This post was originally published on attacomsian.com/blog.

Unlike PHP or JavaScript, Java is a strongly typed programming language. It essentially means that each variable must be declared with a pre-defined data type which can not be changed afterwards. There are two data types in Java:

• Primitive data types - int, double, float, byte, long, boolean etc.
• Reference data types - Integer, Double, Float, Date, String, Object etc.

In this tutorial, we will focus on type conversion for primitive data types.

String to int

There are two methods available for String to int conversion: Integer.parseInt() which returns a primitive int and Integer.valueOf() which return an Integer object.

String str = "1050";

int inum = Integer.parseInt(str);   //return primitive
System.out.println(inum);

Integer onum = Integer.valueOf(str); //return object
System.out.println(onum);

String to long

Similar to int, we can convert a String into a primitive long value using Long.parseLong() or an object Long via Long.valueOf() method.

String longStr = "1456755";

long ilong = Long.parseLong(longStr); //return primitive
System.out.println(ilong);

Long olong = Long.valueOf(longStr); //return object
System.out.println(olong);

String to float

A String can be converted to primitive float value using Float.parseFloat() method. Float.valueOf() method can be used to convert a String into a Float object.

String floatStr = "49.78";

float ifloat = Float.parseFloat(floatStr); //return primitive
System.out.println(ifloat);

Float ofloat = Float.valueOf(floatStr); //return object
System.out.println(ofloat);

String to double

double and float data types may look same but are different in the way that they store the value. float is a single precision (32 bit or 4 bytes) floating point data type whereas double is a double precision (64 bit or 8 bytes) floating point data type.

A String value can be converted to double value using Double.parseDouble() method. Similarly, Double.valueOf() converts a String into a Double object.

String doubleStr = "99.378";

double idouble = Double.parseDouble(doubleStr); //return primitive
System.out.println(idouble);

Double odouble = Double.valueOf(doubleStr); //return object
System.out.println(odouble);

NumberFormatException

If the String does not contain a parsable value during int, float, or double conversion, a NumberFormatException is thrown.

try {
    String exeStr = "14c";
    int exeInt = Integer.parseInt(exeStr);
    System.out.println(exeInt);
} catch (NumberFormatException ex) {
    System.out.println(ex.getMessage());
}

String to boolean

A String value can be converted to primitive boolean value using Boolean.parseBoolean method. For conversion to Boolean object, you can use Boolean.valueOf() method.

String trueStr = "true";
String falseStr = "false";
String randomStr = "java";

System.out.println(Boolean.parseBoolean(trueStr)); //true
System.out.println(Boolean.valueOf(falseStr)); //false
System.out.println(Boolean.parseBoolean(randomStr)); //false

String to Date

Java provides SimpleDateFormat class for formatting and parsing dates. It has the following two important method:

• parse() - It converts a String value into a Date object
• format() - It converts the Date object into a String value

While creating an instance of the SimpleDateFormat classes, you need to pass date and time pattern that tells how the instance should parse or format the dates.

String dateStr = "10/03/2019";

SimpleDateFormat format = new SimpleDateFormat("dd/MM/yyyy");
Date dateObj = format.parse(dateStr);
System.out.println(dateObj);

In the example above, I used dd/MM/yyyy pattern to parse 10/03/2019 string. dd means two digits for the day, MM means two digit for the month and yyyy means 4 digits for the year. Below is a list of the most common date and time patterns used in SimpleDateFormat. For the complete list, please refer to official JavaDoc.

Following are some pattern examples, with examples of how each pattern would parse a date or vice versa:

yyyy/MM/dd  <--> (2019/03/09)

dd-MM-YYYY  <-->  (10-03-2019)

dd-MMM-yy  <-->  (13-Feb-19)

EEE, MMMM dd, yyy  <--> (Fri, March 09, 2019)

yyyy-MM-dd HH:mm:ss <--> (2019-02-28 16:45:23)

hh:mm:ss a <--> (11:23:36 PM)

yyyy-MM-dd HH:mm:ss.SSS Z <--> (2019-01-31 21:05:46.555 +0500)

Date to String

As we discussed above, SimpleDateFormat also supports formatting of dates into strings. Here is an example that formats the date into a string:

Date date = Calendar.getInstance().getTime(); // OR new Date()

SimpleDateFormat dateFormat = new SimpleDateFormat("EEEE, MMMM dd, yyyy HH:mm:ss.SSS Z");

String formatStr = dateFormat.format(date);
System.out.println(formatStr);

The above code snippet will print the following depending on your location:

Sunday, March 10, 2019 20:01:22.417 +0500

Date to ISO 8601 String

ISO 8601 is an international standard that covers the exchange of date- and time-related data. There ere several ways to express date and time in ISO format:

2019-03-30T14:22:15+05:00
2019-03-30T09:22:15Z
20190330T092215Z

Here is an example to convert a date object into an ISO 8601 equivalent string in Java:

TimeZone timeZone = TimeZone.getTimeZone("UTC");
SimpleDateFormat isoFormat = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss'Z'");
isoFormat.setTimeZone(timeZone);
String isoFormatStr = isoFormat.format(new Date());
System.out.println(isoFormatStr);

Following are the date and time patterns for ISO format:

Source code: Download the complete source code from GitHub available under MIT license.

Conclusion

Data type conversions are very common for a developer. Most of these conversions are trivial and are well-known to an experienced programmer. However, string to date conversion is a bit tricky especially for beginners. You may encounter errors if the pattern is not specified correctly. But if you spend some time to remember these patterns, it may save a lot of time while figuring out why a certain conversion is not compiling or executing.

Am I missing any important type conversion in this tutorial? Send me a tweet any time to let me know.

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