Side Effects

• Side effect of a method: any externally observable data modification

public void transfer(double amount, BankAccount other)
{
   balance = balance - amount;
   other.balance = other.balance + amount; // Modifies explicit parameter
}

• Updating explicit parameter can be surprising to programmers;
  it is best to avoid it if possible

• Another example of a side effect is output

public void printBalance() // Not recommended
{
   System.out.println("The balance is now $" + balance);
}

• Bad idea: message is in English, and relies on System.out
  It is best to decouple input/output from the actual work of your classes

• You should minimize side effects that go beyond modification of the implicit parameter

If a refers to a bank account, then the call a.deposit(100) modifies the bank account object. Is that a side effect?

   Answer: No – a side effect of a method is any change outside the implicit parameter.

Consider the DataSet class of Chapter 6. Suppose we add a method
void read(Scanner in)
{
   while (in.hasNextDouble())
      add(in.nextDouble());
}

Does this method have a side effect?

   Answer: Yes – the method affects the state of the Scanner
   parameter.

In Java, a method can never change parameters of primitive type

for example,

double savingsBalance = 1000;
harrysChecking.transfer(500, savingsBalance);      
System.out.println(savingsBalance);      
...
void transfer(double amount, double otherBalance)      
{
   balance = balance - amount;
   otherBalance = otherBalance + amount;
}

savingBalance to otherBalance - ONLY pass by value, no pass by reference in JAVA.

Call by Value and Call by Reference

• Call by value: Method parameters are copied into the parameter variables when a method starts

• Call by reference: Methods can modify parameters

• Java has call by value

• A method can change state of object reference parameters,
  but cannot replace an object reference with another