How to writeToNBT for more advanced objects?

[MrDireball]

So the code below will save an integer to wherever (I just know that it's important to save it or things will get screwed up), but how do I save something such as an ItemStack to in a similar manner? Please tell me that I'm not going to have to break it down into simple data types to save it...

 

Spoiler

    @Override
    public NBTTagCompound writeToNBT(NBTTagCompound compound)
    {
        super.writeToNBT(compound);
        compound.setInteger("int", int);
        return compound;
    }

 

 

 

 

Edited January 10, 2019 by MrDireball

[unassigned]

1 hour ago, MrDireball said:

So the code below will save an integer to wherever (I just know that it's important to save it or things will get screwed up), but how do I save something such as an ItemStack to in a similar manner? Please tell me that I'm not going to have to break it down into simple data types to save it...

 

  Reveal hidden contents

    @Override
    public NBTTagCompound writeToNBT(NBTTagCompound compound)
    {
        super.writeToNBT(compound);
        compound.setInteger("int", int);
        return compound;
    }

 

 

 

 

 

If you'd like to save an item to nbt, you'd need to call ItemStack#writeToNBT, which will write a tag to the nbt under the key "id". If you'd like to store multiple items, you need to create an NBTTagList and write all items to that.

[MrDireball]

3 minutes ago, unassigned said:

If you'd like to save an item to nbt, you'd need to call ItemStack#writeToNBT, which will write a tag to the nbt under the key "id". If you'd like to store multiple items, you need to create an NBTTagList and write all items to that.

Could you give me a code example of that?

[unassigned]

                NBTTagCompound dataCompound = new NBTTagCompound();
                ItemStack newStack = new ItemStack(Items.DIAMOND);

                newStack.writeToNBT(dataCompound); //writes newStack data to dataCompound

                if(dataCompound.hasKey("id")) { ItemStack fromCompoundStack = new ItemStack(dataCompound); } //if the dataCompound has a ItemStack data, make a new stack from that data.

 

[MrDireball]

8 minutes ago, unassigned said:

                NBTTagCompound dataCompound = new NBTTagCompound();
                ItemStack newStack = new ItemStack(Items.DIAMOND);

                newStack.writeToNBT(dataCompound); //writes newStack data to dataCompound

                if(dataCompound.hasKey("id")) { ItemStack fromCompoundStack = new ItemStack(dataCompound); } //if the dataCompound has a ItemStack data, make a new stack from that data.

 

 

[MrDireball]

What should I replace "id" with? "Minecraft:DIAMOND"?

 

Here's what I translated this into:

 

Spoiler

    @Override
    public void readFromNBT(NBTTagCompound compound)
    {
        super.readFromNBT(compound);
        if(compound.hasKey("id"))
        {
        	itemStack = new ItemStack(compound); //itemStack is a member variable of this class.
        }
    }

    @Override
    public NBTTagCompound writeToNBT(NBTTagCompound compound)
    {
        super.writeToNBT(compound);
        return itemStack.writeToNBT(compound);
    }

 

 

 

 

 

 

Edited January 10, 2019 by MrDireball

[diesieben07]

10 minutes ago, MrDireball said:

if(compound.hasKey("id"))

You should not have this check!

Simply using writeToNBT and new ItemStack(nbt) is enough.

Note however that you should not write anything else to the compound used for the stack, so you probably want to store the stack in a sub-compound to avoid interference.

[MrDireball]

Dangit, it's not saving the item. Is there anything that you can see that's clearly wrong with this code? I doubt I'm using the super right.

 

Spoiler

    @Override
    public void readFromNBT(NBTTagCompound compound)
    {
        super.readFromNBT(compound);
        itemStack = new ItemStack(compound);
    }

    @Override
    public NBTTagCompound writeToNBT(NBTTagCompound compound)
    {
        super.writeToNBT(compound);
        return itemStack.writeToNBT(compound);
    }

 

 

 

 

Edited January 10, 2019 by MrDireball

[diesieben07]

Without seeing more of your code: no. We have zero context.

Please clarify also what you mean by "not saving". How exactly did you come to this conclusion, which symptoms did you observe?

[MrDireball]

4 minutes ago, diesieben07 said:

Without seeing more of your code: no. We have zero context.

Please clarify also what you mean by "not saving". How exactly did you come to this conclusion, which symptoms did you observe?

I have a member variable of the type ItemStack in my DataTileEntity class, and I'd like it to still have a reference to whatever it had a reference to before I relogged. But it is null everytime I load up the world again.

