Introduction to Behavioral Patterns in Java

Behavioral patterns: Patterns that organize how objects cooperate, choose behavior, propagate events, or transition through states at runtime.

Java systems often become difficult not because the types are wrong, but because the runtime collaboration is murky. Behavior gets spread across callbacks, conditionals, listeners, flags, and services until it is hard to see who decides what and when.

That is where behavioral patterns help. They give names to recurring ways of structuring runtime decisions and interactions.

    flowchart LR
	    A["Runtime design pressure"] --> B["Choose an algorithm"]
	    A --> C["Behavior changes with state"]
	    A --> D["Coordinate many participants"]
	    A --> E["Notify dependents"]
	    B --> F["Strategy"]
	    C --> G["State"]
	    D --> H["Mediator"]
	    E --> I["Observer or event model"]

What Makes Behavioral Patterns Different

Creational patterns focus on how objects come into existence.

Structural patterns focus on how objects and interfaces are arranged.

Behavioral patterns focus on what happens after the objects already exist:

• who makes a decision
• how behavior is selected
• how collaboration is routed
• how changes are observed
• how runtime rules remain understandable

Java-Specific Forces

Behavioral patterns in Java are shaped by language and ecosystem features:

• lambdas make some Strategy implementations much lighter
• listeners and callbacks make Observer-like designs very common
• frameworks often introduce mediator, command, and template-method behavior implicitly
• sealed types, records, and enums can make some state models more explicit than older class-heavy designs

Because of that, a modern Java guide should not treat every behavioral pattern as if the only valid form were an old UML class tree.

Choose The Pattern By Runtime Pressure

Use Strategy when the system must choose among interchangeable behaviors cleanly.

Use State when the object’s current state changes what operations mean.

Use Mediator when too many components know too much about each other.

Use Observer when change notifications should flow outward without tight point-to-point coupling.

These patterns can overlap, but they should not be collapsed into one vague idea of “flexibility.”

Review Questions

• What runtime decision is hard to see today?
• Is the behavior changing because of caller choice, internal state, or external events?
• Would the proposed pattern reduce conditional sprawl and coupling, or merely rename it?

Behavioral patterns earn their place when they make the runtime story easier to explain.