Methods: How to Write Them: Revision Notes (Computer Science A 3.5)

These notes recap Topic 3.5. For the full explanations and worked examples, read the study guide.

Recap

A void method returns nothing. A non-void method returns a single value whose type is in the header (return by value).
return hands control back to the caller. Code after it on the same path never runs; a return inside an if or a loop exits the whole method.
A non-void method must return on every path.
An accessor returns a copy of an attribute’s value (non-void). A mutator changes attributes (often void).
For a primitive argument the parameter is a copy. Changing the parameter does not change the argument.
Preconditions are the initial conditions a method needs to work as described.

Key relationships

Method kind	Header pattern	Typical body
Accessor	`public int getCoins()`	`return coins;`
Mutator	`public void addCoins(int amount)`	`coins += amount;`
Mutator with result	`public boolean spend(int amount)`	test, change, `return true` / `return false`
Calculation	`public int coinsWithBonus(int percent)`	`return coins + coins * percent / 100;`
Helper	`private boolean isValid(int p)`	`return p > 0;`

Compile errors to recognise

Message	Cause
`missing return statement`	A path through a non-void method has no return
`unreachable statement`	Code placed after a return in the same block
`incompatible types: possible lossy conversion from double to int`	Returning a `double` from an `int` method
`incompatible types: unexpected return value`	`return x;` inside a void method

Worked reminder

Wallet(40); addCoins(10) makes 50; spend(70) returns false and leaves 50; spend(30) returns true and leaves 20; coinsWithBonus(15) is 20 + 20 * 15 / 100 = 23 and does not change the 20. With int n = 5; h.addTen(n); the value of n is still 5; with n = h.addTenAndReturn(n); it becomes 15. And isPrime(15) returns false the moment d reaches 3, without trying d = 4.

Mistakes to avoid

Forgetting the final return after an if / else if chain.
Putting a statement after a return.
Expecting a void method to give back a value.
Expecting a primitive parameter to alter the caller’s variable.
Using integer division where a decimal was wanted (for example a / b with two int values).

Quick self-check

What does public void f(int x) { x = 0; } do to the caller’s argument? Answer: nothing; x is a copy.
A method contains for (...) { if (cond) { return 1; } } then return 0;. When does it return 0? Answer: only if the condition was never true in the loop.
Is public int getX() an accessor or a mutator? Answer: an accessor.
Give a sensible precondition for int average(int total, int count). Answer: count > 0.