Builder模式中的空指针异常：原因与解决方案

public class Engine {

    private String name;
    private Mercedes m; // 假设Mercedes是另一个类

    // 私有构造器，强制通过Builder创建
    private Engine() {
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Mercedes getM() {
        return m;
    }

    public void setM(Mercedes m) {
        this.m = m;
    }

    // 静态工厂方法，返回一个新的EngineBuilder实例
    public static EngineBuilder builder() {
        return new EngineBuilder();
    }

    public static class EngineBuilder {
        private Engine e = null; // 问题所在：Engine对象在此处默认初始化为null

        // 这个builder()方法容易引起混淆，且在示例中未被调用
        public EngineBuilder builder() {
            e = new Engine(); // 期望在这里初始化e，但客户端并未调用此方法
            return this;
        }

        public Engine build() {
            return this.e;
        }

        public EngineBuilder setName(String name) {
            this.e.setName(name); // NullPointerException发生在此处，因为e是null
            return this;
        }

        public EngineBuilder setM(Mercedes m) {
            this.e.setM(m);
            return this;
        }
    }

    public static void main(String[] args) {
        EngineBuilder builder = Engine.builder(); // 创建了一个新的EngineBuilder实例
        builder.setName("test"); // 尝试调用setName，但builder内部的e仍为null

        Engine e = builder.build();
        System.out.println("Engine name: " + e.getName());
    }
}

public class Engine {

    private String name;
    private Mercedes m; // 假设Mercedes是另一个类

    // 私有构造器，强制通过Builder创建
    private Engine() {
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Mercedes getM() {
        return m;
    }

    public void setM(Mercedes m) {
        this.m = m;
    }

    // 静态工厂方法，返回一个新的EngineBuilder实例
    public static EngineBuilder builder() {
        return new EngineBuilder();
    }

    public static class EngineBuilder {
        private Engine e; // 不再默认初始化为null，而是在构造器中初始化

        // EngineBuilder的构造器，负责初始化内部的Engine对象
        public EngineBuilder() {
            this.e = new Engine(); // 关键：在此处初始化Engine对象
        }

        // 移除或重命名原先容易混淆的内部builder()方法，因为它不再需要
        // 或者确保它被正确调用，但更好的实践是直接在构造器中初始化

        public Engine build() {
            // 可以在此处添加验证逻辑，例如检查必要字段是否已设置
            return this.e;
        }

        public EngineBuilder setName(String name) {
            this.e.setName(name);
            return this;
        }

        public EngineBuilder setM(Mercedes m) {
            this.e.setM(m);
            return this;
        }
    }

    public static void main(String[] args) {
        // 现在，当EngineBuilder实例创建时，其内部的Engine对象也已初始化
        EngineBuilder builder = Engine.builder();
        builder.setName("test"); 

        Engine e = builder.build();
        System.out.println("Engine name: " + e.getName()); // 输出: Engine name: test
    }
}