Member Avatar for psylocke

i dont know exactly why constructors are being used, it is used in what purpose?

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A short, practical summary that builds on the replies from , , and .

Constructors set an object up so its methods can rely on a valid, known state. They are special routines (same name as the class, no return type) that run during instantiation. The compiler supplies a default no-argument constructor only when you declare none. You can overload constructors, and use this(...) to chain within a class or super(...) to call a superclass constructor.

A few behaviors worth knowing but not always obvious: static fields and static initializers run once before any instances; when you create an instance, field initializers and instance initializer blocks run before the constructor body; superclass constructors run before subclass constructors. That ordering is why calling overridable methods from a constructor is risky—the subclass part of the object may not be initialized yet. For authoritative detail, see the Java tutorial and the language spec: Java Tutorial: Constructors and JLS — constructors.

Best practices and quick examples:

  • Keep constructors simple and fast; avoid I/O, heavy computation, or starting threads there.
  • Prefer static factory methods or a Builder when there are many optional parameters.
  • Make fields final when possible so invariants are enforced by the constructor.

Example of a minimal immutable holder:

public final class Point {
  private final int x;
  private final int y;

  public Point(int x, int y) {
    this.x = x;
    this.y = y;
  }
}

Troubleshooting tips: add temporary logs or breakpoints inside each constructor to confirm which one runs; check field initializers and any swallowed exceptions; remember some frameworks (serialization, reflection, DI containers) can bypass or change constructor usage. For a deeper practical overview, see Baeldung: Java constructors.

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Member Avatar for Narue

>i dont know exactly why constructors are being used
Not using a constructor is akin to a restaurant giving you a glass without the drink. In the simplest manner, constructors guarantee that your object is created with predictable data for the methods to work with.

Member Avatar for Phaelax

sounds like he hasn't covered objects yet in class. Here's an example where a constructor takes in 1 string.

MyClass person = new MyClass("name");

The object "person" now has an attribute already set to that string. Without a constructor, you would have to do this:

MyClass person = new MyClass();
person.setName("name");

Member Avatar for Narue

sounds like he hasn't covered objects yet in class. Here's an example where a constructor takes in 1 string.

MyClass person = new MyClass("name");

The object "person" now has an attribute already set to that string. Without a constructor, you would have to do this:

MyClass person = new MyClass();
person.setName("name");

>sounds like he hasn't covered objects yet
Right. So giving an example that uses objects, constructors, new, methods, and arguments, and fails to explain what's meant by object, string, and attribute is soooo informative. :rolleyes:

Member Avatar for Phaelax

oops:cheesy:

when i learned about objects, i was told what they were but never understood it until i saw code, but thats just how i learn. seeing code and figuring it out rather than listening to what it is. I wrote a document awhile ago about what objects are and how to get started with java programming using BlueJ. The article's website isn't online anymore, but I can try to locate it later for you.

Member Avatar for nanosani

Constructors simply initialize your program ... intialize means that what do you want your program to do automatically when it starts ... for example if you make a GUI (Graphical User Interface) you'll want your program to display the window and all the buttons and fields etc when the program starts ... so you'll write all these in your constructor.

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