Trying to build a simple calculator

Have this code;

public double result_of(String equation)

{

if (equation.contains("+")){

String[] split_up_equation = String.split([+]);

double result = 0.0;

for (int i=0; i<split_up_equation.length;i++)

{

result += result_of(split_up_equation[i]);

}

return result;

}

}

can anyone tell me why java doesnt like line 4, String.split([+]);

thanks