Constructors

The Big Picture

Constructors initialize objects when you call new. They set initial state, validate inputs, and allocate resources.

• Same name as the class, no return type
• Can be overloaded with different parameters
• Run once per object creation, before any other access

Every class has at least one (default or explicit).

How It Works

No explicit constructor? Java provides a no-arg default. Add any constructor and the default disappears.

• Chain with this(...) to delegate within same class
• super(...) explicitly calls a parent constructor; else Java inserts super()
• Superclass constructor always runs first
• Fields initialize before constructor body executes

In the Java Landscape

Java 16+ records auto-generate canonical constructors.

• Builder pattern avoids telescoping constructors
• Static factory methods (of(), create()) often replace them
• DI frameworks (Spring) call constructors to wire beans
• Lombok @AllArgsConstructor generates constructors
• Copy constructors for defensive copying

Pitfalls and Best Practices

• Validate parameters - fail fast with IllegalArgumentException
• Don't call overridable methods in constructors
• Consider static factories when named creation/validation/caching helps
• Use this(...) chaining to avoid duplicate logic
• Keep constructors simple - complex logic in factories
• Document required vs optional parameters