Why can I chain setters with a class with builder class inside but cannot on a instance of a normal class?

Question

I've got introduced to builder pattern while learning java and I can't understand how builder can chain the setter methods.

//this one compiles
new AlertDialog.Builder().setApplyButton("apply").setText("stop");


//this one doesn't compile
new NormalAlert().setApplyButton("apply").setText("stop");

Answer 1

Builder methods return this (the builder instance itself) so that more methods can be called on it. Generally, the method named build is defined to return the constructed object.

Example:

public class Person {
  private String name;
  private Integer age;

  public static class Builder {
    private String name;
    private Integer age;
        
    public Builder name(String name){
      this.name = name;
      return this;
    }

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

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

  private Person(Builder builder) {
    this.name = builder.name; 
    this.age = builder.age;     
  }
}