Object Creation and Storage (Instantiation): Revision Notes (Computer Science A 1.13)

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

Recap

• A class contains constructors that are called to create objects. They have the same name as the class.
• An object is typically created using the keyword new followed by a call to one of the class’s constructors.
• A constructor signature is the constructor’s name (the class name) plus the ordered list of parameter types. Constructors are overloaded when there are several with different signatures.
• Parameters allow constructors to accept values that establish the initial values of the attributes.
• A constructor argument is a value passed in when the constructor is called. Arguments must match the parameter types in order and number. They are passed by call by value: parameters are initialized with copies of the arguments.
• A variable of a reference type holds an object reference or, if there is no object, null.
• A constructor call interrupts the sequential execution; once its last statement runs, control returns to the point immediately after the call.

Key relationships

Statement	Meaning
Box b;	declares a variable; no object
new Box(3, 5)	calls Box(int, int) and creates an object
Box c = new Box(3, 5);	declare, create and store the reference
Box d = null;	d refers to no object
Box e = c;	e and c refer to the same object
c = new Box(9);	c now refers to a different, new object

Call	Constructor used
new Box()	Box()
new Box(4)	Box(int)
new Box(3, 5)	Box(int, int)
new Box(1, 2, 3)	none: compile-time error
new Box("x") or new Box(2.5)	none: compile-time error

Assumptions and conventions

• A constructor header has no return type, not even void.
• Only a statement containing new creates an object.
• Scanner and String also have constructors listed in the Java Quick Reference.
• Calling a method on a null reference causes a NullPointerException (Topic 1.14).

Mistakes to avoid

1. Confusing declaring a variable with creating an object.
2. Leaving out new.
3. Counting parameter names as part of the signature.
4. Expecting a constructor to change the caller’s variable.
5. Passing arguments that fit no constructor (wrong number or type).
6. Thinking b = a; makes a new object.
7. Forgetting that the constructor runs to its end before the next statement.

Quick self-check

1. What is the signature of public Box(int w, int h)? (Box(int, int))
2. Which constructor does new Box(7) call? (The one with a single int parameter.)
3. After Box b = null; what does b refer to? (No object.)
4. If a constructor adds 5 to its parameter, is the caller’s variable changed? (No: only the copy changes.)
5. How many objects exist after Box a = new Box(2); Box b = a;? (One.)

Next: practice questions.