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Tip of the Day
Language: Java
Expertise: Advanced
Nov 18, 1998

Reflecting on a Classy String

Java's reflection API allows the instantiation of an object using only its String description. A Java application can determine the class of an object from the associated string and then construct an instance of the class. For example:
  
  1. public Object getObject (){
  2. try {
  3. Class MyClass = Class.forName("Hello");
  4. Constructor con = MyClass.getConstructor(new Class[] {});
  5. Object myHello = con.newInstance(new Object[] {});
  6. return myHello;
  7. }
  8. catch (Exception e) {}
  9. }
On Line 3, the class corresponding to the string "Hello" is obtained by calling the method forName() on the Class (java.lang.Class) class and passing "Hello" as a string argument. Note that for this to work, Hello.class must be loadable by Java's ClassLoader; it should be accessible from the CLASSPATH set up for the runtime environment.

The next step, on Line 4, is to obtain the default constructor for the class Hello by calling the method getConstructor() on the Class variable MyClass and passing it a class object with null parameters. The syntax used here is for anonymous classes.

On Line 5, the object is created using the constructor obtained in the previous line. Finally, on Line 6, the newly instantiated object is returned. What you are seeing here is a rudimentary Object Factory.

Ajit Sagar
 
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