import lombok.NonNull; public class WitherExample { private final int age; private @NonNull final String name; public WitherExample(String name, int age) { if (name == null) throw new NullPointerException(); this.name = name; this.age = age; } public WitherExample withAge(int age) { return this.age == age ? this : new WitherExample(name, age); } protected WitherExample withName(@NonNull String name) { if (name == null) throw new java.lang.NullPointerException("name"); return this.name == name ? this : new WitherExample(name, age); } }