Creating Objects
As you know, a class provides the blueprint for objects; you create an object from a class. Each of the following statements taken from the CreateObjectDemo program creates an object and assigns it to a variable:
Point originOne = new Point(23, 94); Rectangle rectOne = new Rectangle(originOne, 100, 200); Rectangle rectTwo = new Rectangle(50, 100);
The first line creates an object of the Point class, and the second and third lines each create an object of the Rectangle class.
Each of these statements has three parts (discussed in detail below):
- Declaration: The code set in bold are all variable declarations that associate a variable name with an object type.
- Instantiation: The new keyword is a Java operator that creates the object.
- Initialization: The new operator is followed by a call to a constructor, which initializes the new object.
Declaring a Variable to Refer to an Object
Previously, you learned that to declare a variable, you write:
This notifies the compiler that you will use name to refer to data whose type is type. With a primitive variable, this declaration also reserves the proper amount of memory for the variable.
You can also declare a reference variable on its own line. For example:
If you declare originOne like this, its value will be undetermined until an object is actually created and assigned to it. Simply declaring a reference variable does not create an object. For that, you need to use the new operator, as described in the next section. You must assign an object to originOne before you use it in your code. Otherwise, you will get a compiler error.
A variable in this state, which currently references no object, can be illustrated as follows (the variable name, originOne , plus a reference pointing to nothing):
Instantiating a Class
The new operator instantiates a class by allocating memory for a new object and returning a reference to that memory. The new operator also invokes the object constructor.
Note: The phrase «instantiating a class» means the same thing as «creating an object.» When you create an object, you are creating an «instance» of a class, therefore «instantiating» a class.
The new operator requires a single, postfix argument: a call to a constructor. The name of the constructor provides the name of the class to instantiate.
The new operator returns a reference to the object it created. This reference is usually assigned to a variable of the appropriate type, like:
Point originOne = new Point(23, 94); The reference returned by the new operator does not have to be assigned to a variable. It can also be used directly in an expression. For example:
int height = new Rectangle().height;
This statement will be discussed in the next section.
Initializing an Object
Here’s the code for the Point class:
This class contains a single constructor. You can recognize a constructor because its declaration uses the same name as the class and it has no return type. The constructor in the Point class takes two integer arguments, as declared by the code (int a, int b). The following statement provides 23 and 94 as values for those arguments:
Point originOne = new Point(23, 94);
The result of executing this statement can be illustrated in the next figure:
Here’s the code for the Rectangle class, which contains four constructors:
public class Rectangle < public int width = 0; public int height = 0; public Point origin; // four constructors public Rectangle() < origin = new Point(0, 0); >public Rectangle(Point p) < origin = p; >public Rectangle(int w, int h) < origin = new Point(0, 0); width = w; height = h; >public Rectangle(Point p, int w, int h) < origin = p; width = w; height = h; >// a method for moving the rectangle public void move(int x, int y) < origin.x = x; origin.y = y; >// a method for computing the area of the rectangle public int getArea() < return width * height; >>
Each constructor lets you provide initial values for the rectangle’s origin, width, and height, using both primitive and reference types. If a class has multiple constructors, they must have different signatures. The Java compiler differentiates the constructors based on the number and the type of the arguments. When the Java compiler encounters the following code, it knows to call the constructor in the Rectangle class that requires a Point argument followed by two integer arguments:
Rectangle rectOne = new Rectangle(originOne, 100, 200);
This calls one of Rectangle ‘s constructors that initializes origin to originOne . Also, the constructor sets width to 100 and height to 200. Now there are two references to the same Point objectan object can have multiple references to it, as shown in the next figure:
The following line of code calls the Rectangle constructor that requires two integer arguments, which provide the initial values for width and height. If you inspect the code within the constructor, you will see that it creates a new Point object whose x and y values are initialized to 0:
Rectangle rectTwo = new Rectangle(50, 100);
The Rectangle constructor used in the following statement doesn’t take any arguments, so it’s called a no-argument constructor:
Rectangle rect = new Rectangle();
All classes have at least one constructor. If a class does not explicitly declare any, the Java compiler automatically provides a no-argument constructor, called the default constructor. This default constructor calls the class parent’s no-argument constructor, or the Object constructor if the class has no other parent. If the parent has no constructor ( Object does have one), the compiler will reject the program.
