Guaranteed initialization with the constructor

Bruce Eckel's Thinking in Java Contents | Prev | Next

with the constructor

You can imagine creating a method called initialize( ) for every class you write. The name is a hint that it should be called before using the object. Unfortunately, this means the user must remember to call the method. In Java, the class designer can guarantee initialization of every object by providing a special method called a constructor. If a class has a constructor, Java automatically calls that constructor when an object is created, before users can even get their hands on it. So initialization is guaranteed.

Here’s a simple class with a constructor: (See page 97 if you have trouble executing this program.)

//: SimpleConstructor.java
// Demonstration of a simple constructor
package c04;
 
class Rock {
  Rock() { // This is the constructor
    System.out.println("Creating Rock");
  }
}
 
public class SimpleConstructor {
  public static void main(String[] args) {
    for(int i = 0; i < 10; i++)
      new Rock();
  }
} ///:~ 

Now, when an object is created:

class Rock {
  Rock(int i) {
    System.out.println(
      "Creating Rock number " + i);
  }
}
 
public class SimpleConstructor {
  public static void main(String[] args) {
    for(int i = 0; i < 10; i++)
      new Rock(i);
  }
}

Constructor arguments provide you with a way to provide parameters for the initialization of an object. For example, if the class Tree has a constructor that takes a single integer argument denoting the height of the tree, you would create a Tree object like this:

Tree t = new Tree(12); // 12-foot tree

If Tree(int) is your only constructor, then the compiler won’t let you create a Tree object any other way.

Constructors eliminate a large class of problems and make the code easier to read. In the preceding code fragment, for example, you don’t see an explicit call to some initialize( ) method that is conceptually separate from definition. In Java, definition and initialization are unified concepts – you can’t have one without the other.



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