NullPointerException的作用及区别

VMNullPointerException在程序试图取消引用引用的位置抛出null，JVM描述了变量（在源代码而言）与空详细消息中NullPointerException。通过更清楚地将动态异常与静态程序代码相关联，它大大提高了对程序的理解。

例如，

import java.util.ArrayList;
import java.util.List;
class Price {
    double basePrice;
    double tax;
    public Price() {
    }
    public Price(double basePrice) {
        this.basePrice = basePrice;
    }
    public Price(double basePrice, double tax) {
        this.basePrice = basePrice;
        this.tax = tax;
    }
}
class Product {
    String name;
    Price price;
    public Product() {
    }
    public Product(String name, Price price) {
        this.name = name;
        this.price = price;
    }
}
class CartEntry {
    Product product;
    int quantity;
    public CartEntry() {
    }
    public CartEntry(Product product, int quantity) {
        this.product = product;
        this.quantity = quantity;
    }
}
class Cart {
    String id;
    List<CartEntry> cartEntries;
    public Cart() {
        cartEntries = new ArrayList<>();
    }

    public Cart(String id) {
        this();
        this.id = id;
    }

    void addToCart(CartEntry entry) {
        cartEntries.add(entry);
    }
}

public class Main {
    public static void main(String[] args) {
        Cart cart = new Cart("cjavapy");
        cart.addToCart(new CartEntry());
        //cart.cartEntries.get(0).product 是 null
        System.out.println(cart.cartEntries.get(0).product.price.basePrice);
    }
}

JDK14(Java 14)之前报错信息：

Exception in thread "main" java.lang.NullPointerException
    at Main.main(Main.java:74)

JDK14(Java 14)的报错信息：

Exception in thread "main" java.lang.NullPointerException: Cannot read field "price" because "java.util.List.get(int).product" is null
    at Main.main(Main.java:74)