Creating New Class Instances
There are two reflective methods for creating instances of classes: java.lang.reflect.Constructor.newInstance() and Class.newInstance() . The former is preferred and is thus used in these examples because:
- Class.newInstance() can only invoke the zero-argument constructor, while Constructor.newInstance() may invoke any constructor, regardless of the number of parameters.
- Class.newInstance() throws any exception thrown by the constructor, regardless of whether it is checked or unchecked. Constructor.newInstance() always wraps the thrown exception with an InvocationTargetException .
- Class.newInstance() requires that the constructor be visible; Constructor.newInstance() may invoke private constructors under certain circumstances.
Sometimes it may be desirable to retrieve internal state from an object which is only set after construction. Consider a scenario where it is necessary to obtain the internal character set used by java.io.Console . (The Console character set is stored in a private field and is not necessarily the same as the Java virtual machine default character set returned by java.nio.charset.Charset.defaultCharset() ). The ConsoleCharset example shows how this might be achieved:
import java.io.Console; import java.nio.charset.Charset; import java.lang.reflect.Constructor; import java.lang.reflect.Field; import java.lang.reflect.InvocationTargetException; import static java.lang.System.out; public class ConsoleCharset < public static void main(String. args) < Constructor[] ctors = Console.class.getDeclaredConstructors(); Constructor ctor = null; for (int i = 0; i < ctors.length; i++) < ctor = ctors[i]; if (ctor.getGenericParameterTypes().length == 0) break; >try < ctor.setAccessible(true); Console c = (Console)ctor.newInstance(); Field f = c.getClass().getDeclaredField("cs"); f.setAccessible(true); out.format("Console charset : %s%n", f.get(c)); out.format("Charset.defaultCharset(): %s%n", Charset.defaultCharset()); // production code should handle these exceptions more gracefully >catch (InstantiationException x) < x.printStackTrace(); >catch (InvocationTargetException x) < x.printStackTrace(); >catch (IllegalAccessException x) < x.printStackTrace(); >catch (NoSuchFieldException x) < x.printStackTrace(); >> > Class.newInstance() will only succeed if the constructor is has zero arguments and is already accessible. Otherwise, it is necessary to use Constructor.newInstance() as in the above example.
Example output for a UNIX system:
$ java ConsoleCharset Console charset : ISO-8859-1 Charset.defaultCharset() : ISO-8859-1
Example output for a Windows system:
C:\> java ConsoleCharset Console charset : IBM437 Charset.defaultCharset() : windows-1252
Another common application of Constructor.newInstance() is to invoke constructors which take arguments. The RestoreAliases example finds a specific single-argument constructor and invokes it:
import java.lang.reflect.Constructor; import java.lang.reflect.Field; import java.lang.reflect.InvocationTargetException; import java.util.HashMap; import java.util.Map; import java.util.Set; import static java.lang.System.out; class EmailAliases < private Setaliases; private EmailAliases(HashMap h) < aliases = h.keySet(); >public void printKeys() < out.format("Mail keys:%n"); for (String k : aliases) out.format(" %s%n", k); >> public class RestoreAliases < private static MapdefaultAliases = new HashMap(); static < defaultAliases.put("Duke", "duke@i-love-java"); defaultAliases.put("Fang", "fang@evil-jealous-twin"); >public static void main(String. args) < try < Constructor ctor = EmailAliases.class.getDeclaredConstructor(HashMap.class); ctor.setAccessible(true); EmailAliases email = (EmailAliases)ctor.newInstance(defaultAliases); email.printKeys(); // production code should handle these exceptions more gracefully >catch (InstantiationException x) < x.printStackTrace(); >catch (IllegalAccessException x) < x.printStackTrace(); >catch (InvocationTargetException x) < x.printStackTrace(); >catch (NoSuchMethodException x) < x.printStackTrace(); >> > This example uses Class.getDeclaredConstructor() to find the constructor with a single argument of type java.util.HashMap . Note that it is sufficient to pass HashMap.class since the parameter to any get*Constructor() method requires a class only for type purposes. Due to type erasure, the following expression evaluates to true :
HashMap.class == defaultAliases.getClass()
The example then creates a new instance of the class using this constructor with Constructor.newInstance() .
$ java RestoreAliases Mail keys: Duke Fang