URLDecoder class in Java

java.net.URLDecoder

Utility class for HTML form decoding. This class contains static methods for decoding a String from the application/x-www-form-urlencodedMIME format.

The conversion process is the reverse of that used by the URLEncoder class. It is assumed that all characters in the encoded string are one of the following: "a" through "z", "A" through "Z", "0" through "9", and"-", "_",".", and "*". The character "%" is allowed but is interpreted as the start of a special escaped sequence.

The following rules are applied in the conversion:

1. The alphanumeric characters "a" through"z", "A" through"Z" and "0" through "9" remain the same.

2. The special characters ".","-", "*", and"_" remain the same.

3. The plus sign "+" is converted into a space character "   " .

4. A sequence of the form "%xy" will be treated as representing a byte where xy is the two-digit hexadecimal representation of the 8 bits. Then, all substrings that contain one or more of these byte sequences consecutively will be replaced by the character(s) whose encoding would result in those consecutive bytes. The encoding scheme used to decode these characters may be specified, or if unspecified, the default encoding of the platform will be used.

There are two possible ways in which this decoder could deal with illegal strings. It could either leave illegal characters alone or it could throw a java.lang.IllegalArgumentException.

Which approach the decoder takes is left to the implementation.

Approach

Java

package com.URLDecoder;

import java.net.URLDecoder;

public class URLDecoder1 {
    public static void main(String[] args) {

        URLDecoder urlDecoder = new URLDecoder();

        System.out.println(urlDecoder);
    }
}

Output:

java.net.URLDecoder@53bd815b

Methods

1. URLDecoder.decode(String s, Charset charset)

String java.net.URLDecoder.decode(String s, Charset charset)

This method takes two arguments. This method decodes an application/x-www-form-urlencoded string using a specific Charset. The supplied charset is used to determine what characters are represented by any consecutive sequences of the form "%xy".

Note: The World Wide Web Consortium Recommendation states that UTF-8 should be used. Not doing so may introduce incompatibilities.

Parameters: Two parameters are required for this method.

s: the String to decode.

charset: the given charset.

Returns: the newly decoded String.

Throws:

1. NullPointerException - if s or charset is null.

2. IllegalArgumentException - if the implementation encounters illegal characters.

Approach 1: When no exception

Java

package com.URLDecoder;

import java.net.URLDecoder;
import java.nio.charset.Charset;

public class URLDecoderdecode {
    public static void main(String[] args) {

        String s = "hello";

        System.out.println(URLDecoder.decode(s, Charset.defaultCharset()));
    }
}

Output:

hello

Approach 2: NullPointerException

Java

package com.URLDecoder;

import java.net.URLDecoder;
import java.nio.charset.Charset;

public class URLDecoderdecode {
    public static void main(String[] args) {

        String s = null;

        System.out.println(URLDecoder.decode(s, Charset.defaultCharset()));
    }
}

Output:

Exception in thread "main" java.lang.NullPointerException: Cannot invoke "String.length()" because "s" is null
	at java.base/java.net.URLDecoder.decode(URLDecoder.java:176)
	at com.URLDecoder.URLDecoderdecode.main(URLDecoderdecode.java:11)

Approach 3: IllegalArgumentException 

Java

package com.URLDecoder;

import java.net.URLDecoder;
import java.nio.charset.Charset;

public class URLDecoderdecode {
    public static void main(String[] args) {

        String s = "%%%%%";

        System.out.println(URLDecoder.decode(s, Charset.defaultCharset()));
    }
}

Output:

Exception in thread "main" java.lang.IllegalArgumentException: URLDecoder: Illegal hex characters in escape (%) pattern - Error at index 0 in: "%%"
	at java.base/java.net.URLDecoder.decode(URLDecoder.java:232)
	at com.URLDecoder.URLDecoderdecode.main(URLDecoderdecode.java:11)

2. URLDecoder.decode(String s, String enc)

String java.net.URLDecoder.decode(String s, String enc) throws UnsupportedEncodingException

This method takes two arguments. This method decodes an application/x-www-form-urlencoded string using a specific encoding scheme.

Parameters: Two parameters are required for this method.

s: the String to decode.

enc: The name of a supported character encoding.

Returns: the newly decoded String.

Throws:

1. UnsupportedEncodingException - If character encoding needs to be consulted, but named character encoding is not supported.

Approach 1: When no exception

Java

package com.URLDecoder;

import java.io.UnsupportedEncodingException;
import java.net.URLDecoder;

public class URLDecoderdecode2 {
    public static void main(String[] args) throws UnsupportedEncodingException {

        String s = "hello", enc = "UTF-8";
        System.out.println(URLDecoder.decode(s, enc));
    }
}

Output:

hello

Approach 2: UnsupportedEncodingException 

Java

package com.URLDecoder;

import java.io.UnsupportedEncodingException;
import java.net.URLDecoder;

public class URLDecoderdecode2 {
    public static void main(String[] args) throws UnsupportedEncodingException {

        String s = "hello", enc = "UTF919199";
        System.out.println(URLDecoder.decode(s, enc));
    }
}

Output:

Exception in thread "main" java.io.UnsupportedEncodingException: UTF919199
	at java.base/java.net.URLDecoder.decode(URLDecoder.java:144)
	at com.URLDecoder.URLDecoderdecode2.main(URLDecoderdecode2.java:10)