 

Spoiler

import net.minecraft.init.Items;
import net.minecraft.item.ItemStack;
import net.minecraft.nbt.NBTTagCompound;
import net.minecraft.tileentity.TileEntity;

public class DataTileEntity extends TileEntity {
    
    ItemStack itemStack;

    public void setItemStackment(ItemStack itemStack)
    {
        this.itemStack = itemStack;
        markDirty();
    }
    
    public ItemStack getItemStackment() {
        return itemStack;
    }

    @Override
    public void readFromNBT(NBTTagCompound compound)
    {
        super.readFromNBT(compound);
        itemStack = new ItemStack(compound);
    }

    @Override
    public NBTTagCompound writeToNBT(NBTTagCompound compound)
    {
        super.writeToNBT(compound);
        return itemStack.writeToNBT(compound);
    }
}

 

 

 

Edited January 10, 2019 by MrDireball

[diesieben07]

1 minute ago, MrDireball said:

But it is null everytime I load up the world again.

How did you check this?

[diesieben07]

Actually, I now know why.

You need to follow this advice:

22 minutes ago, diesieben07 said:

Note however that you should not write anything else to the compound used for the stack, so you probably want to store the stack in a sub-compound to avoid interference.

 

The default NBT for tile entities contains a key "id", which conflicts with the ItemStack serialization.

[MrDireball]

1 minute ago, diesieben07 said:

Actually, I now know why.

You need to follow this advice:

 

The default NBT for tile entities contains a key "id", which conflicts with the ItemStack serialization.

I'm checking it by right-clicking a block, and it swaps the item in the player's hand with the item in the DataTileEntity, which works fine until I reload, then whatever was in it is gone.

 

So how can I get around the default setting?

[diesieben07]

Like I said, store the stack in a sub-compound. I.e. create a new NBTTagCompound, write the ItemStack (and only the ItemStack) to it and then add it to "your own" compound.

[MrDireball]

5 minutes ago, diesieben07 said:

Like I said, store the stack in a sub-compound. I.e. create a new NBTTagCompound, write the ItemStack (and only the ItemStack) to it and then add it to "your own" compound.

Pardon my bad understanding, are you saying that I should create a new instance of NBTTagCompound or a new type that derives from NBTTagCompound?

[diesieben07]

A new instance.

[MrDireball]

1 hour ago, diesieben07 said:

A new instance.

I told it to read from and write to a file called "test.bat", but for some reason, it's still not working or even creating the file.

 

Spoiler

    @Override
    public void readFromNBT(NBTTagCompound compound)
    {
    	readStackFromFile("test.dat");
    }

    @Override
    public NBTTagCompound writeToNBT(NBTTagCompound compound)
    {
    	return writeStackToFile("test.dat");
    }
    
    public NBTTagCompound writeStackToFile(String fileName)
    {
        if (itemStack != null)
        {
            NBTTagCompound nbt = new NBTTagCompound();
            itemStack.writeToNBT(nbt);
            File file = new File(fileName);
            try
            {
                file.createNewFile();
                FileOutputStream fileoutputstream = new FileOutputStream(file);
                CompressedStreamTools.writeCompressed(nbt, fileoutputstream);
                fileoutputstream.close();
            }
            catch (Exception exception)
            {
                exception.printStackTrace();
            }
            return nbt;
        }
        return null;
    }
        
    public void readStackFromFile(String file)
    {
    	FileInputStream fileinputstream = null;
    	NBTTagCompound nbt = null;
		try
		{
			fileinputstream = new FileInputStream(file);
		} catch (FileNotFoundException e)
		{
			e.printStackTrace();
		}
		try
		{
			nbt = CompressedStreamTools.readCompressed(fileinputstream);
	    	itemStack = new ItemStack(nbt);
		} catch (IOException e)
		{
			e.printStackTrace();
		}
    }
}

 

 

 

[unassigned]

48 minutes ago, MrDireball said:

I told it to read from and write to a file called "test.bat", but for some reason, it's still not working or even creating the file.

