java - 为什么我在应用 applyCoupon() 方法后收到 java.lang.NullPointerException

Question

这个问题已经有答案了:

What is a NullPointerException, and how do I fix it? (12 个回答)

已关闭 3 年前。

这是我的代码..但是每当我尝试在 applyCoupon() 方法中访问“Category”类的私有(private)变量到“Product”类时，我都会遇到空指针异常错误...

//主类

public class basicjava{

        public static void main(String args[]){
            Product product=new Product();
            Category category=new Category();
            category.setCategoryName("book");
            product.applyCoupon();
    }

    }

//二等

 public class Product {
        private Category category;
        Product(){

        }

        public Category getCategory() {
            return category;
        }

        public void setCategory(Category category) {
            this.category = category;
        }

        public void applyCoupon(){
            if (category.getCategoryName()=="book") {
                System.out.println("book");
            }
            else {
                System.out.println("page");
            }
        }

    }

//第三类

public class Category {
    private String categoryName;
    Category(){

    }

    public String getCategoryName() {
        return categoryName;
    }

    public void setCategoryName(String categoryName) {
        this.categoryName = categoryName;
    }
}

Answer 1

在使用非原始数据类型之前，您必须在构造函数中或通过 set 方法初始化它们

public static void main(String args[]) {
    Product product = new Product();
    Category category = new Category(); 
    category.setCategoryName("book");
    product.setCategory(category);
    product.applyCoupon();
}