java.util

## Class Properties

- java.lang.Object
- - java.util.Dictionary<K,V>
  - - java.util.Hashtable<Object,Object>
    - - java.util.Properties

- All Implemented Interfaces:
    Serializable, Cloneable, Map<Object,Object>

      public class Properties
      extends Hashtable<Object,Object>

  The `Properties` class represents a persistent set of properties. The `Properties` can be saved to a stream or loaded from a stream. Each key and its corresponding value in the property list is a string.

  A property list can contain another property list as its "defaults"; this second property list is searched if the property key is not found in the original property list.

  Because `Properties` inherits from `Hashtable`, the `put` and `putAll` methods can be applied to a `Properties` object. Their use is strongly discouraged as they allow the caller to insert entries whose keys or values are not `Strings`. The `setProperty` method should be used instead. If the `store` or `save` method is called on a "compromised" `Properties` object that contains a non-`String` key or value, the call will fail. Similarly, the call to the `propertyNames` or `list` method will fail if it is called on a "compromised" `Properties` object that contains a non-`String` key.

  The `load(Reader)` `/` `store(Writer, String)` methods load and store properties from and to a character based stream in a simple line-oriented format specified below. The `load(InputStream)` `/` `store(OutputStream, String)` methods work the same way as the load(Reader)/store(Writer, String) pair, except the input/output stream is encoded in ISO 8859-1 character encoding. Characters that cannot be directly represented in this encoding can be written using Unicode escapes as defined in section 3.3 of The Java™ Language Specification; only a single 'u' character is allowed in an escape sequence. The native2ascii tool can be used to convert property files to and from other character encodings.

  The `loadFromXML(InputStream)` and `storeToXML(OutputStream, String, String)` methods load and store properties in a simple XML format. By default the UTF-8 character encoding is used, however a specific encoding may be specified if required. An XML properties document has the following DOCTYPE declaration:

       <!DOCTYPE properties SYSTEM "http://java.sun.com/dtd/properties.dtd">

  Note that the system URI (http://java.sun.com/dtd/properties.dtd) is not accessed when exporting or importing properties; it merely serves as a string to uniquely identify the DTD, which is:

          <?xml version="1.0" encoding="UTF-8"?>

          <!-- DTD for properties -->

          <!ELEMENT properties ( comment?, entry* ) >

          <!ATTLIST properties version CDATA #FIXED "1.0">

          <!ELEMENT comment (#PCDATA) >

          <!ELEMENT entry (#PCDATA) >

          <!ATTLIST entry key CDATA #REQUIRED>

  This class is thread-safe: multiple threads can share a single `Properties` object without the need for external synchronization.

  Since:
    JDK1.0
  Author:
    Arthur van Hoff, Michael McCloskey, Xueming Shen
  See Also:
    native2ascii tool for Solaris, native2ascii tool for Windows, Serialized Form

- - ### Field Summary

    Fields
    Modifier and Type | Field and Description
    --- | ---
    `protected Properties` | `defaults` A property list that contains default values for any keys not found in this property list.

  - ### Constructor Summary

    Constructors
    Constructor and Description
    `Properties()` Creates an empty property list with no default values.
    `Properties(Properties defaults)` Creates an empty property list with the specified defaults.

  - ### Method Summary

    All Methods Instance Methods Concrete Methods Deprecated Methods
    Modifier and Type | Method and Description
    --- | ---
    `String` | `getProperty(String key)` Searches for the property with the specified key in this property list.
    `String` | `getProperty(String key, String defaultValue)` Searches for the property with the specified key in this property list.
    `void` | `list(PrintStream out)` Prints this property list out to the specified output stream.
    `void` | `list(PrintWriter out)` Prints this property list out to the specified output stream.
    `void` | `load(InputStream inStream)` Reads a property list (key and element pairs) from the input byte stream.
    `void` | `load(Reader reader)` Reads a property list (key and element pairs) from the input character stream in a simple line-oriented format.
    `void` | `loadFromXML(InputStream in)` Loads all of the properties represented by the XML document on the specified input stream into this properties table.
    `Enumeration<?>` | `propertyNames()` Returns an enumeration of all the keys in this property list, including distinct keys in the default property list if a key of the same name has not already been found from the main properties list.
    `void` | `save(OutputStream out, String comments)` Deprecated. This method does not throw an IOException if an I/O error occurs while saving the property list. The preferred way to save a properties list is via the `store(OutputStream out, String comments)` method or the `storeToXML(OutputStream os, String comment)` method.
    `Object` | `setProperty(String key, String value)` Calls the `Hashtable` method `put`.
    `void` | `store(OutputStream out, String comments)` Writes this property list (key and element pairs) in this `Properties` table to the output stream in a format suitable for loading into a `Properties` table using the `load(InputStream)` method.
    `void` | `store(Writer writer, String comments)` Writes this property list (key and element pairs) in this `Properties` table to the output character stream in a format suitable for using the `load(Reader)` method.
    `void` | `storeToXML(OutputStream os, String comment)` Emits an XML document representing all of the properties contained in this table.
    `void` | `storeToXML(OutputStream os, String comment, String encoding)` Emits an XML document representing all of the properties contained in this table, using the specified encoding.
    `Set<String>` | `stringPropertyNames()` Returns a set of keys in this property list where the key and its corresponding value are strings, including distinct keys in the default property list if a key of the same name has not already been found from the main properties list.

