Main Class File:
package me.scherer.Zach;
import java.util.logging.Logger;

import org.bukkit.event.Event;
import org.bukkit.event.Listener;
import org.bukkit.plugin.PluginManager;
import org.bukkit.plugin.java.JavaPlugin;

public class Zach extends JavaPlugin  {

    private ZachBlockListener blockListener = new ZachBlockListener(this);
    Logger log = Logger.getLogger("Minecraft");
    @Override
    public void onEnable(){
        log.info("Your plugin has been enabled.");
        PluginManager ps = this.getServer().getPluginManager();
        ps.registerEvent(Event.Type.BLOCK_BREAK, (Listener) blockListener, Event.Priority.Normal, this);
    }
    @Override
    public void onDisable(){
        log.info("Plugin Disabled!");
    }
}

Block Listener Class File:
package me.scherer.Zach;

import org.bukkit.Material;
import org.bukkit.block.Block;
import org.bukkit.event.block.BlockBreakEvent;
import org.bukkit.event.block.BlockListener;
import org.bukkit.inventory.ItemStack;

public class ZachBlockListener extends BlockListener{
    private Zach plugin;
    public ZachBlockListener(Zach instance){
        this.plugin = instance;
    }

    public void onBlockBreak(BlockBreakEvent event){
         Block block;
         block = event.getBlock();
         if(block.getType().equals(Material.LEAVES)) {
             if(Math.random() * 100 <= 50){
                 block.getWorld().dropItemNaturally(block.getLocation(), new ItemStack(Material.APPLE, 1));
         }
     }
}
}
Plugin.yml:
name: Zach
main: me.scherer.Zach.Zach
version: 1.4