Stack Implementation in Java

Below is my code to implement Stack from ground-up. I tried (in a very short time) to emulate the behavior of Stack in the Java Util package. The methods I have written are:
size()
get(int index)
empty()
clear()
pop()
push(Object input)
bottom()
top()

I call my Stack implementation as CustomStack as it may evolve over time. I have also provided some test cases to validate that the implementation actually works. The sample output is shown below the codes.

import java.lang.reflect.Array;

public class CustomStack {

 private Object[] stackElements;
 private int stackIndex = -1;
 private int stackSize = 0;
 private static final int INIT_SIZE = 3;
 private boolean stackSizeGiven = false;
 private int sizeHolder = 0;

 public CustomStack(){
  stackElements = new Object[ INIT_SIZE ];
  stackSizeGiven = false;
 }

 public CustomStack(int inputSize){
  sizeHolder = inputSize;
  stackElements = new Object[ sizeHolder ];
  stackSizeGiven = true;
 }
 
 public int size(){
  return stackSize;
 }

 public Object get(int index){
  if ((index < 0) || (index > (stackSize))) {
   throw new RuntimeException("Index out of bound. Supplied " + index);
  }
  return stackElements[ index ];
 }
 
 public boolean empty() {
  return stackIndex == -1;
 }

 public void clear() {
  if (stackSizeGiven) {
   stackElements = new Object[ sizeHolder ];
  } else {
   stackElements = new Object[ INIT_SIZE ];
  }
  stackIndex = -1;
  stackSize = 0;
 }
 
 public void pop() {
  if(empty()){
   throw new RuntimeException( "Stack is already empty. No elements to pop." );
  }
  stackIndex--;
  stackSize--;
 }

 public void push(Object input) {
  if( stackIndex + 1 == stackElements.length ) {
   int arrayLength = Array.getLength(stackElements);
   Object [] newArray = (Object[]) Array.newInstance(stackElements.getClass().getComponentType(), (arrayLength + arrayLength ));
   System.arraycopy(stackElements, 0, newArray, 0, arrayLength);
   stackElements = newArray;
  }
  stackElements[ ++stackIndex ] = input;
  stackSize++;
 }

 public Object bottom(){
  if(empty())
   throw new RuntimeException( "Stack is already empty. No elements at bottom." );
  return stackElements[ 0 ];
 }
 
 public Object top() {
  if(empty())
   throw new RuntimeException( "Stack is already empty. No elements at top." );
  return stackElements[ stackIndex ];
 }

 public static void testCases(){
  CustomStack aStack = new CustomStack();
  System.out.println("Stack is empty: " + aStack.empty());
  aStack.push("Nicholas Key");
  aStack.push(3.142);
  aStack.push(false);
  // Prints false
  System.out.println("Element at top of Stack: " + aStack.top());
  System.out.println("Size of current Stack: " + aStack.size());

  aStack.push("Java Developer");
  aStack.push("Android App Developer");
  // Prints Android App Developer
  System.out.println("Element at top of Stack: " + aStack.top());
  
  aStack.push("Web App Developer");
  aStack.push("Blogger");
  aStack.push(2009);
  // Prints 2009
  System.out.println("Element at top of Stack: " + aStack.top());
  System.out.println("Size of current Stack: " + aStack.size());
  
  // Prints Stack elements
  for (int i= 0; i< aStack.size(); i++) {
   System.out.println("Index " + i + ": " + aStack.get(i));
  }

  // Pops 2009
  aStack.pop();
  // Prints Blogger
  System.out.println("Element at top of Stack: " + aStack.top());
  
  // Pops Blogger
  aStack.pop();
  // Prints Web App Developer
  System.out.println("Element at top of Stack: " + aStack.top());

  // Prints Stack elements
  for (int i= 0; i< aStack.size(); i++) {
   System.out.println("Index " + i + ": " + aStack.get(i));
  }

  System.out.println("Element at bottom of Stack: " + aStack.bottom());

  // Pops out all Stack elements
  aStack.clear();  
  System.out.println("Stack is empty: " + aStack.empty());

  System.out.println(aStack.get(1));
  
  System.out.println(aStack.get(aStack.size() + 300));
 }
 
 public static void main(String[] args) {
  testCases(); 
 }
}

Stack is empty: true
Element at top of Stack: false
Size of current Stack: 3
Element at top of Stack: Android App Developer
Element at top of Stack: 2009
Size of current Stack: 8
Index 0: Nicholas Key
Index 1: 3.142
Index 2: false
Index 3: Java Developer
Index 4: Android App Developer
Index 5: Web App Developer
Index 6: Blogger
Index 7: 2009
Element at top of Stack: Blogger
Element at top of Stack: Web App Developer
Index 0: Nicholas Key
Index 1: 3.142
Index 2: false
Index 3: Java Developer
Index 4: Android App Developer
Index 5: Web App Developer
Element at bottom of Stack: Nicholas Key
Stack is empty: true
Exception in thread "main" java.lang.RuntimeException: Index out of bound. Supplied 1
 at CustomStack.get(CustomStack.java:29)
 at CustomStack.testCases(CustomStack.java:128)
 at CustomStack.main(CustomStack.java:136)

Update: I've added several methods to make my CustomStack more robust ;) Click here to look at the improvements.