instance vs instanceof: Both are very confusing words and few people know about instanceof. Let make the concept crystal clear.
A) instance:
Everyone knows what an instance is. Any OOPs guy says, "an instance of a class is known as object". At an outset, instance and object one and the same. By the by, instance is not a keyword of Java.
public class Demo
{
public static void main(String args[])
{
Demo d1 = new Demo();
}
}
In the above code, d1 is known as instance or object of class Demo. Is there any way, in Java, to check d1 is really an instance of Demo class? Yes, there is. Use instanceof keyword. The next explanation says.
B) instanceof:
instanceof is a keyword of Java used to check whether a particular object belongs to which class. instanceof evaluates to true when an object belongs to a specific class.
public class Demo
{
public static void main(String args[])
{ // Java API classes
String str1 = "hello";
StringBuffer sb1 = new StringBuffer();
// user-defined class
Demo d1 = new Demo();
System.out.print("str1 instanceof String: ");
System.out.println(str1 instanceof String);
System.out.print("sb1 instanceof StringBuffer: ");
System.out.println(sb1 instanceof StringBuffer);
System.out.print("d1 instanceof Demo: ");
System.out.println(d1 instanceof Demo);
// super class Object class
System.out.print("str1 instanceof Object: ");
System.out.println(str1 instanceof Object);
System.out.print("d1 instanceof Object: ");
System.out.println(d1 instanceof Object);
}
}
Output Screenshot on instance vs instanceof Java
In the above code, str1 is an instance of class String. For this reason. "str1 instanceof String" printed true. Similarly with StringBuffer and Demo classes also.
instanceof evaluation also gives true even with its super class. The super class of String is Object class. That is, "str1 instanceof Object" also prints true. It is more clear in the next program.
class Test { }
public class Demo extends Test
{
public static void main(String args[])
{ // Java API classes
Demo d1 = new Demo();
System.out.println();
System.out.print("d1 instanceof Demo: ");
System.out.println(d1 instanceof Demo);
System.out.print("d1 instanceof Test: ");
System.out.println(d1 instanceof Test);
}
}
Output Screenshot on instance vs instanceof Java
Demo class super class is Test class. d1 is an object of Demo class. "d1 instanceof Demo" and "d1 instanceof Test", both print true.
What Java rule says:
An instance of a class is also an instance of its super classes. By this rule, every instance of any Java class is also an instance of Object class. Here, d1 instance (or object) of Demo class is also an instance of its super class Test (and also of Object, not shown in the code).
