Class Type Variables Storing References

Java Tutorials - Herong's Tutorial Examples

∟Class Type Variables Storing References

This section describes how references of class objects are stored in class variables of the same class type.

When a variable is declared as a class type, it will be used to store references of objects of the same class type. The object class type must match the variable class type. Examples are listed below:

   String msg = new String("Hello");
      // assigns the reference of a String object

   java.util.Date today = new java.util.Date();
      // assigns the reference of a Data object

   java.util.ArrayList<String>
      = new java.util.ArrayList<String>();
      // assigns the reference of an ArrayList<String> object

Since class type variables stores references of class objects, it is very common that the reference of a single object can be stored in multiple variables. Here is a tutorial example program that shows you how the reference of a single object is stored in to variables:

/* ClassTypeVariable.java
 * Copyright (c) HerongYang.com. All Rights Reserved.
 */
import java.util.*;
class ClassTypeVariable {
   public static void main(String[] a) {
      java.io.PrintStream o = System.out;

      // assigning the reference of a Date object
      Date myTime = new Date();

      // showing the object content
      o.println("myTime before = "+myTime);

      // assigning the reference to another variable
      Date hisTime = myTime;

      // modifying the object content
      hisTime.setTime(hisTime.getTime()+1000*60*60);

      // showing the object content
      o.println("myTime after = "+myTime);
      o.println("hisTime after = "+hisTime);
   }
}

Compile and run the example, you will get:

myTime before = Sun May 04 22:14:47 PDT 2018
myTime after = Sun May 04 23:14:47 PDT 2018
hisTime after = Sun May 04 23:14:47 PDT 2018

The result proves that both "myTime" and "hisTime" are references of the same object. When the object value is modified, time moved to 1 hour later, through "hisTime" reference, "myTime" reference can also see the change.