Calling Instance Methods: Revision Notes (Computer Science A 1.14)

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

Recap

• Instance methods are called on objects of the class. The dot operator is used along with the object name: box.area().
• A class method is called with the class name and needs no object; an instance method needs an object and cannot be called with just the class name.
• Each object has its own attribute values, so one method can give different results for different objects.
• A void method is called as a statement and can change the object. A non-void method returns a value to store or use in an expression.
• Arguments must match parameters in number, order and compatible type, and are passed by value.
• A method call on a null reference results in a NullPointerException: a run-time error.
• String methods such as length() and indexOf(...) are instance methods called on String objects.

Key relationships

Call	Kind	Example
Math.sqrt(16)	class method	called through the class name
small.area()	instance method, non-void	an expression with the returned value
small.grow(1);	instance method, void	a statement that changes small
word.length()	instance method of String	on a String object
Rectangle.area()	error	instance method called through a class name
t.area() with t == null	run-time error	NullPointerException

Situation	Result
Rectangle s = r; then s.grow(4);	r sees the change: same object
two separate objects	each keeps its own values
int c = r.grow(2);	compile-time error: void
r.grow("2");	compile-time error: wrong argument type

Assumptions and conventions

• The object name before the dot is a reference variable that must refer to an object when the call runs.
• Exceptions interrupt the normal flow: nothing after the failing call runs (Topic 1.1).
• The compiler checks that the method exists for the variable’s class, not that the variable is non-null.
• No calculator is allowed on the exam; trace the objects with a table.

Mistakes to avoid

1. Calling an instance method with the class name.
2. Forgetting that a void method changes the object but returns nothing.
3. Assuming a call on one object changes all objects of the class.
4. Missing that two names can refer to one object.
5. Expecting a compile error for a null call.
6. Storing the result of a void method.
7. Passing the wrong number, order or type of arguments.

Quick self-check

1. How do you call area on a Rectangle named r? (r.area())
2. Does r.grow(2); return a value? (No: it is a void method that changes r.)
3. If s and r refer to the same object and s.grow(1); is called, what does r.area() see? (The changed object.)
4. What happens when t.area() runs and t is null? (A NullPointerException.)
5. What does "banana".length() return? (6)

Next: practice questions.