Functions Summary

Computation Abstraction: Functions

• Functions group instructions and give them a name
• Allow composition at higher levels of abstraction
• Basic Java function syntax:

  return_type function_name(param_type1 param1, param_type2 param2) {
    function body
  }
  

• Return type is mandatory (use void if no return value)
• Java doesn't support returning multiple values

Benefits of Functions

Compartmentalization

• Isolates computation and effects
• Limits interactions to parameters and return values
• Reduces number of variables to track
• Contains complexity within function body

Implementation Hiding

• Hides how a task is performed
• Caller only needs to know what function does
• Reduces information needed between programmers
• Allows implementation changes without affecting callers

Code Reuse

• Reduces repeated code through parameterization
• Makes code more succinct and readable
• Reduces places that need modification when code evolves
• Decreases chance of introducing bugs

Abstraction Barrier

• Separates code that calls function from code that implements function
• Divides programmer roles:
  • Implementer: provides implementation
  • Client: uses the function
• Enforces separation of concerns
• Allows implementation changes without affecting client code
• Protects implementation details from client

Pure Functions

• Functions that behave like mathematical functions
• Given same input, always produce same output
• Have no side effects (don't modify external state)
• Example:

  int square(int i) {
    return i * i;
  }
  

Non-Pure Functions

• May throw exceptions
• May modify external state
• May depend on external state
• May not return a value
• Example:

  void addToQueue(Queue<Integer> queue, int i) {
    queue.enq(i);  // has side effects on queue
  }
  

Variable Arguments (Varargs)

• Allows methods to accept variable number of arguments
• Uses ellipsis (...) in parameter declaration
• Must be the last parameter in method signature
• Internally treated as an array
• Example:

  void printAll(String... messages) {
    for (String msg : messages) {
      System.out.println(msg);
    }
  }
  

• Can be called with:

  printAll("Hello");                    // one argument
  printAll("Hello", "World");          // two arguments
  printAll("A", "B", "C", "D");        // multiple arguments
  printAll();                          // zero arguments
  

• Can also be called with an array:

  String[] messages = {"A", "B", "C"};
  printAll(messages);                   // passing array directly