I was looking through some code about textures, and I found these lines: 

StringBuilder builder = new StringBuilder();
builder.append(" \"all\": \"" + BaseMod.MODID + ":blocks/" + texture + "\"");

The variable`texture` is  an image file.

What I understand is that the first line declares a StringBuilder (I'm not really sure what that is). Then, the second line applies textures to all sides of the block because of the `all` part. I need help understanding this, and how would I change the code so that the second line only applies textures to the front side?

Applying textures is done through JSON files ever since Minecraft 1.8

 

Normally, for blocks with specific "front" sides, you want your block to dynamically rotate depending on it's relative position to the player (most commonly face the player when placed).
For that, you need to work with BlockState's. The vanilla furnace is a good example, and the BlockFurnace class should show you how to use PropertyDirection to alter the BlockState.

 

As for the textures, you can look at the furnace.json located in forgeSrc-<mc.version>-<forge.version>.jar/assets/minecraft/models/block. Use your IDE to find that file.

 

This model.JSON file has to exist inside src/main/resources/assets/modid/models/block, and if needed, a blockstate.JSON has to exist inside src/main/resources/assets/modid/blockstates

 

The JSON files have to be completely lowercase in 1.11, and their names have to be identical to your block's RegistryName.

Edited June 21, 20178 yr by Matryoshika

You're misunderstanding that code. A StringBuilder is a kind of mutable string object. All that line of code does is put together a string from those components - the result is (almost*) equivalent to just using the + operator on string literals directly.

 

(*StringBuilder tutorial)

 

Simple model code is usually controlled entirely in the json model files. If you want more help understanding or adapting the code that you found, we need (lots) more context to understand what it's doing.