TileEntityVendingMachine

import net.minecraft.entity.player.EntityPlayer;
import net.minecraft.inventory.IInventory;
import net.minecraft.item.ItemStack;
import net.minecraft.nbt.*;
import net.minecraft.tileentity.TileEntity;

public class TileEntityVendingMachine extends TileEntity implements IInventory {
    private ItemStack[] items;

    public TileEntityVendingMachine() {
        items = new ItemStack[ 20 ];
    }

    @Override
    public int getSizeInventory() {
        return items.length;
    }

    @Override
    public ItemStack getStackInSlot( int slot ) {
        return items[ slot ];
    }

    @Override
    public ItemStack decrStackSize( int slot, int count ) {
        ItemStack itemStack = getStackInSlot( slot );
        if( itemStack != null ) {
            if( itemStack.stackSize <= count ) {
                setInventorySlotContents( slot, null );
            } else {
                itemStack.splitStack( count );
                onInventoryChanged();
            }
        }
        return itemStack;
    }

    @Override
    public ItemStack getStackInSlotOnClosing( int slot ) {
        ItemStack itemStack = getStackInSlot( slot );
        setInventorySlotContents( slot, null );
        return itemStack;
    }

    @Override
    public void setInventorySlotContents( int slot, ItemStack itemStack ) {
        items[ slot ] = itemStack;
        onInventoryChanged();
    }

    @Override
    public String getInvName() {
        return "Vending Machine";
    }

    @Override
    public boolean isInvNameLocalized() {
        return false;
    }

    @Override
    public int getInventoryStackLimit() {
        return 64;
    }

    @Override
    public boolean isUseableByPlayer( EntityPlayer entityplayer ) {
        return entityplayer.getDistanceSq( xCoord + 0.5, yCoord + 0.5, zCoord + 0.5 ) <= 64;
    }

    @Override
    public void openChest() {
    }

    @Override
    public void closeChest() {
    }

    @Override
    public boolean isItemValidForSlot( int slot, ItemStack itemStack ) {
        ItemStack sellStack = getStackInSlot( 18 );
        ItemStack buyStack = getStackInSlot( 19 );
        if( slot == 18 ) {
            return !( buyStack != null && itemStack.itemID == buyStack.itemID );
        } else if( slot == 19 ) {
            return !( sellStack != null && itemStack.itemID == sellStack.itemID );
        } else if( slot < 9 ) {
            return ( sellStack != null && itemStack.itemID == sellStack.itemID );
        } else if( slot >= 9 && slot <= 17 ) {
            return ( buyStack != null && itemStack.itemID == buyStack.itemID );
        }
        return false;
    }

    @Override
    public void writeToNBT( NBTTagCompound compound ) {
        super.writeToNBT( compound );
        NBTTagList items = new NBTTagList();
        for( int i = 0; i < this.items.length; i++ ) {
            ItemStack stack = getStackInSlot( i );
            if( stack != null ) {
                NBTTagCompound item = new NBTTagCompound();
                item.setByte( "slot", (byte)i );
                stack.writeToNBT( item );
                items.appendTag( item );
            }
        }
        compound.setTag( "items", items );
    }

    @Override
    public void readFromNBT( NBTTagCompound compound ) {
        super.readFromNBT( compound );
        NBTTagList items = compound.getTagList( "items" );
        for( int i = 0; i < items.tagCount(); i++ ) {
            NBTTagCompound item = (NBTTagCompound)items.tagAt( i );
            int slot = item.getByte( "slot" );
            if( slot >= 0 && slot < getSizeInventory() ) {
                setInventorySlotContents( slot, ItemStack.loadItemStackFromNBT( item ) );
            }
        }
    }
}