java - NullPointerException 的问题

Question

今天我遇到了这个评估问题，我必须创建两个类:Dress 和 TestClass。我完成了这些类(class)，但是当我尝试运行该程序时，我收到了 NullPointerException 消息。这是我的类(class):

类(class)着装:

public class Dress {
    String colors []; 
    int sizes [];

    public Dress ( String colors [], int sizes []){
       this.colors = new String [colors.length];
       this.sizes = new int [sizes.length] ;
       this.colors = colors;
       this.sizes = sizes;
    }

    public boolean search (String color){
       for (int i =0; i<colors.length;i++)
           if (colors [i].equals(color))
              return true;
       return false;
    }
    public boolean search (int size){
       for (int i =0; i<sizes.length;i++)
           if (sizes [i] == size)
              return true;
       return false;
    }
}

类测试:

public class Tests {
    public static void main (String args []){
       String color[] = {"Pink","Blue","Red"};
       int size[] = {8,9,7};
       Dress d = new Dress (color, size);
       System.out.println(d.search("Pink"));
       System.out.println(d.search(8));
    }
}

Answer 1

仅供引用 - 将可变引用分配给私有(private)数据成员不是一个好主意:

this.colors = new String [colors.length];  // The new reference is discarded after reassignment on next line
this.colors = colors;  // The program that passes this reference can modify it; changes will be visible to your class instance.

任何获得该引用并更改其状态的人都将更改您的实例数据成员，而不考虑其私有(private)状态。

这是正确的方法(为了清楚起见，仅采用一种方法):

public Dress(String [] colors) {
    if (colors == null) throw new IllegalArgumentException("colors cannot be null");
    this.colors = new String[colors.length];
    // Copy the values from the parameter array into the new, private array.
    System.arraycopy(colors, 0, this.colors, 0, this.colors.length);
}

您应该始终为私有(private)、可变数据制作防御性副本。