State pattern in LLD - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding the State Pattern Purpose

Which of the following best describes the main purpose of the State pattern in system design?

A To allow an object to change its behavior when its internal state changes, appearing as if it changed its class.

B To create a single instance of a class to be shared across the system.

C To separate the construction of a complex object from its representation.

D To provide a way to access elements of a collection sequentially without exposing its underlying representation.

💡 Hint

Think about how an object behaves differently based on its current condition or mode.

The State pattern allows an object to alter its behavior when its internal state changes, making it appear as if the object changed its class. This helps manage state-specific behavior cleanly.

❓ Architecture

intermediate

Identifying Components in State Pattern Architecture

In a typical State pattern architecture, which component is responsible for defining the interface for encapsulating the behavior associated with a particular state?

A Client

B Concrete State

C Context

D State Interface or Abstract State

💡 Hint

Consider which part defines the contract for all states to implement.

The State Interface or Abstract State defines the common interface for all concrete states, ensuring they implement the required behavior methods.

❓ scaling

advanced

Scaling State Pattern for Many States

When a system using the State pattern needs to support a very large number of states, which approach best helps maintain scalability and manageability?

A Avoid using the State pattern and switch to a simple enum-based state variable.

B Use a single state class with many conditional statements to handle all states.

C Combine related states into groups and use a hierarchical state machine with nested states.

D Implement all states as separate classes and instantiate them on demand in the Context.

💡 Hint

Think about organizing states to reduce complexity and reuse behavior.

Hierarchical state machines group related states into nested states, reducing the number of classes and improving manageability when many states exist.

❓ tradeoff

advanced

Tradeoffs of Using State Pattern

What is a common tradeoff when applying the State pattern in system design?

A It simplifies the code but makes it impossible to add new states later.

B It reduces code duplication but increases the number of classes and complexity.

C It improves performance but reduces code readability.

D It eliminates the need for interfaces but increases memory usage.

💡 Hint

Consider how the pattern affects class count and code organization.

The State pattern reduces code duplication by encapsulating state-specific behavior but increases the number of classes, which can add complexity to the system.

❓ component

expert

Request Flow in State Pattern with Context and States

Consider a system where a Context object delegates requests to its current State object. Which sequence best describes the flow when the Context receives a request that causes a state transition?

A 1,2,3,4

B 1,3,2,4

C 2,1,3,4

D 1,2,4,3

💡 Hint

Think about the order of delegation and state update during a request.

The Context first receives the request, then delegates it to the current State. The State processes it and may decide to change the Context's state. Finally, the Context updates its reference to the new State.

Practice

1. What is the main purpose of the State pattern in system design?

easy

A. To provide a global point of access to a resource

B. To create multiple instances of a class efficiently

C. To allow an object to change its behavior when its internal state changes

D. To separate the construction of a complex object from its representation

Solution

1. Step 1: Understand the role of the State pattern

   The State pattern helps an object change its behavior based on its internal state without changing its class.

2. Step 2: Compare with other design patterns

   Other options describe Singleton (A), Prototype (B), and Builder (C) patterns, which are unrelated to state behavior changes.

3. Final Answer:

   To allow an object to change its behavior when its internal state changes -> Option C

4. Quick Check:

   State pattern = behavior change by internal state [OK]

Hint: State pattern changes behavior with state, not object creation [OK]

Common Mistakes:

• Confusing State pattern with Singleton or Builder patterns
• Thinking it manages object creation instead of behavior
• Assuming it provides global access to resources

2. Which of the following is the correct way to define a state interface in a typical State pattern implementation?

easy

A. interface State { void handle(); }

B. class State { void handle() {} }

C. enum State { START, STOP }

D. struct State { int status; }

Solution

1. Step 1: Identify the correct interface syntax

   The State pattern requires a State interface with a method like handle() to define behavior.

2. Step 2: Eliminate incorrect options

   class State { void handle() {} } is a class, not an interface; C is an enum, not behavior; D is a struct without behavior.

