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Thinking in Java
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The this keyword

If you have two objects of the same type called a and b, you might wonder how it is that you can call a method f( ) for both those objects:

class Banana { void f(int i) { /* ... */ } }
Banana a = new Banana(), b = new Banana();
a.f(1);
b.f(2);


If there’s only one method called f( ), how can that method know whether it’s being called for the object a or b?

To allow you to write the code in a convenient object-oriented syntax in which you “send a message to an object,” the compiler does some undercover work for you. There’s a secret first argument passed to the method f( ), and that argument is the reference to the object that’s being manipulated. So the two method calls become something like:

Banana.f(a,1);
Banana.f(b,2);


This is internal and you can’t write these expressions and get the compiler to accept them, but it gives you an idea of what’s happening.

Suppose you’re inside a method and you’d like to get the reference to the current object. Since that reference is passed secretly by the compiler, there’s no identifier for it. However, for this purpose there’s a keyword: this. The this keyword—which can be used only inside a method—produces the reference to the object the method has been called for. You can treat this reference just like any other object reference. Keep in mind that if you’re calling a method of your class from within another method of your class, you don’t need to use this. You simply call the method. The current this reference is automatically used for the other method. Thus you can say:

class Apricot {
  void pick() { /* ... */ }
  void pit() { pick(); /* ... */ }
}


Inside pit( ), you could say this.pick( ) but there’s no need to.[20] The compiler does it for you automatically. The this keyword is used only for those special cases in which you need to explicitly use the reference to the current object. For example, it’s often used in return statements when you want to return the reference to the current object:

//: c04:Leaf.java
// Simple use of the "this" keyword.
import com.bruceeckel.simpletest.*;

public class Leaf {
  static Test monitor = new Test();
  int i = 0;
  Leaf increment() {
    i++;
    return this;
  }
  void print() {
    System.out.println("i = " + i);
  }
  public static void main(String[] args) {
    Leaf x = new Leaf();
    x.increment().increment().increment().print();
    monitor.expect(new String[] {
      "i = 3"
    });
  }
} ///:~


Because increment( ) returns the reference to the current object via the this keyword, multiple operations can easily be performed on the same object.

Calling constructors from constructors

When you write several constructors for a class, there are times when you’d like to call one constructor from another to avoid duplicating code. You can make such a call by by using the this keyword.

Normally, when you say this, it is in the sense of “this object” or “the current object,” and by itself it produces the reference to the current object. In a constructor, the this keyword takes on a different meaning when you give it an argument list. It makes an explicit call to the constructor that matches that argument list. Thus you have a straightforward way to call other constructors:

//: c04:Flower.java
// Calling constructors with "this."
import com.bruceeckel.simpletest.*;

public class Flower {
  static Test monitor = new Test();
  int petalCount = 0;
  String s = new String("null");
  Flower(int petals) {
    petalCount = petals;
    System.out.println(
      "Constructor w/ int arg only, petalCount= "
      + petalCount);
  }
  Flower(String ss) {
    System.out.println(
      "Constructor w/ String arg only, s=" + ss);
    s = ss;
  }
  Flower(String s, int petals) {
    this(petals);
//!    this(s); // Can't call two!
    this.s = s; // Another use of "this"
    System.out.println("String & int args");
  }
  Flower() {
    this("hi", 47);
    System.out.println("default constructor (no args)");
  }
  void print() {
//! this(11); // Not inside non-constructor!
    System.out.println(
      "petalCount = " + petalCount + " s = "+ s);
  }
  public static void main(String[] args) {
    Flower x = new Flower();
    x.print();
    monitor.expect(new String[] {
      "Constructor w/ int arg only, petalCount= 47",
      "String & int args",
      "default constructor (no args)",
      "petalCount = 47 s = hi"
    });
  }
} ///:~


The constructor Flower(String s, int petals) shows that, while you can call one constructor using this, you cannot call two. In addition, the constructor call must be the first thing you do, or you’ll get a compiler error message.

This example also shows another way you’ll see this used. Since the name of the argument s and the name of the member data s are the same, there’s an ambiguity. You can resolve it using this.s, to say that you’re referring to the member data. You’ll often see this form used in Java code, and it’s used in numerous places in this book.

In print( ) you can see that the compiler won’t let you call a constructor from inside any method other than a constructor.

The meaning of static

With the this keyword in mind, you can more fully understand what it means to make a method static. It means that there is no this for that particular method. You cannot call non-static methods from inside static methods[21] (although the reverse is possible), and you can call a static method for the class itself, without any object. In fact, that’s primarily what a static method is for. It’s as if you’re creating the equivalent of a global function (from C). However, global functions are not permitted in Java, and putting the static method inside a class allows it access to other static methods and to static fields.

Some people argue that static methods are not object-oriented, since they do have the semantics of a global function; with a static method, you don’t send a message to an object, since there’s no this. This is probably a fair argument, and if you find yourself using a lot of static methods, you should probably rethink your strategy. However, statics are pragmatic, and there are times when you genuinely need them, so whether or not they are “proper OOP” should be left to the theoreticians. Indeed, even Smalltalk has the equivalent in its “class methods.”
Thinking in Java
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   Reproduced courtesy of Bruce Eckel, MindView, Inc. Design by Interspire