View source for Execute Brain****/Java

{{implementation|Brainf***}}{{collection|RCBF}}
In this implementation of [[Brainf***]] in [[Java]], the code is read in all at once and checked for uneven brackets (unequal amounts of [ and ] commands). If that error occurs, the code will obviously not be run.

Under the hood, the program memory is an <tt>ArrayList</tt> of Integers which expands "infinitely" (limited by your system's memory) to the right (still [http://en.wikipedia.org/wiki/Turing_completeness Turing complete]). So, if the pointer moves past zero to the left, the program will exit and a "Pointer out of range" error message will be displayed.

Due to the <tt>BufferedReader</tt> input class, return characters ([http://www.asciitable.com ASCII] 10 and 13) are ignored on input (the , command), but are not ignored on output (the . command). In order to input anything, you'll need to append a new line.

Loops are handled in the <tt>jumpForward</tt> and <tt>jumpBack</tt> methods. In each method, the bracket that the code pointer is on is counted as 1 (the counter is initialized to 1). The method then moves the code pointer forward or back and counts other brackets along the way. If it finds a bracket of the same type, 1 is added to the counter; if it finds a bracket of the other type, 1 is subtracted from the counter. The search continues until the counter reaches 0. For jumping forward, the code pointer must be allowed to skip the ending bracket, or else it will jump back on the next execution iteration. For jumping back, the pointer can remain at the starting bracket. So, <tt>jumpBack</tt> offsets the next code pointer increment to make sure the check for 0 happens.

More detailed information about the rest of the code can be found in the comments throughout it.<br clear=all>
{{works with|Java|1.5+}}
<syntaxhighlight lang="java5">import java.io.BufferedReader;
import java.io.FileNotFoundException;
import java.io.FileReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.ArrayList;

/**
 * Interprets Brainf**k source from std in or a file.
 * 
 * @author Mike Neurohr
 */
public class BF{
	private static int pointer;//memory pointer
	private static int codeIndex;//instruction number
	private static final int INITIAL_SIZE= 1000;//initial size of memory
	private static final String instChars= "[]<>,.+-";//valid code characters

	/**
	 * The main program. Holds the input loop and error-handling.
	 * 
	 * @param args 0: Source file    others: ignored
	 */
	public static void main(String[] args){
		BufferedReader source;
		try{
			if(args.length == 0){//if no file specified
				source= new BufferedReader(new InputStreamReader(System.in));
			}else{
				source= new BufferedReader(new FileReader(args[0]));
			}
			char instruction;

			//holds the cleaned up code
			StringBuilder code= new StringBuilder();

			//count for loop syntax errors
			int bCnt= 0;

			//read whole source in
			for(instruction= (char)source.read(); source.ready(); instruction=
					(char)source.read()){
				//remove extra characters
				if(instChars.contains(instruction + "")){
					//add the instruction to the clean code
					code.append(instruction);
					if(instruction == '[') bCnt++;
					if(instruction == ']') bCnt--;
				}
			}
			if(bCnt != 0){//if there is a bracket with no match
				System.err.println("Error: Uneven brackets.");
				return;
			}

			//set up input for ,
			BufferedReader input=
					new BufferedReader(new InputStreamReader(System.in));
			//set up memory
			ArrayList<Integer> memory= new ArrayList<Integer>(INITIAL_SIZE);
			memory.add(new Integer(0));
			
			pointer= 0; //initialize pointer
			//loop through the cleaned up code and execute
			for(codeIndex= 0;codeIndex < code.length();codeIndex++){
				instruction= code.charAt(codeIndex);
				switch(instruction){
				case '+':
					memory.set(pointer, memory.get(pointer) + 1);
					break;
				case '-':
					memory.set(pointer, memory.get(pointer) - 1);
					break;
				case '.':
					System.out.print((char)memory.get(pointer).intValue());
					break;
				case ',':
					input(memory, input);
					break;
				case '[':
					if(memory.get(pointer) == 0){
						//skip the code inside the loop
						jumpForward(code);
					}
					//if the loop can continue...
					//we don't need to check other things
					break;
				case ']':
					if(memory.get(pointer) != 0){
						//go back to the corresponding [ to check
						jumpBack(code);
					}
					//if the loop should stop...
					//we don't need to check other things
					break;
				case '>':
					pointer++;
					//gets rid of NullPointerExceptions
					while(pointer + 1 > memory.size()) memory.add(0);
					break;
				case '<':
					if(--pointer < 0){
						//can't do it
						System.err.println("Pointer out of range (negative).");
						return;
					}
				default:
					//No other characters left after the cleaning
				}
			}
		}catch(FileNotFoundException e){
			System.err.println("Error opening file: " + args[0] + ".");
		}catch(IOException e){
			System.err.println("Error on input.");
		}
	}

	public static void jumpBack(StringBuilder code){
		//initial count for the bracket we're on
		int bracketCnt= 1;
		//count brackets until the corresponding [
		while(codeIndex >= 0 && bracketCnt != 0){
			codeIndex--;
			char inst= code.charAt(codeIndex);
			if(inst == '[') bracketCnt--;
			if(inst == ']') bracketCnt++;
		}
		//"- 1" to offset the next "codeIndex++".
		codeIndex= codeIndex - 1;
	}

	public static void jumpForward(StringBuilder code){
		//initial count for the bracket we're on
		int bracketCnt= 1;
		//count brackets until the corresponding ]
		while(codeIndex < code.length() && bracketCnt != 0){
			codeIndex++;
			char inst= code.charAt(codeIndex);
			if(inst == ']') bracketCnt--;
			if(inst == '[') bracketCnt++;
		}
	}

	public static void input(ArrayList<Integer> mem, BufferedReader input)
			throws IOException{
		int val;
		//read until something comes in other than return chars
		for(val= input.read(); val == 10 || val == 13; val= input.read());
		mem.set(pointer, new Integer(val));
 	}
}</syntaxhighlight>