3. Final Answer:

   interface State { void handle(); } -> Option A

4. Quick Check:

   State interface defines behavior method [OK]

Hint: State pattern needs interface with behavior method [OK]

Common Mistakes:

• Using enum or struct instead of interface/class for behavior
• Defining empty methods without interface
• Confusing class and interface roles

3. Consider this simplified code snippet using the State pattern:

class Context {
  State state;
  void request() { state.handle(this); }
  void setState(State s) { state = s; }
}

class State {
  void handle(Context c) { c.setState(new StateB()); }
}

class StateB extends State {
  void handle(Context c) { c.setState(new State()); }
}

Context ctx = new Context();
ctx.setState(new State());
ctx.request();
ctx.request();

What is the final state of ctx after these two requests?

medium

A. An instance of State

B. An instance of StateB

C. Null (no state)

D. An error occurs

Solution

1. Step 1: Trace first request()

   Initially, ctx.state = State instance. Calling request() calls State.handle(ctx), which sets state to new StateB.

2. Step 2: Trace second request()

   Now ctx.state = StateB instance. Calling request() calls StateB.handle(ctx), which sets state back to new State.

3. Final Answer:

   An instance of State -> Option A

4. Quick Check:

   State and StateB toggle on requests [OK]

Hint: State and StateB toggle on each request call [OK]

Common Mistakes:

• Assuming state stays the same after first request
• Confusing which handle method is called
• Ignoring state changes inside handle methods

4. In the following State pattern code, what is the main issue causing incorrect behavior?

interface State {
  void handle(Context c);
}

class Context {
  State state;
  void request() {
    state.handle(this);
  }
}

class ConcreteStateA implements State {
  void handle(Context c) {
    // Missing state transition
    System.out.println("State A handling");
  }
}

Context ctx = new Context();
ctx.state = new ConcreteStateA();
ctx.request();
ctx.request();

medium

A. State interface method signature is incorrect

B. Context's state is never updated inside handle, so state never changes

C. Context does not initialize state before request

D. ConcreteStateA does not implement handle method

Solution

1. Step 1: Analyze state transitions in handle()

   ConcreteStateA.handle() prints a message but does not update Context's state, so no state change occurs.

2. Step 2: Check other options

   State interface method is correct; Context initializes state before request; ConcreteStateA implements handle properly.

3. Final Answer:

   Context's state is never updated inside handle, so state never changes -> Option B

4. Quick Check:

   State transition missing inside handle() [OK]

Hint: State must update Context's state inside handle() [OK]

Common Mistakes:

• Forgetting to update state inside handle method
• Assuming printing is enough for state change
• Ignoring initialization of state before request

5. You are designing a traffic light system using the State pattern. The traffic light cycles through Green, Yellow, and Red states. Which design choice best applies the State pattern to handle state transitions and behavior?

hard

A. Use a global variable to track color and update it externally without encapsulating behavior

B. Use a single class with a variable holding the current color and switch behavior using if-else statements

C. Implement the traffic light as a simple timer without state classes

D. Create separate state classes for Green, Yellow, and Red, each implementing a next() method to switch to the next state

Solution

1. Step 1: Understand State pattern application

   The pattern suggests encapsulating each state in its own class with behavior and transitions.

2. Step 2: Evaluate options for scalability and clarity

   Create separate state classes for Green, Yellow, and Red, each implementing a next() method to switch to the next state cleanly separates states and their transitions. Options B, C, and D mix logic or lack encapsulation, reducing maintainability.

3. Final Answer:

   Create separate state classes for Green, Yellow, and Red, each implementing a next() method to switch to the next state -> Option D

4. Quick Check:

   Separate classes with transitions = State pattern [OK]

Hint: Separate states as classes with next() method for transitions [OK]

Common Mistakes:

• Using if-else instead of separate state classes
• Not encapsulating state behavior inside classes
• Relying on external variables without behavior