Java integer overflow

Question

I'm totally new to Java and I'm implementing a simple function to convert string to integer on Leetcode.

public int myAtoi(String str) {
 if(str.length() == 0){
 return 0;
 }
 str = str.trim();
 int n = str.length();
 int signal = 0;
 if(n == 1 && str.equals("+") || str.equals("-")){
 return 0;
 }
 if(str.charAt(0) == '+'){
 signal = 1;
 }else if(str.charAt(0) == '-'){
 signal = -1;
 }
 int i = (signal != 0)? 1 : 0;
 if(signal == 0){
 signal = 1;//default
 }
 int res = 0;
 while(i < n){
 char c = str.charAt(i);
 if(!Character.isDigit(c)){
 return res * signal;
 }
 //res = res * 10 + c - '0';
 if(signal * res > Integer.MAX_VALUE){
 return Integer.MAX_VALUE;
 }
 if(signal * res < Integer.MIN_VALUE){
 return Integer.MIN_VALUE;
 }
 res = res * 10 + c - '0';
 ++i;
 }
 return res * signal;
}

I know java integer has the MAX_VALUE of 2147483647. When my input is 2147483648 the output should be 2147483647 but indeed it's -214748648. I really have no idea what's wrong in here. Can anybody help me to understand this?

• Alejandro Alcalde: This happens when you overflow the integer capacity: en.wikipedia.org/wiki/Integer_overflow
• zanussi: Possible duplicate of why Integer.MAX_VALUE + 1 == Integer.MIN_VALUE?

Answer 1

Consider this example

public static void main(String args[]) {
 int i=2147483647;
 System.out.println("i="+i);
 int j=++i;
 System.out.println("Now i is="+i);
 System.out.println("j="+j);
}

What happens? output will be :

i = 2147483647
Now i is=-2147483648
j=-2147483648

The maximum value of integer is 2,147,483,647 and the minimum value is -2,147,483,648. Here in j (with post increment of i), we have crossed the maximum limit of an integer

This is exactly what is happening in your case too .

Because the integer overflows. When it overflows, the next value is Integer.MIN_VALUE

Why?

Integer values are represented in binary form, and there is binary addition in java. It uses a representation called two's complement, in which the first bit of the number represents its sign. Whenever you add 1 to the largest Integer(MAX INT), which has a bit sign of 0, then its bit sign becomes 1 and the number becomes negative.

So, don't put> MAX INT as input, else put a condition in your code to check it on input itself.

Answer 2

The input is never [phone] since that value can't be represented as a Java int.

It will wrap around to the negative number you observe, so accounting for that result.

• Jason Cheng: Yeah I know it's right. But actually the input is a string and I want to convert it to the int.