3.2 Constructors

What is a constructor?

A constructor is a special method that initializes a new object's instance variables when it's created with `new`. It has the same name as the class and no return type (not even `void`). Constructors run automatically when an object is instantiated, setting up its initial state.

Default vs. parameterized constructors

If no constructor is written, Java provides a default constructor that sets fields to default values (0, false, null). A parameterized constructor accepts arguments to set instance variables to specific values, often using `this.field = parameter` when names match. Classes can have multiple constructors (overloading) with different parameter lists.

Using this in constructors

The keyword `this` refers to the current object. It resolves naming conflicts between parameters and instance variables (`this.x = x;`) and can call another constructor in the same class via `this(args)` as the first statement, avoiding duplicate initialization code.

Key terms

Constructor

A special method that initializes a new object's instance variables; shares the class name and has no return type.

Default constructor

A constructor automatically provided by Java when none is written, setting fields to default values.

this keyword (in constructors)

Refers to the current object; used to distinguish instance variables from parameters or to call another constructor.

Constructor overloading

Defining multiple constructors in a class, each with a different parameter list, to allow varied object initialization.

this(args) call

A call to another constructor in the same class; must be the first statement in the calling constructor.

Try an AP-style question

public class Book {
    private String title;
    public Book(String t) { title = t; }
}

What does the constructor do?

1. Sets the title instance variable
2. Returns the title
3. Creates a new String class
4. Prints the title