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44_[자바] 초기화 위치 (composition)

It makes sense that the compiler doesn’t just create a default object for every reference, because that would incur unnecessary overhead in many cases. If you want the references initialized, you can do it:

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 is often called lazy initialization. It can reduce overhead in situations where object creation is expensive and the object doesn’t need to be created every time.

4. Using instance initialization.

 

All four approaches are shown here:

 

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

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 reference—except for the inevitable run-time exception.

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

[Thinking in Java, 166~]

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