Question

函数findRecipe返回一个包含String名称的对象，在我捕获返回的对象并使用getName（）方法显示其String名称后，它显示空指针Exception ..Why？我第一次发帖问题..如果不是细节的话。谢谢

public class Cookbook {

private Recipe listOfRecipes[] = new Recipe[100];
private static int numberOfRecipes = 0;

public void setListOfRecipes(Recipe a)
{
    listOfRecipes[numberOfRecipes] = a;
    numberOfRecipes++;
}

public Recipe[] getListOfRecipes()
{
    return listOfRecipes;
}

public void addRecipe(Recipe b)
{
    Recipe temp[] = new Recipe[listOfRecipes.length];
    int count = 0;

    for(int i = 0; i < listOfRecipes.length; i++)
    {
       temp[i] = listOfRecipes[i];  
       count++;
    }

    temp[count] = b;  
}

public Recipe findRecipe(String c)
{
    Recipe temp1 = new Recipe();
    Recipe temp2[] = new Recipe[numberOfRecipes - 1];
    for(int j = 0; j < temp2.length; j++)
    {
    temp2[j] = listOfRecipes[j];
    }

    String a;
    for(int i = 0; i < temp2.length; i++)
    {
        a = temp2[i].getName();
        if(c.equals(a))
        {
          temp1 = temp2[i]; 
        }
        else
        {
          temp1 = null;
        }
    }

    return temp1;
}

public static void main(String[]args)
{
    Recipe butterCake = new Recipe("ButterCake");

    butterCake.setIngredients("3 cup butter");
    butterCake.setIngredients("4 1/2 cup flour");
    butterCake.setIngredients("3 cup sugar");
    butterCake.setIngredients("4 eggs");

    butterCake.setInstructions("Add butter with flour");
    butterCake.setInstructions("Mix butter and flour");
    butterCake.setInstructions("Now add 3 cup sugar");
    butterCake.setInstructions("Mix 4 eggs until yellow pale colour appears");
    butterCake.setInstructions("Bake the mixture for 30 minutes");

    System.out.println(butterCake.getName());
    butterCake.numberOfInstructions();
    butterCake.numberOfIngredients();
    butterCake.showIngredients();
    butterCake.showInstructions();

    Recipe vanillaCake = new Recipe("VanillaCake");

    vanillaCake.setIngredients("3 cup butter");
    vanillaCake.setIngredients("4 1/2 cup flour");
    vanillaCake.setIngredients("3 cup sugar");
    vanillaCake.setIngredients("4 eggs");

    vanillaCake.setInstructions("Add butter with flour");
    vanillaCake.setInstructions("Mix butter and flour");
    vanillaCake.setInstructions("Now add 3 cup sugar");
    vanillaCake.setInstructions("Mix 4 eggs until yellow pale colour appears");
    vanillaCake.setInstructions("Bake the mixture for 30 minutes");

    System.out.println(vanillaCake.getName());
    vanillaCake.numberOfInstructions();
    vanillaCake.numberOfIngredients();
    vanillaCake.showIngredients();
    vanillaCake.showInstructions();

    Cookbook cookBook1 = new Cookbook();

    cookBook1.setListOfRecipes(butterCake);
    cookBook1.setListOfRecipes(vanillaCake);

    Recipe q[] = new Recipe[2];
    q = cookBook1.getListOfRecipes();
    System.out.println(q[0].getName());
    System.out.println(q[1].getName());

    Recipe y = new Recipe();

    y = cookBook1.findRecipe("VanillaCake");

    System.out.println(y.getName()); // ------> GIVING ERROR

  }
}

output :

run:
Name of the Recipe: ButterCake
Number of instructions: 5
Number of ingredients: 4

Ingredients: 
1. 3 cup butter
2. 4 1/2 cup flour
3. 3 cup sugar
4. 4 eggs

Instructions: 
1. Add butter with flour

Exception in thread "main" java.lang.NullPointerException
2. Mix butter and flour
3. Now add 3 cup sugar
at Cookbook.main(Cookbook.java:120)
4. Mix 4 eggs until yellow pale colour appears
5. Bake the mixture for 30 minutes
Name of the Recipe: VanillaCake
Number of instructions: 5
Number of ingredients: 4

Ingredients: 
1. 3 cup butter
2. 4 1/2 cup flour
3. 3 cup sugar
4. 4 eggs

Instructions: 
1. Add butter with flour
2. Mix butter and flour
3. Now add 3 cup sugar
4. Mix 4 eggs until yellow pale colour appears
5. Bake the mixture for 30 minutes
Name of the Recipe: ButterCake
Name of the Recipe: VanillaCake
Java Result: 1
BUILD SUCCESSFUL (total time: 0 seconds)

Answer 1

问题1：一旦找到匹配项，您就不会破坏。因此，只有当您在最后一个索引处发生匹配时，才会返回非空值。

解决方案：找到匹配后添加break。

问题2 ：班级getName中的Recipe功能返回Name of the Recipe: RECIPE NAME

解决方案：更改getName返回配方名称的方式（摆脱Name of the Recipe:）或更改您的通话方式。

在函数for loop

中修改了getRecipe

String a;  
temp1=null;
for(int i = 0; i < temp2.length; i++)
{
    a = temp2[i].getName();
    if(c.equals(a))
    {
      temp1 = temp2[i]; 
      break; //<----------
    }
    else
    {
      temp1 = null;
    }
}

 return temp1;
}

将此功能称为（主要）

y = cookBook1.findRecipe("Name of the Recipe: VanillaCake");

如何查看收到的对象的内容？

1 个答案: