Composition syntax


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Until now, composition has been used quite frequently. You simply place object handles inside new classes. For example, suppose you’d like an object that holds several String objects, a couple of primitives and an object of another class. For the non-primitive objects, just put handles inside your new class, and for the primitives just define them inside your class: (See page 97 if you have trouble executing this program.)

//: SprinklerSystem.java
// Composition for code reuse
package c06;
class WaterSource {
  private String s;
  WaterSource() {
    s = new String("Constructed");
  public String toString() { return s; }
public class SprinklerSystem {
  private String valve1, valve2, valve3, valve4;
  WaterSource source;
  int i;
  float f;
  void print() {
    System.out.println("valve1 = " + valve1);
    System.out.println("valve2 = " + valve2);
    System.out.println("valve3 = " + valve3);
    System.out.println("valve4 = " + valve4);
    System.out.println("i = " + i);
    System.out.println("f = " + f);
    System.out.println("source = " + source);
  public static void main(String[] args) {
    SprinklerSystem x = new SprinklerSystem();
} ///:~ 

One of the methods defined in WaterSource is special: toString( ). You will learn later that every non-primitive object has a toString( ) method, and it’s called in special situations when the compiler wants a String but it’s got one of these objects. So in the expression:

valve1 = null
valve2 = null
valve3 = null
valve4 = null
i = 0
f = 0.0
source = null

  1. At the point the objects are defined. This means that they’ll always be initialized before the constructor is called.
  2. In the constructor for that class
  3. Right before you actually need to use the object. This can reduce overhead, if there are situations where the object doesn’t need to be created.
All three approaches are shown here:

//: Bath.java
// Constructor initialization with composition
class Soap {
  private String s;
  Soap() {
    s = new String("Constructed");
  public String toString() { return s; }
public class Bath {
  private String 
    // Initializing at point of definition:
    s1 = new String("Happy"), 
    s2 = "Happy", 
    s3, s4;
  Soap castille;
  int i;
  float toy;
  Bath() {
    System.out.println("Inside Bath()");
    s3 = new String("Joy");
    i = 47;
    toy = 3.14f;
    castille = new Soap();
  void print() {
    // Delayed initialization:
    if(s4 == null)
      s4 = new String("Joy");
    System.out.println("s1 = " + s1);
    System.out.println("s2 = " + s2);
    System.out.println("s3 = " + s3);
    System.out.println("s4 = " + s4);
    System.out.println("i = " + i);
    System.out.println("toy = " + toy);
    System.out.println("castille = " + castille);
  public static void main(String[] args) {
    Bath b = new Bath();
} ///:~ 

Note that in the Bath constructor a statement is executed before any of the initializations take place. When you don’t initialize at the point of definition, there’s still no guarantee that you’ll perform any initialization before you send a message to an object handle – except for the inevitable run-time exception.

Here’s the output for the program:

Inside Bath()
s1 = Happy
s2 = Happy
s3 = Joy
s4 = Joy
i = 47
toy = 3.14
castille = Constructed

When print( ) is called it fills in s4 so that all the fields are properly initialized by the time they are used.


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