UndoStack.java

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package hospitalmanagementsystem;



/**
 *
 * @author egehanhatipoglu
 */

public class UndoStack {
    
    
    // INNER CLASS: StackNode
   
    // Represents a node in the stack. We store 'Object data' alongside the operation string
    // to allow saving entire objects (like a deleted Patient) for restoration later.
    public class StackNode {
        public String operation;
        public Object data;
        StackNode next;
        
        StackNode(String operation, Object data) {
            this.operation = operation;
            this.data = data;
            this.next = null;
        }
    }
    
    private StackNode top;
    
    public UndoStack() {
        this.top = null;
    }
    
    // Overloaded push for simple operations that don't need object data
    public void push(String operation) {
        push(operation, null);
    }
    
    // Core Push Method: Adds a new operation to the stack.
    // The 'data' parameter allows us to save the state of an object before it is modified or deleted,
    // which is critical for implementing the Undo logic for "Delete" operations.
    public void push(String operation, Object data) {
        StackNode newNode = new StackNode(operation, data);
        newNode.next = top;
        top = newNode;
    }
    
    // Standard Pop: Returns only the operation string.
    // Useful for simple undo logic where we only need to parse an ID (e.g., "PATIENT_ADD:5").
    public String pop() {
        if (isEmpty()) return null;
        String operation = top.operation;
        top = top.next;
        return operation;
    }
    
    // Critical Method: Returns the entire node including the stored 'data' object.
    // This is required when we need to restore a complex object (like adding a deleted Patient back into the system).
    public StackNode popWithData() {
        if (isEmpty()) return null;
        StackNode node = top;
        top = top.next;
        return node;
    }
    
    // Peeks at the stored data object without removing the node.
    public Object peekData() {
        if (isEmpty()) return null;
        return top.data;
    }
    
    public boolean isEmpty() {
        return top == null;
    }
}