 

  Reveal hidden contents

    @Override
    public void readFromNBT(NBTTagCompound compound)
    {
    	readStackFromFile("test.dat");
    }

    @Override
    public NBTTagCompound writeToNBT(NBTTagCompound compound)
    {
    	return writeStackToFile("test.dat");
    }
    
    public NBTTagCompound writeStackToFile(String fileName)
    {
        if (itemStack != null)
        {
            NBTTagCompound nbt = new NBTTagCompound();
            itemStack.writeToNBT(nbt);
            File file = new File(fileName);
            try
            {
                file.createNewFile();
                FileOutputStream fileoutputstream = new FileOutputStream(file);
                CompressedStreamTools.writeCompressed(nbt, fileoutputstream);
                fileoutputstream.close();
            }
            catch (Exception exception)
            {
                exception.printStackTrace();
            }
            return nbt;
        }
        return null;
    }
        
    public void readStackFromFile(String file)
    {
    	FileInputStream fileinputstream = null;
    	NBTTagCompound nbt = null;
		try
		{
			fileinputstream = new FileInputStream(file);
		} catch (FileNotFoundException e)
		{
			e.printStackTrace();
		}
		try
		{
			nbt = CompressedStreamTools.readCompressed(fileinputstream);
	    	itemStack = new ItemStack(nbt);
		} catch (IOException e)
		{
			e.printStackTrace();
		}
    }
}

 

 

 

What? Why are you creating a file? Just create a new NBTTagCompound (NBTTagCompound itemTag = new NBTTagCompound), write the data to that tag, then add that tag to your TEs compound tag (in writeToNBT) (via NBTTagCompound#setTag). Then access it when you go to read it via NBTagCompound#getCompoundTag and create a new ItemStack off this.

 

Here is an example:

    @Override
    public NBTTagCompound writeToNBT(NBTTagCompound compound) {
        super.writeToNBT(compound);
        NBTTagCompound itemData = new NBTTagCompound(); // This will store the item data seperate from the TE's data
        ItemStack stack = new ItemStack(Items.DIAMOND); // You might not need to create a new item.
        stack.writeToNBT(itemData); //writes the Item's NBT to the 'itemData'.

        compound.setTag("ItemData", itemData); //writes the itemData tag to the TEs compound. This will be attached to the TE.

        return compound;
    }

    @Override
    public void readFromNBT(NBTTagCompound compound) {
        super.readFromNBT(compound);

        NBTTagCompound itemData = compound.getCompoundTag("ItemData"); //this will get the compound
        ItemStack stack = new ItemStack(itemData); //this will read the itemData and turn it into what it was.
        //From here, the stack will be loaded back into the TE and can be accessed again.
    }

 

Edited January 10, 2019 by unassigned
fixed dumb spaces in parentheses

[MrDireball]

29 minutes ago, unassigned said:

What? Why are you creating a file? Just create a new NBTTagCompound ( NBTTagCompound itemTag = new NBTTagCompound ), write the data to that tag, then add that tag to your TEs compound tag (in writeToNBT) (via NBTTagCompound#setTag ). Then access it when you go to read it via NBTagCompound#getTag and create a new ItemStack off this.

I'm trying to use that method of doing it, but I'm getting two errors.

 

Code:

Spoiler

    @Override
    public void readFromNBT(NBTTagCompound compound)
    {
    	super.readFromNBT(compound);
        if (compound.hasKey("item"))
        {
            itemStack.deserializeNBT((NBTTagCompound) compound.getTag("item"));
        }
    }

    @Override
    public NBTTagCompound writeToNBT(NBTTagCompound compound)
    {
        super.writeToNBT(compound);
        compound.setTag("item", itemStack.serializeNBT());
        return compound;
    }

 

 

 

First error, occurs upon placing block:

Spoiler

[19:03:26] [Server thread/FATAL] [minecraft/MinecraftServer]: Error executing task
java.util.concurrent.ExecutionException: java.lang.NullPointerException
    at java.util.concurrent.FutureTask.report(Unknown Source) ~[?:1.8.0_192]
    at java.util.concurrent.FutureTask.get(Unknown Source) ~[?:1.8.0_192]
    at net.minecraft.util.Util.runTask(Util.java:54) [Util.class:?]
    at net.minecraft.server.MinecraftServer.updateTimeLightAndEntities(MinecraftServer.java:798) [MinecraftServer.class:?]
    at net.minecraft.server.MinecraftServer.tick(MinecraftServer.java:743) [MinecraftServer.class:?]
    at net.minecraft.server.integrated.IntegratedServer.tick(IntegratedServer.java:192) [IntegratedServer.class:?]
    at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:592) [MinecraftServer.class:?]
    at java.lang.Thread.run(Unknown Source) [?:1.8.0_192]
Caused by: java.lang.NullPointerException
    at davidgingerich.davesmod.blocks.advancedblocks.datablock.DataTileEntity.writeToNBT(DataTileEntity.java:43) ~[DataTileEntity.class:?]
    at net.minecraftforge.common.util.BlockSnapshot.getTileNBT(BlockSnapshot.java:132) ~[BlockSnapshot.class:?]
    at net.minecraftforge.common.util.BlockSnapshot.<init>(BlockSnapshot.java:61) ~[BlockSnapshot.class:?]
    at net.minecraftforge.common.util.BlockSnapshot.<init>(BlockSnapshot.java:82) ~[BlockSnapshot.class:?]
    at net.minecraftforge.common.util.BlockSnapshot.getBlockSnapshot(BlockSnapshot.java:113) ~[BlockSnapshot.class:?]
    at net.minecraft.world.World.setBlockState(World.java:394) ~[World.class:?]
    at davidgingerich.davesmod.blocks.advancedblocks.datablock.DataBlock.onBlockPlacedBy(DataBlock.java:90) ~[DataBlock.class:?]
    at net.minecraft.item.ItemBlock.placeBlockAt(ItemBlock.java:205) ~[ItemBlock.class:?]
    at net.minecraft.item.ItemBlock.onItemUse(ItemBlock.java:57) ~[ItemBlock.class:?]
    at net.minecraftforge.common.ForgeHooks.onPlaceItemIntoWorld(ForgeHooks.java:884) ~[ForgeHooks.class:?]
    at net.minecraft.item.ItemStack.onItemUse(ItemStack.java:200) ~[ItemStack.class:?]
    at net.minecraft.server.management.PlayerInteractionManager.processRightClickBlock(PlayerInteractionManager.java:507) ~[PlayerInteractionManager.class:?]
    at net.minecraft.network.NetHandlerPlayServer.processTryUseItemOnBlock(NetHandlerPlayServer.java:769) ~[NetHandlerPlayServer.class:?]
    at net.minecraft.network.play.client.CPacketPlayerTryUseItemOnBlock.processPacket(CPacketPlayerTryUseItemOnBlock.java:68) ~[CPacketPlayerTryUseItemOnBlock.class:?]
    at net.minecraft.network.play.client.CPacketPlayerTryUseItemOnBlock.processPacket(CPacketPlayerTryUseItemOnBlock.java:13) ~[CPacketPlayerTryUseItemOnBlock.class:?]
    at net.minecraft.network.PacketThreadUtil$1.run(PacketThreadUtil.java:21) ~[PacketThreadUtil$1.class:?]
    at java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source) ~[?:1.8.0_192]
    at java.util.concurrent.FutureTask.run(Unknown Source) ~[?:1.8.0_192]
    at net.minecraft.util.Util.runTask(Util.java:53) ~[Util.class:?]
    ... 5 more

 

Second error, occurs upon trying to save game:

 

Spoiler

[19:03:27] [Server thread/ERROR] [FML]: A TileEntity type davidgingerich.davesmod.blocks.advancedblocks.datablock.DataTileEntity has throw an exception trying to write state. It will not persist. Report this to the mod author
java.lang.NullPointerException: null
    at davidgingerich.davesmod.blocks.advancedblocks.datablock.DataTileEntity.writeToNBT(DataTileEntity.java:43) ~[DataTileEntity.class:?]
    at net.minecraft.world.chunk.storage.AnvilChunkLoader.writeChunkToNBT(AnvilChunkLoader.java:414) [AnvilChunkLoader.class:?]
    at net.minecraft.world.chunk.storage.AnvilChunkLoader.saveChunk(AnvilChunkLoader.java:185) [AnvilChunkLoader.class:?]
    at net.minecraft.world.gen.ChunkProviderServer.saveChunkData(ChunkProviderServer.java:214) [ChunkProviderServer.class:?]
    at net.minecraft.world.gen.ChunkProviderServer.saveChunks(ChunkProviderServer.java:242) [ChunkProviderServer.class:?]
    at net.minecraft.world.WorldServer.saveAllChunks(WorldServer.java:1061) [WorldServer.class:?]
    at net.minecraft.server.MinecraftServer.saveAllWorlds(MinecraftServer.java:468) [MinecraftServer.class:?]
    at net.minecraft.server.integrated.IntegratedServer.saveAllWorlds(IntegratedServer.java:274) [IntegratedServer.class:?]
    at net.minecraft.server.integrated.IntegratedServer.tick(IntegratedServer.java:177) [IntegratedServer.class:?]
    at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:592) [MinecraftServer.class:?]
    at java.lang.Thread.run(Unknown Source) [?:1.8.0_192]
 

 

 

 

3

 

 

Edited January 11, 2019 by MrDireball

[unassigned]

You can use NBTagCompound#getCompoundTag instead of NBTTagCompound#getTag

 

Aswell, are you sure that the itemStack field is not null? Are you assigning it anywhere? 

