Create ItemGUI class that will be used to change all required and reward item stacks.

src/main/java/world/bentobox/challenges/panel/util/ItemGUI.java

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+package world.bentobox.challenges.panel.util;
+
+
+import org.bukkit.event.inventory.InventoryClickEvent;
+import org.bukkit.event.inventory.InventoryCloseEvent;
+import org.bukkit.inventory.ItemStack;
+import java.util.Collections;
+import java.util.List;
+
+import world.bentobox.bentobox.api.panels.PanelItem;
+import world.bentobox.bentobox.api.panels.PanelListener;
+import world.bentobox.bentobox.api.panels.builders.PanelBuilder;
+import world.bentobox.bentobox.api.user.User;
+
+
+/**
+ * This class allows to change Input ItemStacks to different ItemStacks.
+ */
+public class ItemGUI
+{
+	public ItemGUI(User user, List<ItemStack> itemStacks)
+	{
+		this.user = user;
+		this.itemStacks = itemStacks;
+		this.build();
+	}
+
+
+	/**
+	 * This method builds panel that allows to change given number value.
+	 */
+	private void build()
+	{
+		PanelBuilder panelBuilder = new PanelBuilder().name(this.user.getTranslation("challenges.gui.change-items"));
+		panelBuilder.listener(new CustomPanelListener());
+
+		for (ItemStack itemStack : this.itemStacks)
+		{
+			panelBuilder.item(new CustomPanelItem(itemStack));
+		}
+
+		panelBuilder.build().open(this.user);
+	}
+
+
+// ---------------------------------------------------------------------
+// Section: Private classes
+// ---------------------------------------------------------------------
+
+
+	/**
+	 * This CustomPanelItem does no lose Item original MetaData. After PanelItem has been
+	 * created it restores original meta data. It also does not allow to change anything that
+	 * could destroy meta data.
+	 */
+	private class CustomPanelItem extends PanelItem
+	{
+		CustomPanelItem(ItemStack item)
+		{
+			super(item.clone(), "", Collections.emptyList(), false, null, false);
+			this.getItem().setItemMeta(item.getItemMeta());
+		}
+
+
+		@Override
+		public void setGlow(boolean glow)
+		{
+		}
+
+
+		@Override
+		public void setDescription(List<String> description)
+		{
+		}
+
+
+		@Override
+		public void setName(String name)
+		{
+		}
+
+
+		@Override
+		public void setHead(ItemStack itemStack)
+		{
+		}
+	}
+
+
+	/**
+	 * This CustomPanelListener allows to move items in current panel.
+	 */
+	private class CustomPanelListener implements PanelListener
+	{
+		@Override
+		public void setup()
+		{
+		}
+
+
+		@Override
+		public void onInventoryClose(InventoryCloseEvent inventoryCloseEvent)
+		{
+		}
+
+
+		@Override
+		public void onInventoryClick(User user, InventoryClickEvent event)
+		{
+			event.setCancelled(false);
+		}
+	}
+
+
+// ---------------------------------------------------------------------
+// Section: Variables
+// ---------------------------------------------------------------------
+
+
+	/**
+	 * User who opens current gui.
+	 */
+	private User user;
+
+	/**
+	 * List with original items.
+	 */
+	private List<ItemStack> itemStacks;
+}