为Builder模式中的属性设置默认值，避免空指针异常的发生。

Builder模式是一种创建型设计模式，在该模式中，我们使用Builder对象逐步构建复杂对象。然而，如果在Builder模式中某一属性没有被正常设置，可能会导致空指针异常的发生。例如：

public class User { private String name; private int age; private String email;

// private constructor
private User(Builder builder) {
    this.name = builder.name;
    this.age = builder.age;
    this.email = builder.email;
}

public static class Builder {
    private String name;
    private int age;
    private String email;

    public Builder name(String name) {
        this.name = name;
        return this;
    }

    public Builder age(int age) {
        this.age = age;
        return this;
    }

    public Builder email(String email) {
        this.email = email;
        return this;
    }

    public User build() {
        return new User(this);
    }
}

// public getters
public String getName() {
    return name;
}
public int getAge() {
    return age;
}
public String getEmail() {
    return email;
}

}

在上述代码中，如果在创建Builder对象的同时没有设置属性值，例如：

User.Builder builder = new User.Builder(); User user = builder.build();

那么构造User对象时就会抛出空指针异常。为避免这种情况，可以为属性设置默认值：

public static class Builder { private String name = ""; private int age = 0; private String email = "";

public Builder name(String name) {
    this.name = name;
    return this;
}

public Builder age(int age) {
    this.age = age;
    return this;
}

public Builder email(String email) {
    this.email = email;
    return this;
}

public User build() {
    return new User(this);
}

}

这样，当属性没有被设置时，会使用默认值，而不是抛出