3.4 Constructors · CSA Exam Prep

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Unit 3 · Class Creation · Topic 3.4

Constructors

Skill 2.BSkill 3.B

Big idea: A constructor gives a brand-new object its starting state. Any instance variable it doesn't set keeps its default value.

Learning objectives

Develop code to declare instance variables for the attributes to be initialized in the body of the constructors of a class.

Setting the initial state

An object's state is its attributes and their current values. The constructor's job is to give every instance variable a sensible value, usually from its parameters.

Instance variables you don't assign get defaults: 0 for int, 0.0 for double, false for boolean, and null for references.

public class Student {
  private String name;
  private int grade;
  private boolean active;

  public Student(String n) {
    name = n;
    // grade and active get default values: 0 and false
  }

  public Student(String n, int g) {
    name = n;
    grade = g;
    active = true;
  }

  public String toString() {
    return name + " " + grade + " " + active;
  }
}

Overloaded constructors and defaults

Code

Student a = new Student("Mo");
Student b = new Student("Ivy", 11);
System.out.println(a);
System.out.println(b);

Output

Mo 0 false
Ivy 11 true

The default constructor

If you write no constructor at all, Java provides a no-parameter one that leaves every instance variable at its default. Once you write any constructor, that free one disappears.

Code

Item it = new Item();
System.out.println(it.getQty() + " " + it.getName());

Output

0 null

The local-variable trap

Writing a type in front of a name inside the constructor declares a new local variable. The instance variable is left at its default.

Code

Box b = new Box(5);
System.out.println(b.getSize());

Output

Key points

A constructor sets the initial state, giving every instance variable a value.
Instance variables form a has-a relationship (a Student has a name).
Defaults: int 0, double 0.0, boolean false, references null.
With no constructor written, Java supplies a no-parameter default constructor.
When given a mutable object, a constructor should store a copy so outside code can't change the object's state.

Watch out

A constructor has no return type, not even void. public void Box() is a method, not a constructor.
int size = s; in a constructor creates a local variable and leaves the instance variable at 0.

Quick check

Is public Cube(int side) a valid constructor for class Cube? What about public void Cube()?

Show answer

The first is. The second has a return type, so it is an ordinary method named Cube.

Practice 3.4 (12 questions)