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先上结论：如果类中用了@Builder注解，而属性没有任何注解话，那么在你初始化这个类的时候，如果你的属性赋值了默认值，则在你初始化该类后，属性的默认值则无效即获取会产生空指针异常
接下来我们来剖析下这中间发生了什么
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从上面的例子，可以发现Teacher 的address属性为空，这正是我们很常规初始化操作，获取这个address，接着对它进行操作， 如果此时它是null，则会出现空指针异常；
比较Student中的address则是我们理想中的正常执行过程，是有一个默认值的对象，同时观察Student中的name 和age两个属性值默认值也出现了如同Teacher中的address现象，默认值消失了；
从表面来看，Student多了@Builder.Default的注解，这个注解确实就是解决这个问题关键，让你想要赋值的默认值来进行正确的初始化了。知道了这个注解的使用只是做到了知其然，我们要做做知其所以然，所以来看看下他们生成的class有什么区别？ 以下代码反编译删除了equal和hashcode方法
Teacher.class
public class Teacher {
private String name;
private List<String> address = new ArrayList();
Teacher(String name, List<String> address) {
this.name = name;
this.address = address;
}
public static Teacher.TeacherBuilder builder() {
return new Teacher.TeacherBuilder();
}
public String getName() {
return this.name;
}
public List<String> getAddress() {
return this.address;
}
public void setName(String name) {
this.name = name;
}
public void setAddress(List<String> address) {
this.address = address;
}
protected boolean canEqual(Object other) {
return other instanceof Teacher;
}
public String toString() {
return "Teacher(name=" + this.getName() + ", address=" + this.getAddress() + ")";
}
public static class TeacherBuilder {
private String name;
private List<String> address;
TeacherBuilder() {
}
public Teacher.TeacherBuilder name(String name) {
this.name = name;
return this;
}
public Teacher.TeacherBuilder address(List<String> address) {
this.address = address;
return this;
}
public Teacher build() {
return new Teacher(this.name, this.address);
}
public String toString() {
return "Teacher.TeacherBuilder(name=" + this.name + ", address=" + this.address + ")";
}
}
Student.class
public class Student {
private String name = "c";
private int age = 25;
private long num;
private List<String> address;
private static List<String> $default$address() {
return new ArrayList();
}
Student(String name, int age, long num, List<String> address) {
this.name = name;
this.age = age;
this.num = num;
this.address = address;
}
public static Student.StudentBuilder builder() {
return new Student.StudentBuilder();
}
public String getName() {
return this.name;
}
public int getAge() {
return this.age;
}
public long getNum() {
return this.num;
}
public List<String> getAddress() {
return this.address;
}
public void setName(String name) {
this.name = name;
}
public void setAge(int age) {
this.age = age;
}
public void setNum(long num) {
this.num = num;
}
public void setAddress(List<String> address) {
this.address = address;
}
protected boolean canEqual(Object other) {
return other instanceof Student;
}
public String toString() {
return "Student(name=" + this.getName() + ", age=" + this.getAge() + ", num=" + this.getNum() + ", address=" + this.getAddress() + ")";
}
public static class StudentBuilder {
private String name;
private int age;
private long num;
private boolean address$set;
private List<String> address;
StudentBuilder() {
}
public Student.StudentBuilder name(String name) {
this.name = name;
return this;
}
public Student.StudentBuilder age(int age) {
this.age = age;
return this;
}
public Student.StudentBuilder num(long num) {
this.num = num;
return this;
}
public Student.StudentBuilder address(List<String> address) {
this.address = address;
this.address$set = true;
return this;
}
public Student build() {
List address = this.address;
if(!this.address$set) {
address = Student.$default$address();
}
return new Student(this.name, this.age, this.num, address);
}
public String toString() {
return "Student.StudentBuilder(name=" + this.name + ", age=" + this.age + ", num=" + this.num + ", address=" + this.address + ")";
}
}
}
看两个类的build方法，Student类在调用builde方法时，会判断this.address$set 这个变量是否为false，如果为false，则为这个address对象进行赋值默认值，这个变量就是由@Builder.Default注解产生的.
而如果你直接对address方法进行赋值话，则会将这个this.address$set进行赋值，这样调用build方法时，就不会再对address进行赋值了.
比较Teacher，没有对address属性增加@Builder.Default注解，所以在调用build方法时候，就不会产生判断是否要对address进行默认值的初始化了，所以你获取到的address就是null.
这下你知道你程序为什么会出现空指针异常了，为什么添加@Builder.Default注解就能解决问题了。所以对你用的东西进行深入了解，出现问题才能做到知其然知其所以然
