Implementation of Queue with Array (Java)

ArrayQueue1.java

public class ArrayQueue1 implements Queue {
	private static Object []	arr;
	private static int	currentSize;
	private static int	front;
	private static int	back;

	private static final int DEFAULT_CAPACITY = 10;

	public ArrayQueue1()
	{
		arr = new Object[ DEFAULT_CAPACITY ];
		makeEmpty( );
	}
    
	public void enqueue(Object x) {
		if( currentSize == arr.length ) 	// jika besaran entry data melebihi kapasitas
			doubleQueue( );             	// buat array yang baru dengan memperbesar kapasitas 2x
		back++;
		arr[back] = x;
		currentSize++;
	}
 
	public Object getFront() {
		if(isEmpty()) throw new UnderflowException("Array Underflow!");
		return arr[front];
	}
 
	public Object dequeue() {
		if(isEmpty()) throw new UnderflowException( "ArrayQueue dequeue" );
		currentSize--;

		Object returnValue = arr[front];
		front++;
		return returnValue;
	}
 
	public boolean isEmpty() {
		return currentSize == 0;
	}
 
	public void makeEmpty() {
		currentSize	= 0;
		front		= 0;
		back		= -1;
	}
	
	private void doubleQueue( ) {
		Object [] arrBaru;

		arrBaru = new Object[ arr.length * 2 ];
		for( int i = 0; i < currentSize; i++, front++ )
			arrBaru[ i ] = arr[ front ];

		arr 	= arrBaru;
		front 	= 0;
		back 	= currentSize - 1;
	}
	
	public static void display() {
		if (back > front) {
			System.out.println(">> Elemen di antrian:");
			for (int i = front; i <= back; i++) System.out.println(arr[i]);
		}
	}

	public static void main(String[] args) {  
		ArrayQueue1 q = new ArrayQueue1( );

		try {
			q.enqueue(1); display();
			q.enqueue(2); display();
			q.enqueue(3); display(); 
			q.enqueue(4); display(); 
			q.dequeue();  display(); // remove 1st element from list
			q.enqueue(5); display(); 
			q.enqueue(6); display(); 
			q.dequeue();  display(); // remove 2nd element from list
		}
		catch( UnderflowException e ) { System.out.println( "Unexpected overflow" ); }
        
		System.out.println("Final result:");
		while( !q.isEmpty() )
			System.out.println( q.dequeue() ); 
	}
}

interface Queue {
	void		enqueue(Object x);
	Object		getFront();
	Object		dequeue();
	boolean		isEmpty();
	void 		makeEmpty();
}