創建給出問題的用戶定義數據類型

Question

我創建了一個包含兩個字段（時間和度量）的數據類型。我創建了這些數據類型的數組。當我然後試圖分配一個值的數組[0]的字段之一，我得到一個空指針異常。誰能告訴我什麼是錯的？

Public class MeasurementPoint{ 

private int time; 
private double measurement; 

public MeasurementPoint(){ 
this.time =0; 
this.measurement =0;}} 

public void setMeausurement(double value){ 
this.measurement = value;} 
public double getMeasurement(){ 
return this.measurement;} 

public void setTime(int value){ 
this.time = value;} 
public int getMeasurement(){ 
return this.time;} 
} 

，我用它來創建數組的代碼如下

MeasurementPoint [] controlValues = new MeasurementPoint [points]; 
System.out.println(controlValues.length(); // this gives me 16 so I know the array is created 

controlValues[0].setMeasurement(aCtivity);// this is where I get the exception. 

Answer 1

您初始化您的數組，但你永遠不初始化您數組元素，這樣你可以獲得NullPointerException當您嘗試接取零'你的數組的索引作爲yourarray [0]返回null，你嘗試在null上調用setMe ...（）..你必須實際初始化你的數組元素。

MeasurementPoint [] controlValues = new MeasurementPoint [points]; 
controlValues[0] = new Measurementpoint(val1,val2);