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Java Converting to and from Strings

From WikiOD

Converting String to other datatypes.[edit | edit source]

You can convert a numeric string to various Java numeric types as follows:

String to int:

String number = "12";
int num = Integer.parseInt(number);

String to float:

String number = "12.0";
float num = Float.parseFloat(number);

String to double:

String double = "1.47";
double num = Double.parseDouble(double);

String to boolean:

String falseString = "False";
boolean falseBool = Boolean.parseBoolean(falseString);   // falseBool = false 

String trueString = "True";
boolean trueBool = Boolean.parseBoolean(trueString);     // trueBool = true

String to long:

String number = "47";
long num = Long.parseLong(number);

String to BigInteger:

String bigNumber = "21";
BigInteger reallyBig = new BigInteger(bigNumber);

String to BigDecimal:

String bigFraction = "17.21455";
BigDecimal reallyBig = new BigDecimal(bigFraction);

Conversion Exceptions:

The numeric conversions above will all throw an (unchecked) NumberFormatException if you attempt to parse a string that is not a suitably formatted number, or is out of range for the target type. The Exceptions topic discusses how to deal with such exceptions.

If you wanted to test that you can parse a string, you could implement a tryParse... method like this:

boolean tryParseInt (String value) {  
    try {  
        String somechar = Integer.parseInt(value);
        return true;  
     } catch (NumberFormatException e) { 
        return false;  

However, calling this tryParse... method immediately before parsing is (arguably) poor practice. It would be better to just call the parse... method and deal with the exception.

Conversion to / from bytes[edit | edit source]

To encode a string into a byte array, you can simply use the String#getBytes() method, with one of the standard character sets available on any Java runtime:

byte[] bytes = "test".getBytes(StandardCharsets.UTF_8);

and to decode:

String testString = new String(bytes, StandardCharsets.UTF_8);

you can further simplify the call by using a static import:

import static java.nio.charset.StandardCharsets.UTF_8;
byte[] bytes = "test".getBytes(UTF_8);

For less common character sets you can indicate the character set with a string:

byte[] bytes = "test".getBytes("UTF-8");

and the reverse:

String testString = new String (bytes, "UTF-8");

this does however mean that you have to handle the checked UnsupportedCharsetException.

The following call will use the default character set. The default character set is platform specific and generally differs between Windows, Mac and Linux platforms.

byte[] bytes = "test".getBytes();

and the reverse:

String testString = new String(bytes);

Note that invalid characters and bytes may be replaced or skipped by these methods. For more control - for instance for validating input - you're encouraged to use the CharsetEncoder and CharsetDecoder classes.

Base64 Encoding / Decoding[edit | edit source]

Occasionally you will find the need to encode binary data as a base64-encoded string.

For this we can use the DatatypeConverter class from the javax.xml.bind package:

import javax.xml.bind.DatatypeConverter;
import java.util.Arrays;

// arbitrary binary data specified as a byte array
byte[] binaryData = "some arbitrary data".getBytes("UTF-8");

// convert the binary data to the base64-encoded string
String encodedData = DatatypeConverter.printBase64Binary(binaryData);
// encodedData is now "c29tZSBhcmJpdHJhcnkgZGF0YQ=="

// convert the base64-encoded string back to a byte array
byte[] decodedData = DatatypeConverter.parseBase64Binary(encodedData);

// assert that the original data and the decoded data are equal
assert Arrays.equals(binaryData, decodedData);

Apache commons-codec

Alternatively, we can use Base64 from Apache commons-codec.

import org.apache.commons.codec.binary.Base64;

// your blob of binary as a byte array
byte[] blob = "someBinaryData".getBytes();

// use the Base64 class to encode
String binaryAsAString = Base64.encodeBase64String(blob);

// use the Base64 class to decode
byte[] blob2 = Base64.decodeBase64(binaryAsAString);

// assert that the two blobs are equal
System.out.println("Equal : " + Boolean.toString(Arrays.equals(blob, blob2)));

If you inspect this program wile running, you will see that someBinaryData encodes to c29tZUJpbmFyeURhdGE=, a very managable UTF-8 String object.

Java SE 8

Details for the same can be found at Base64

// encode with padding
String encoded = Base64.getEncoder().encodeToString(someByteArray);

// encode without padding
String encoded = Base64.getEncoder().withoutPadding().encodeToString(someByteArray);

// decode a String
byte [] barr = Base64.getDecoder().decode(encoded);


Converting other datatypes to String[edit | edit source]

You can get the value of other primitive data types as a String using one the String class's valueOf methods.

For example:

int i = 42;
String string = String.valueOf(i);
//string now equals "42”.

This method is also overloaded for other datatypes, such as float, double, boolean, and even Object.

You can also get any other Object (any instance of any class) as a String by calling .toString on it. For this to give useful output, the class must override toString(). Most of the standard Java library classes do, such as Date and others.

For example:

Foo foo = new Foo(); //Any class.
String stringifiedFoo = foo.toString().

Here stringifiedFoo contains a representation of foo as a String.

You can also convert any number type to String with short notation like below.

int i = 10;
String str = i + "";

Or just simple way is

String str = 10 + "";

Getting a `String` from an `InputStream`[edit | edit source]

A String can be read from an InputStream using the byte array constructor.


public String readString(InputStream input) throws IOException {
    byte[] bytes = new byte[50]; // supply the length of the string in bytes here;
    return new String(bytes);

This uses the system default charset, although an alternate charset may be specified:

return new String(bytes, Charset.forName("UTF-8"));

Parsing Strings to a Numerical Value[edit | edit source]

String to a primitive numeric type or a numeric wrapper type:

Each numeric wrapper class provides a parseXxx method that converts a String to the corresponding primitive type. The following code converts a String to an int using the Integer.parseInt method:

String string = "59";
int primitive = Integer.parseInteger(string);

To convert to a String to an instance of a numeric wrapper class you can either use an overload of the wrapper classes valueOf method:

String string = "59";
Integer wrapper = Integer.valueOf(string);

or rely on auto boxing (Java 5 and later):

String string = "59";
Integer wrapper = Integer.parseInteger(string);  // 'int' result is autoboxed

The above pattern works for byte, short, int, long, float and double and the corresponding wrapper classes (Byte, Short, Integer, Long, Float and Double).

String to Integer using radix:

String integerAsString = "0101"; // binary representation
int parseInt = Integer.parseInt(integerAsString,2);
Integer valueOfInteger = Integer.valueOf(integerAsString,2);
System.out.println(valueOfInteger); // prints 5 
System.out.println(parseInt); // prints 5


The unchecked NumberFormatException exception will be thrown if a numeric valueOf(String) or parseXxx(...) method is called for a string that is not an acceptable numeric representation, or that represents a value that is out of range.