如何在String数组为null时抛出异常？Truck构造器校验实现

问题描述 #

我希望在通过构造器创建Truck实例时，若String数组参数materials为null，则抛出新的TruckException。请问在if语句中应如何校验该数组是否为null？相关代码如下：

{
  private String[] materials = new String[5];

  private long permit = 0;

  private String company = "";


  public Truck(String[] materials, long permit, String company, String ownerName, String ownerAddress, String brand, String model, String licencePlate, float value) throws TruckException, VehicleException
  {
    super(ownerName, ownerAddress, brand, model, licencePlate, value);

    setMaterials(materials); //on ne fait pas setMaterial(this.materials) car setMaterials modifie this.materials 
    if(materials.length == 0 || materials.length>5 // ||???) //this if statement
      throw new TruckException("materials");
    
    this.company = company;
    if(company.equals(""))
      throw new VehicleException("company");

//rest of code...


}

解决方案 #

要实现null校验，必须先判断数组是否为null——直接访问null数组的length属性会触发NullPointerException。利用逻辑或（||）的短路特性（只要前面的条件为真，后面的条件就不会执行），可以避免空指针异常。

修改后的if语句如下：

if(materials == null || materials.length == 0 || materials.length > 5)
    throw new TruckException("materials");

额外优化建议 #

你当前代码先调用setMaterials(materials)再做参数校验，如果setMaterials方法未处理null情况，会在校验前触发空指针异常。建议将校验逻辑提前到setMaterials调用之前，确保参数合法后再执行赋值操作，调整后的完整代码片段：

public Truck(String[] materials, long permit, String company, String ownerName, String ownerAddress, String brand, String model, String licencePlate, float value) throws TruckException, VehicleException
{
    super(ownerName, ownerAddress, brand, model, licencePlate, value);

    // 先执行参数合法性校验
    if(materials == null || materials.length == 0 || materials.length > 5)
        throw new TruckException("materials");
    
    setMaterials(materials); // 参数合法后再赋值
    this.company = company;
    if(company.equals(""))
        throw new VehicleException("company");

    // rest of code...
}

内容的提问来源于stack exchange，提问作者Nicholas Spektor