Builder Pattern — Construct complex objects step by step
Step-by-step construction; validate at build(). · Fluent APIs chain and read like prose.
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In code
class Query {
private cols = "*";
private tbl = "";
select(c: string) {
this.cols = c;
return this;
}
from(t: string) {
this.tbl = t;
return this;
}
build() {
if (!this.tbl) throw new Error("from() required");
return `SELECT ${this.cols} FROM ${this.tbl}`;
}
}
new Query().select("id").from("users").build();📘 Key ideas
Problem it solves
Constructors with 8 parameters are unreadable. Optional parameters cause constructor explosion. Telescoping constructors are a maintenance nightmare.
Fluent interface
Each setter returns 'this', enabling method chaining: QueryBuilder.select('*').from('users').where('active=1').limit(10).build()
Build is terminal
build() is the last call. It validates required fields (e.g. from() is mandatory) and constructs the final immutable object.
When to use
Objects with many optional parameters, especially when some combinations are invalid. Common for HTTP clients, SQL builders, and config objects.
🧠 Practice — Apply What You Learned
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