Topic 3.4 — Constructors

Goal: explain what constructors do, recognize constructor headers, and write constructors that initialize instance variables (including overloaded constructors).

The big idea

A constructor runs when you create an object with new. Its job is to set up the object’s starting state (initialize instance variables).

If you don’t write any constructor, Java provides a default no-arg constructor (in some cases).

Constructor rules you must know

Constructor name == class name
No return type (not even void)
Usually public
Used to initialize instance variables

Basic example (no-arg constructor)

public class Counter {
  private int value;

  public Counter() {   // constructor
    value = 0;
  }
}

When you do new Counter(), the constructor runs and sets value to 0.

Overloaded constructors

A class can have multiple constructors as long as their parameter lists are different.

public class Counter {
  private int value;

  public Counter() {
    value = 0;
  }

  public Counter(int start) {
    value = start;
  }
}

Same constructor name, different parameters → that’s overloading.

Calling one constructor from another

You can reuse initialization code by calling this(...) inside a constructor. (If your teacher allows it, it’s a clean design.)

public Counter() {
  this(0);           // calls Counter(int start)
}

public Counter(int start) {
  value = start;
}

If used, this(...) must be the first statement in the constructor.

Common mistakes

Writing a return type (turns it into a method, not a constructor).
Forgetting to initialize instance variables.
Parameter names confusing you: use this.field when needed (Topic 3.9).

// Not a constructor (has return type!)
public void Counter() { ... }  // ❌

Quick self-check

When does a constructor run?
What two things must match for a constructor header to be valid?
What does it mean to “overload” constructors?
Why do we write constructors instead of leaving variables uninitialized?
Spot the bug: why is public void MyClass() not a constructor?

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