There is no Sample class in your code . The one which you have declared is a private method .

// private method which takes an int as parameter and returns another int
private int Sample(int c)
{
  int a = 1;
  int b = 2;
  c = a + b;
  return c;
}

With the current snippet , You need to instantiate the Testing class and make use of the Sample method. Notice your class definition is preceded by the keyword class , in this case class Testing.

public class Testing{
  private int Sample(int c)
  {
    int a = 1;
    int b = 2;
    c = a + b;
    return c;
 }
  public static void main(String []args)
 {
    Testing t = new Testing(); // instantiate a Testing class object
    int result = t.Sample(1); // use the instance t to invoke a method on it
    System.out.println(result);
 }
}

But that doesn't really make sense, your Sample method always returns 3 .

Are you trying to do something like this :

class Sample {
 int a;
 int b;

 Sample(int a, int b) {
    this.a = a;
    this.b = b;
 }

 public int sum() {
    return a + b;
 }
}

public class Testing {
 public static void main(String[] args) {
    Sample myTest = new Sample(1, 2);
    int sum = myTest.sum();
    System.out.println(sum);
 }
}

In Java 9 and afterward, if there's a declared zero-parameter ("nullary") constructor, you'd use Class.getDeclaredConstructor() to get it, then call newInstance() on it:

Object foo(Class type) throws InstantiationException, IllegalAccessException, InvocationTargetException {
    return type.getDeclaredConstructor().newInstance();
}

Prior to Java 9, you would have used Class.newInstance:

Object foo(Class type) throws InstantiationException, IllegalAccessException {
    return type.newInstance();
}

...but it was deprecated as of Java 9 because it threw any exception thrown by the constructor, even checked exceptions, but didn't (of course) declare those checked exceptions, effectively bypassing compile-time checked exception handling. Constructor.newInstance wraps exceptions from the constructor in InvocationTargetException instead.

Both of the above assume there's a zero-parameter constructor. A more robust route is to go through Class.getDeclaredConstructors or Class.getConstructors, which takes you into using the Reflection stuff in the java.lang.reflect package, to find a constructor with the parameter types matching the arguments you intend to give it.

You can use Class.getConstructors (or Class.getConstructor) to get a list of available constructors, and invoke any of them with Constructor.newInstance, which does accept parameters.

To instantiate something is to create an object of that type. You are doing that here:

NsLookup lookup = new NsLookup();

and here:

inet = InetAddress.getByName(host);

Two ways:

Method 1 - only for classes having a no-arg constructor

If your class has a no-arg constructor, you can get a Class object using Class.forName() and use the newInstance() method to create an instance (though beware that this method is often considered evil because it can defeat Java's checked exceptions).

For example:

Class<?> clazz = Class.forName("java.util.Date");
Object date = clazz.newInstance();

Method 2

An alternative safer approach which also works if the class doesn't have any no-arg constructors is to query your class object to get its Constructor object and call a newInstance() method on this object:

Class<?> clazz = Class.forName("com.foo.MyClass");
Constructor<?> constructor = clazz.getConstructor(String.class, Integer.class);
Object instance = constructor.newInstance("stringparam", 42);

Both methods are known as reflection. You will typically have to catch the various exceptions which can occur, including things like:

• the JVM can't find or can't load your class
• the class you're trying to instantiate doesn't have the right sort of constructors
• the constructor itself threw an exception
• the constructor you're trying to invoke isn't public
• a security manager has been installed and is preventing reflection from occurring

Both are correct and both mean the exact same thing. Objects are instances of classes. Whether you "instantiate class X [to create an object of class X]" or "instantiate an object [of class X]" is just semantic nitpicking.