Question

I was wondering if someone could help me with this.

I have defined my interface as:

interface Model 
{
public String  toString();
public Model add (Model m);
}

There are 2 classes implementing the interface (ClassA and ClassB):

class ClassA implements Model
{
private int val;

public ClassA(int x) 
{
    val = x;
}

public String toString() 
{
 return ""+ "value of object of class A is " + val;
}

public Model add (Model m) 
{
 if (m instanceof ClassA)
   return new ClassA(val + ( (ClassA) m).val);
 else
   return null;
}
}

class ClassB implements Model 
{

private String str;

public  ClassB(String s)
{
 str = s;
}

public String toString()
{
 return str;
 }

 public Model add (Model m) 
 {
 if (m instanceof ClassB)                      
 return new ClassB(str + ((ClassB) m).str); 
 else
 return null;
 }
 }

My main defines objects of ClassA and ClassB and calls their tostring() methods.

public class Example {
public static void main (String args[]) {
 ClassA a = new ClassA(5);
 ClassB b= new ClassB("Hi");

 Model m = b;
 System.out.println(m.toString());

 ClassA a1 = new ClassA(7);

 m = a.add(a1);

 System.out.println(m);
}
}

When I try to build this file it compiles fine but, upon trying to run the application I get an error message:

“Usage: java [-options] class [args…]
(to execute a class)
or java [-options] -jar jarfile [args…]
(to execute a jar file)…..etc…etc”

Can anyone help me with this? It’s probably something simple. I’m a beginner Java student.

Answer 1

There is no error in your program.Your program is absolutely ok. But , I do not know which command you are writing for execution .Try once again with absolute path setting to JDK and JRE.

Command like:-

for Compile -javac Example.java
for Run -java Example

It will successfully run. Hope it will help you.