How to convert a String to int in Java

The only requirement to convert a string to an integer in java is that it must contain only digits that you want to put in a variable of type int, except for the first character that indicates a negative "-" or positive "+" value. For example, the result of "-123" is the number -123.
Many will cast, but java doesn't allow it.

cannot Cast from string to int

The exception Cannot cast from String to int means that the Java compiler does not allow a String to cast to int. This makes sense since the two are not of the same type.

The Integer

The Integer object contains a single attribute of type int. This class contains several methods to convert an int to a String and a String to an int.

1) By instantiating a new Integer(String s)

This constructor allocates a new object containing an integer:

public class Cast {

 public static void main(String[] args) {
 String nbs = "12"; 
 int nb; 
 nb = new Integer(nbs); 
 System.out.println(nb); 
 }
}
Output:

12

2) With the Integer.valueOf(String s)

The valueOf() returns a new instance of java.lang.Integer which contains the value represented in the argument String.

public class Cast {

 public static void main(String[] args) {
 String nbs = "12"; 
 int nb; 
 nb = new Integer(nbs); 
 System.out.println(nb); 
 }
}
Output:

12

3) With the Integer.parseInt(String s)

The parseInt() returns an integer value, just as if the argument was passed in the valueOf(String s).

public class Cast {

 public static void main(String[] args) {
 String nbs = "12"; 
 int nb; 
 nb = Integer.parseInt(nbs); 
 System.out.println(nb); 
 }
}
Output:

12

Exception NumberFormatException thrown

NumberFormatException is called if the argument passed is not in the correct format. The causes are:
  • The passed argument contains at least one non-numeric character.
  • The number is not decimal like the float 12.4f.

Improving our procedure

To avoid the program terminating, we will return a default value if the format is not respected.

public class Cast {

 public static void main(String[] args) {
 String nbs = "12.4s"; 
 int nb; 
 System.out.println(stringToInt(nbs,0)); 
 }
 
 public static int stringToInt(String value, int _default) {
 try {
 return Integer.parseInt(value); 
 } catch (NumberFormatException e) {
 return _default; 
 }
 }
}
Output: 

0
In the case where the variable nbs is not numeric, the function stringToInt() returns 0 by default.

References:
Oracle.com -  The Integer
JRJC-Convert string to int