Machine langauge simulation

I am new to java.I am learning from a book called "Java how to program" from Dietel. First, Is it a good book?

Second, I want help about an exersice the exersice is to make a simulation to a mchine langauge hat supports the following instructions:

final int READ = 10; Read a word from the keyboard into a specific location in memory.
final int WRITE = 11; Write a word from a specific location in memory to the screen.
final int LOAD = 20; Load a word from a specific location in memory into the accumulator.
final int STORE = 21; Store a word from the accumulator into a specific location in memory.
final int ADD = 30; Add a word from a specific location in memory to the word in the accumulator (leave the result in the accumulator).
final int SUBTRACT = 31; Subtract a word from a specific location in memory from the word in the accumulator (leave the result in the accumulator).
final int DIVIDE = 32; Divide a word from a specific location in memory into the word in the accumulator (leave result in the accumulator).
final int MULTIPLY = 33; Multiply a word from a specific location in memory by the word in the accumulator (leave the result in the accumulator).
final int BRANCH = 40; Branch to a specific location in memory.
final int BRANCHNEG = 41; Branch to a specific location in memory if the accumulator is negative.
final int BRANCHZERO = 42; Branch to a specific location in memory if the accumulator is zero.
final int HALT = 43; Halt. The program has completed its task

and here is my code i want to know if there r any logical errors. and if there any optimization for the code

import java.util.Scanner;

public class SML
{
private static int acc, instcounter, opcode, operand, instregister;
private static int[] memory;
private static Scanner input = new Scanner ( System.in );

public static void load()
{
int index =0;

System.out.println("*** Welcome to Simpletron! ***\n" +
"*** Please enter your program one instruction ***\n" +
"*** (or data word) at a time into the input ***\n" +
"*** text field. I will display the location ***\n" +
"*** number and a question mark (?). You then ***\n" +
"*** type the word for that location. Press the ***\n" +
"*** Done button to stop entering your program. ***" );
while( true )
{
System.out.printf("%02d ", index);
memory[index] = input.nextInt();
if( memory[index] == -99999 )
break;
else
index++;

System.out.print("\n");
}
System.out.print("*** Program loading completed ***\n" +
"*** Program execution begins ***\n" );

}

public static void fetch()
{
instregister = memory[instcounter];
opcode = instregister / 100;
operand = instregister % 100;
}

public static void execute()
{
switch( opcode )
{
case 10: // Read from Keyboard
System.out.print("Input an integer: ");
memory[operand] = input.nextInt();
//System.out.printf("\n%d\n", memory[operand]);
instcounter ++;
break;
case 11: // print value of a memory location
System.out.printf("%d\n", memory[operand]);
instcounter ++;
break;
case 20: //Load accumulator
acc = memory[operand];
//System.out.printf("\n%d\n", acc);
instcounter ++;
break;
case 21: //Store accumulator
memory[operand] = acc;
//System.out.printf("\n%d\n", memory[operand]);
instcounter ++;
break;
case 30: //Add
acc += memory[operand];
//System.out.printf("\n%d\n", acc);
instcounter ++;
break;
case 31: //Sub
acc -= memory[operand];
instcounter ++;
break;
case 32: //Div
acc /= memory[operand];
instcounter ++;
break;
case 33: //Multiply
acc *= memory[operand];
instcounter ++;
break;
case 40: //unconditional branch
instcounter = operand;
break;
case 41: //Branch if negative
if( acc < 0)
instcounter = operand;
else
instcounter ++;
break;
case 42: //Branch if Zero
if( acc == 0)
instcounter = operand;
else
instcounter ++;
break;
case 43: //Halt
System.out.print("*** Simpletron execution terminated ***");
instcounter ++;
break;
}
}

public static void main(String[] args)
{
memory = new int[100];

load();

while ( memory[instcounter] != -99999)
{
fetch();
execute();
}

}

}