[MrDireball]

32 minutes ago, unassigned said:

You can use NBTagCompound#getCompoundTag instead of NBTTagCompound#getTag

 

Aswell, are you sure that the itemStack field is not null? Are you assigning it anywhere? 

Ah, yes, I forgot to check whether it was null first. I also fixed the other error by setting itemStack to initialize to a stack of air rather than null. So although there doesn't seem to be any more errors, the problem with it not saving the value of itemStack after unloading and reloading still persists.

 

DataBlock

Spoiler

public class DataBlock extends BlockBase implements ITileEntityProvider {

    public static final PropertyDirection FACING = PropertyDirection.create("facing", EnumFacing.Plane.HORIZONTAL);

    public DataBlock() {
        super("data_block", Material.ROCK);
		setCreativeTab(CreativeTabs.BUILDING_BLOCKS);
    }

    @SideOnly(Side.CLIENT)
    public void initModel() {
    	ModelLoader.setCustomModelResourceLocation(Item.getItemFromBlock(this), 0, new ModelResourceLocation(getRegistryName(), "inventory"));
    }

    @Override
    public TileEntity createNewTileEntity(World worldIn, int meta) {
        return new DataTileEntity();
    }

    private DataTileEntity getTE(World world, BlockPos pos) {
        return (DataTileEntity) world.getTileEntity(pos);
    }

    @Override
    public boolean onBlockActivated(World world, BlockPos pos, IBlockState state, EntityPlayer player, EnumHand hand,
                EnumFacing side, float hitX, float hitY, float hitZ) {
        if (!world.isRemote) {
    		DataTileEntity dataEntity = (DataTileEntity)getTE(world, pos);
    	    ItemStack itemStack = dataEntity.getItemStack();
    	    dataEntity.setItemStack(player.getHeldItemMainhand());
    	    player.setHeldItem(player.getActiveHand(), itemStack);
        }
        return true;
    }

    @Override
    public void onBlockPlacedBy(World world, BlockPos pos, IBlockState state, EntityLivingBase placer, ItemStack stack) {
        world.setBlockState(pos, state.withProperty(FACING, placer.getHorizontalFacing().getOpposite()), 2);
    }

    @Override
    public IBlockState getStateFromMeta(int meta) {
        return getDefaultState().withProperty(FACING, EnumFacing.getFront((meta & 3) + 2));
    }

    @Override
    public int getMetaFromState(IBlockState state) {
        return state.getValue(FACING).getIndex()-2;
    }

    @Override
    protected BlockStateContainer createBlockState() {
        return new BlockStateContainer(this, FACING);
    }

 

DataTileEntity

Spoiler

public class DataTileEntity extends TileEntity {
    
    ItemStack itemStack = new ItemStack(Items.AIR);

    public void setItemStack(ItemStack itemStack)
    {
        this.itemStack = itemStack;
        markDirty();
    }
    
    public ItemStack getItemStack() {
        return itemStack;
    }

    @Override
    public void readFromNBT(NBTTagCompound compound)
    {
    	super.readFromNBT(compound);
        if (compound.hasKey("item"))
        {
            itemStack.deserializeNBT((NBTTagCompound) compound.getCompoundTag("item"));
        }
    }

    @Override
    public NBTTagCompound writeToNBT(NBTTagCompound compound)
    {
        super.writeToNBT(compound);
        if (itemStack != null)compound.setTag("item", itemStack.serializeNBT());
        return compound;
    }

 

 

 

 

 

[unassigned]

24 minutes ago, MrDireball said:

Ah, yes, I forgot to check whether it was null first. I also fixed the other error by setting itemStack to initialize to a stack of air rather than null. So although there doesn't seem to be any more errors, the problem with it not saving the value of itemStack after unloading and reloading still persists.

