How to Convert String to Integer in Java

How to convert String to int in Java

You need to convert String into int if you want to perform a mathematical operation on string which contains digits. To do so, you need to perform the conversion from String to int.

You can convert String to int in two ways:

• Using Integer.parseInt() method
• Using Integer.valueOf() method

Using Integer.parseInt() method

The paresInt() is the static method of Integer wrapper class under java.lang package.

Example-

In the following example, number is a variable of type String that contains digits “10” as string. num1 is a variable of type integer that contains value 20. num2 is the variable of type integer which stores the converted value of str. Integer is a class. paresInt() is the static method that takes str as an argument and returns a primitive int value. sum is a variable of type integer which stores the value after addition. The println statement prints the value of sum.

public class StringTointExample
{
public static void main(String args[])
{
String str="10";
int num1 = 20;
int num2 = Integer.parseInt(str);
int sum = num1+num2;                   //num2 adds the converted value of str into num1
System.out.println("Befor conversion: "+str);
System.out.println("After conversion Sum is:  "+sum);
}
}

Output-

Before conversion:  10
After conversion Sum is:  30

Remember: All characters must be in digits; the number can have a minus sign.

Valid Conversion

• String number=”-369”;

Invalid Conversion

• String number=”369a”;
• String number=”abcd”;

If you define a String as above, Integer.parseInt() throws “NumberFormatException”, because it is not a valid conversion. 

Exception

Exception in thread "main" java.lang.NumberFormatException: For input string: "369a"
at java.lang.NumberFormatException.forInputString(NumberFormatException.java:65)
at java.lang.Integer.parseInt(Integer.java:580)
at java.lang.Integer.parseInt(Integer.java:615)
at StringToIntExample.main(StringToIntExample.java:6)

Using Integer.valueOf() method

Integer.valueOf() works same as Integer.parseInt(String). It also converts a String to int value. It also throws NumberFormatException.

Example-

In the following example, str is a variable of type String that contains digits “-10” as string. num1 is a variable of type integer that contains value 110. num2 is a variable of type integer that stores the converted value of str. Integer is a wrapper class. valueOf() is the method that returns an integer instance representing the specified int value.  val is a variable that stores the sum of num1 and num2. First println statement prints the String str and second println prints the value of val after performing addition.

public class StringTointExample1
{
public static void main(String args[])
{
String str="-10";
int num1 = 110;
int num2 = Integer.valueOf(str);
int val = num1+num2;           //num2 adds the converted value of str into num1
System.out.println("Before conversion: "+str);
System.out.println("After addition value is: "+val);
}
}

Output-

Before conversion:  -10
After addition value is:  100

Difference Between Integer.valueOf() and Integer.parseInt()

There is a difference between Integer.valueOf() and Integer.parseInt(). The Integer.valueOf() method returns an object of Integer class whereas the Integer.parseInt() method returns a primitive int value.

If you need the primitive int datatype, then use parseInt() method. If you want wrapper Integer object, then use the valueOf() method.