In mods such as Tinker's Construct and Thermal Expansion when you hover over a liquid in a gui it tells you the name of the liquid and the amount in mB. How is this accomplished?

To accomplish this, you'll need to check where the mouse is on the screen then, draw the tooltip. Considering this took me hours of research to figure out a few months back; here's some code to help you out.

 

public void drawToolTips(int mouseX, int mouseY) {
    int boxX = (this.width - this.xSize) / 2; //These are just like the center of the screen. 
    int boxY = (this.height - this.ySize) / 2;//You make these in order to get a starting point on where to check (With these, it would be the top left of your GUI)

	int defaultX = 16; //These are like the boxes of the area to check.
	int defaultY = 16; //So, if you're rendering an Item to hover over, it would be 16x16. 

	//This checks if the mouse is in the correct position on screen. You can add and subtract from boxX and Y to determine where to be.
	if(mouseX > boxX && mouseX < boxX + defaultX && mouseY > boxY && mouseY < boxY + defaultY) { 
                           List list = new ArrayList(); //Just the list of text to be added. It is just like the addInformation method now. 
                           list.add("SOMETHING") //Each time you add something, it's gonna be a new line in the tooltip.
                           this.drawHoveringText(list, mouseX, mouseY, fontRenderer);
	}
}

 

Then you just add that method in your drawScreen AFTER calling super.drawScreen(). If you add it before, you will get some pretty weird rendering errors. The first two params of drawScreen are the mouseX and mouseY so use those. Hope it helped!

where do i get fontRenderer from?

Edit: Found it, It's ment to be fontRendererObj

Minecraft.getMinecraft().getFontRenderer() or getFontRendererObj()

 

Or you know, if you are using some kind of GUI - 1st check if there is no ready-to-use fontRenderer or Minecraft instance reference.

If you extend GuiScreen you get a ready-to-use instance of Minecraft. Just thought I'd point it out.