(neighborStates[i].config, modeFlag), neighborStates[i].opSequence+=o.name());
            //neighborStates[i].config=move(config,o.name().charAt(0));
            //neighborStates[i].realCost++;
            //neighborStates[i].opSequence+=o.name();
            //neighborStates[i].heuristicCost=getHeuristicCost(neighborStates[i].config, modeFlag);
            i++;
        }

类State定义为：

public class State {
    public State(String config, int realCost, int heuristicCost, String opSequence){
        this.config = config;
        this.realCost = realCost;
        this.heuristicCost = heuristicCost;
        this.opSequence = opSequence;
    }

Answer 1

您需要在State数组中实例化neighborStates（s）。您创建了一个包含7个插槽的阵列，但它们最初都是null。假设你有一个默认的构造函数，它应该看起来像，

for (Operation o : Operation.values()){
  neighborStates[i] = new State();
  // ...

另外，根据neighborStates

确定Operation.values()的大小可能是个好主意

State[] neighborStates = new State[Operation.values().length];

Answer 2

使用时创建指定大小的数组 State[] neighborStates = new State[7]你分配了那个记忆。无论类型的默认值是什么，都会初始化数组的每个索引。布尔值的默认值为false。数字的默认值为0。这是重要的部分：引用的默认值是null。

数组中的所有值都将为null，直到您使用neighborState[i] = new State();

实例化它们为止

Answer 3

当你编写这行代码时 State [] neighborStates = new State [7];
你刚收到一个对象的引用，其中7个项目都是null，这行只初始化State Array对象，里面没有任何项目。

因此在for循环中，所有项目仍然为null，您必须先使用它初始化每个项目。 neighborStates [i] = new State（）;

Answer 4

对于您的问题版本更新后。

您正在评估此表达式 -

new State(move(config,o.name().charAt(0)),neighborStates[i].realCost++,
                            getHeuristicCost(neighborStates[i].config, modeFlag), neighborStates[i].opSequence+=o.name());

在为neighborStates[i]分配值之前。因此，neighborStates[i].realCost++表示触发空指针异常的部分 - neighborStates[i]仍为null。

Answer 5

你只是初始化一个数组，但你还没有在State []数组中构造成员在使用它之前，需要调用构造函数来构造State类。尝试更改您的代码：

public State[] neighborStates(String config, int modeFlag){
    State[] neighborStates=new State[7];
    int i=0;
    for (Operation o : Operation.values()){
        neighborStates[i] = new State(move(config,o.name().charAt(0)),
                                      someRealCost, //what's the value of this parameter?
                                      getHeuristicCost(config, modeFlag),
                                      modeFlag
                                      );
        ....
        i++;
    }
}

初始化空值项时出现空指针异常

5 个答案: