ClassOne.java

/*
 * To change this license header, choose License Headers in Project Properties.
 * To change this template file, choose Tools | Templates
 * and open the template in the editor.
 */
package javaapplication1;

/**
 *
 * @author Dean
 */
public class ClassOne {
    // Change this to public if you want the JavaApplication1 class
    // to be able to access this object.
    private ClassTwo classTwo;
    
    public ClassOne() {
        classTwo = new ClassTwo();
        classTwo.sayHello();
    }
    
    public void callClassTwo() {
     classTwo.sayHello();
    }
}

ClassTwo.java

/*
 * To change this license header, choose License Headers in Project Properties.
 * To change this template file, choose Tools | Templates
 * and open the template in the editor.
 */
package javaapplication1;

/**
 *
 * @author Dean
 */
public class ClassTwo {
    
    public void sayHello() {
        System.out.println("Hello!");
    }
}

JavaApplication1.java

package javaapplication1;

/**
 * @author Dean
 */
public class JavaApplication1 {

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        // This will work just fine.
        ClassOne classOne = new ClassOne();
        
        // So will this:
        classOne.callClassTwo();
        
        // This Won't Work unless private ClassTwo is changed to public:
        classOne.classTwo.sayHello();
    }
    
}