Calling Class Methods: Revision Notes (Computer Science A 1.10)

Short on time? This page is the recap. For explanations and worked examples, use the full study guide.

Recap

• Class methods are associated with the class, not with instances of the class.
• Class methods include the keyword static in the header, before the method name.
• Class methods are typically called using the class name and the dot operator: Converter.toFahrenheit(100).
• When the call occurs in the defining class, the use of the class name is optional.
• Everything about parameters, arguments, return types, void and non-void methods from Topic 1.9 still applies.
• The Math class contains only class methods, so it is never instantiated.

Key relationships

Situation	Form of the call
Calling from a different class	ClassName.method(args)
Calling from inside the defining class	method(args) or ClassName.method(args)
Library class method	Math.sqrt(16)
Void method	a statement: Converter.banner("x");
Non-void method	an expression: double f = Converter.toFahrenheit(100);
Nested call	f(g(x)): g(x) first, then f with that value
Argument is an int, parameter is a double	allowed (converted)
Argument is a double, parameter is an int	compile-time error

Compile-time error	Typical cause
cannot find symbol	method defined in another class and the class name is missing, or a misspelt name
cannot be applied to given types	wrong number of arguments
incompatible types	wrong argument type, or a returned value stored in the wrong type
void cannot be converted	using the result of a void method

Assumptions and conventions

• static is shown in headers of class methods; non-static (instance) methods come in Topic 1.14.
• The class name is the name of the class that defines the method, not the name of a variable.
• Arguments are evaluated before the method is entered.
• No calculator is allowed on the exam; trace nested calls on paper from the inside out.

Class methods and instance methods

• Class method: needs no object; static in the header; called through the class name (Math.abs(-3)). Used for work that does not depend on any one object.
• Instance method: belongs to an object; called on the object (word.length()); you cannot call it with only the class name (Topics 1.12 to 1.14).
• main is itself a class method, which is why it can start a program before any object exists.
• Class names are case-sensitive: Converter is not converter.

Mistakes to avoid

1. Calling a method from another class without its class name.
2. Reading static as “constant”.
3. Creating an object just to call a class method (such as new Math()).
4. Passing a double to an int parameter.
5. Putting arguments in the wrong order.
6. Storing the result of a void method.
7. Working a nested call from the outside in.

Quick self-check

1. What keyword marks a class method? (static)
2. How do you call wholeMinutes of class Converter from another class, with the argument 200? (Converter.wholeMinutes(200))
3. Is the class name optional inside Converter itself? (Yes.)
4. What does Math.sqrt(Math.abs(-9)) return? (3.0)
5. Why does Converter.wholeMinutes(2.5) not compile? (The parameter is an int and a double argument cannot be passed to it.)

Next: practice questions.