 

DataBlock

  Reveal hidden contents

public class DataBlock extends BlockBase implements ITileEntityProvider {

    public static final PropertyDirection FACING = PropertyDirection.create("facing", EnumFacing.Plane.HORIZONTAL);

    public DataBlock() {
        super("data_block", Material.ROCK);
		setCreativeTab(CreativeTabs.BUILDING_BLOCKS);
    }

    @SideOnly(Side.CLIENT)
    public void initModel() {
    	ModelLoader.setCustomModelResourceLocation(Item.getItemFromBlock(this), 0, new ModelResourceLocation(getRegistryName(), "inventory"));
    }

    @Override
    public TileEntity createNewTileEntity(World worldIn, int meta) {
        return new DataTileEntity();
    }

    private DataTileEntity getTE(World world, BlockPos pos) {
        return (DataTileEntity) world.getTileEntity(pos);
    }

    @Override
    public boolean onBlockActivated(World world, BlockPos pos, IBlockState state, EntityPlayer player, EnumHand hand,
                EnumFacing side, float hitX, float hitY, float hitZ) {
        if (!world.isRemote) {
    		DataTileEntity dataEntity = (DataTileEntity)getTE(world, pos);
    	    ItemStack itemStack = dataEntity.getItemStack();
    	    dataEntity.setItemStack(player.getHeldItemMainhand());
    	    player.setHeldItem(player.getActiveHand(), itemStack);
        }
        return true;
    }

    @Override
    public void onBlockPlacedBy(World world, BlockPos pos, IBlockState state, EntityLivingBase placer, ItemStack stack) {
        world.setBlockState(pos, state.withProperty(FACING, placer.getHorizontalFacing().getOpposite()), 2);
    }

    @Override
    public IBlockState getStateFromMeta(int meta) {
        return getDefaultState().withProperty(FACING, EnumFacing.getFront((meta & 3) + 2));
    }

    @Override
    public int getMetaFromState(IBlockState state) {
        return state.getValue(FACING).getIndex()-2;
    }

    @Override
    protected BlockStateContainer createBlockState() {
        return new BlockStateContainer(this, FACING);
    }

 

DataTileEntity

  Reveal hidden contents

public class DataTileEntity extends TileEntity {
    
    ItemStack itemStack = new ItemStack(Items.AIR);

    public void setItemStack(ItemStack itemStack)
    {
        this.itemStack = itemStack;
        markDirty();
    }
    
    public ItemStack getItemStack() {
        return itemStack;
    }

    @Override
    public void readFromNBT(NBTTagCompound compound)
    {
    	super.readFromNBT(compound);
        if (compound.hasKey("item"))
        {
            itemStack.deserializeNBT((NBTTagCompound) compound.getCompoundTag("item"));
        }
    }

    @Override
    public NBTTagCompound writeToNBT(NBTTagCompound compound)
    {
        super.writeToNBT(compound);
        if (itemStack != null)compound.setTag("item", itemStack.serializeNBT());
        return compound;
    }

 

 

 

 

 

Yeah, just remember ItemStacks should never be null. Instead, if you want an ItemStack to be empty, use ItemStack.EMPTY. 

 

What are you trying to achieve with this block? What it seems like you want to add the player's hand stack to its inventory. Instead of implementing your own inventory system, try exposing the IItemHandler capability. This will give your TE an inventory, and will most likely solve many of your problems. Not to mention, the capability will provide a lot better compat with other mods that access inventories. You shouldn't be creating an inventory from an ItemStack field.

 

Here is an example of what I'm saying. (You should also reference the official forge documentation)

First, create the handler itself (This will be a field will be in your TE):

    public final ItemStackHandler inv = new ItemStackHandler(1);

The constructor to ItemStackHandler is how many slots the inventory should have.

Now, you need to expose the capability for forge to access. 

    @Override
    public boolean hasCapability(Capability<?> capability, @Nullable EnumFacing facing) {
        if(capability == CapabilityItemHandler.ITEM_HANDLER_CAPABILITY){
            return true;  //this is also where you'd check the facing, incase you only want to insert on certain sides.
        }

        return super.hasCapability(capability, facing);
    }

    @Nullable
    @Override
    public <T> T getCapability(Capability<T> capability, @Nullable EnumFacing facing) {
        if(capability == CapabilityItemHandler.ITEM_HANDLER_CAPABILITY){ //this will check to see if the capability being 'get' is, in fact, the ItemHandler
            return (T) this.inv; //this will be the inventory field that you have attached to your TE.
        }
        return super.getCapability(capability, facing);
    }

 

To save the data of the inventory, use ItemStackHandler#serializeNBT and ItemStackHandler#deserializeNBT.

 

From the block class, you need to get the capability (using TileEntity#getCapability) and use ItemStackHandler#insertItem to put the item into the TE's inventory.

Edited January 11, 2019 by unassigned
stack correction