Objects, Constructors, and Calling Methods

Classes and objects

A class is a blueprint; an object is an instance created from it. A variable of a class type holds a reference to an object, or null.

Constructors

Objects are created with new and a constructor:

Rectangle r = new Rectangle(3, 5);

A class can have several constructors with different parameter lists (overloading). The arguments must match a constructor's parameters in number, order and type.

Calling methods

• Instance methods are called on an object: r.getArea().
• Static methods are called on the class: Math.sqrt(16).
• A void method performs an action and returns nothing; it is used as a statement.
• A non-void method returns a value that should be used: int area = r.getArea();

Method signatures

public int getArea()
public void setWidth(int w)

The signature tells you the return type, the name and the parameters. Calling r.setWidth(4.5) does not compile because a double cannot be passed to an int parameter.

null and NullPointerException

Calling a method on a variable that is null throws a NullPointerException.

Application program interfaces

On the exam you read documentation (an API) for a class you have never seen and use its constructors and methods correctly.

Quick summary

• new ClassName(args) creates an object.
• Instance methods use an object; static methods use the class name.
• Use the return value of non-void methods; arguments must match parameters.