    - ### Methods inherited from class java.util.Hashtable

      `clear, clone, contains, containsKey, containsValue, elements, entrySet, equals, get, hashCode, isEmpty, keys, keySet, put, putAll, rehash, remove, size, toString, values`

    - ### Methods inherited from class java.lang.Object

      `finalize, getClass, notify, notifyAll, wait, wait, wait`

    - ### Methods inherited from interface java.util.Map

      `compute, computeIfAbsent, computeIfPresent, forEach, getOrDefault, merge, putIfAbsent, remove, replace, replace, replaceAll`

- - ### Field Detail

    - #### defaults

          protected Properties defaults

      A property list that contains default values for any keys not found in this property list.

  - ### Constructor Detail

    - #### Properties

          public Properties()

      Creates an empty property list with no default values.

    - #### Properties

          public Properties(Properties defaults)

      Creates an empty property list with the specified defaults.
      Parameters:
        `defaults` - the defaults.

  - ### Method Detail

    - #### setProperty

          public Object setProperty(String key,
                                    String value)

      Calls the `Hashtable` method `put`. Provided for parallelism with the `getProperty` method. Enforces use of strings for property keys and values. The value returned is the result of the `Hashtable` call to `put`.
      Parameters:
        `key` - the key to be placed into this property list.
        `value` - the value corresponding to `key`.
      Returns:
        the previous value of the specified key in this property list, or `null` if it did not have one.
      Since:
        1.2
      See Also:
        `getProperty(java.lang.String)`

    - #### load

          public void load(Reader reader)
                    throws IOException

      Reads a property list (key and element pairs) from the input character stream in a simple line-oriented format.

      Properties are processed in terms of lines. There are two kinds of line, natural lines and logical lines. A natural line is defined as a line of characters that is terminated either by a set of line terminator characters (`\n` or `\r` or `\r\n`) or by the end of the stream. A natural line may be either a blank line, a comment line, or hold all or some of a key-element pair. A logical line holds all the data of a key-element pair, which may be spread out across several adjacent natural lines by escaping the line terminator sequence with a backslash character `\`. Note that a comment line cannot be extended in this manner; every natural line that is a comment must have its own comment indicator, as described below. Lines are read from input until the end of the stream is reached.

      A natural line that contains only white space characters is considered blank and is ignored. A comment line has an ASCII `'#'` or `'!'` as its first non-white space character; comment lines are also ignored and do not encode key-element information. In addition to line terminators, this format considers the characters space (`' '`, `'\u0020'`), tab (`'\t'`, `'\u0009'`), and form feed (`'\f'`, `'\u000C'`) to be white space.

      If a logical line is spread across several natural lines, the backslash escaping the line terminator sequence, the line terminator sequence, and any white space at the start of the following line have no affect on the key or element values. The remainder of the discussion of key and element parsing (when loading) will assume all the characters constituting the key and element appear on a single natural line after line continuation characters have been removed. Note that it is not sufficient to only examine the character preceding a line terminator sequence to decide if the line terminator is escaped; there must be an odd number of contiguous backslashes for the line terminator to be escaped. Since the input is processed from left to right, a non-zero even number of 2n contiguous backslashes before a line terminator (or elsewhere) encodes n backslashes after escape processing.

      The key contains all of the characters in the line starting with the first non-white space character and up to, but not including, the first unescaped `'='`, `':'`, or white space character other than a line terminator. All of these key termination characters may be included in the key by escaping them with a preceding backslash character; for example,

      `\:\=`

      would be the two-character key `":="`. Line terminator characters can be included using `\r` and `\n` escape sequences. Any white space after the key is skipped; if the first non-white space character after the key is `'='` or `':'`, then it is ignored and any white space characters after it are also skipped. All remaining characters on the line become part of the associated element string; if there are no remaining characters, the element is the empty string `""`. Once the raw character sequences constituting the key and element are identified, escape processing is performed as described above.

      As an example, each of the following three lines specifies the key `"Truth"` and the associated element value `"Beauty"`:

           Truth = Beauty
            Truth:Beauty
           Truth                    :Beauty

      As another example, the following three lines specify a single property:

           fruits                           apple, banana, pear, \
                                            cantaloupe, watermelon, \
                                            kiwi, mango

      The key is `"fruits"` and the associated element is:

          "apple, banana, pear, cantaloupe, watermelon, kiwi, mango"

      Note that a space appears before each `\` so that a space will appear after each comma in the final result; the `\`, line terminator, and leading white space on the continuation line are merely discarded and are not replaced by one or more other characters.

      As a third example, the line:

          cheeses

      specifies that the key is `"cheeses"` and the associated element is the empty string `""`.

      Characters in keys and elements can be represented in escape sequences similar to those used for character and string literals (see sections 3.3 and 3.10.6 of The Java™ Language Specification). The differences from the character escape sequences and Unicode escapes used for characters and strings are:

      - Octal escapes are not recognized.
      - The character sequence `\b` does not represent a backspace character.
      - The method does not treat a backslash character, `\`, before a non-valid escape character as an error; the backslash is silently dropped. For example, in a Java string the sequence `"\z"` would cause a compile time error. In contrast, this method silently drops the backslash. Therefore, this method treats the two character sequence `"\b"` as equivalent to the single character `'b'`.
      - Escapes are not necessary for single and double quotes; however, by the rule above, single and double quote characters preceded by a backslash still yield single and double quote characters, respectively.
      - Only a single 'u' character is allowed in a Uniocde escape sequence.

      The specified stream remains open after this method returns.

      Parameters:
        `reader` - the input character stream.
      Throws:
        `IOException` - if an error occurred when reading from the input stream.
        `IllegalArgumentException` - if a malformed Unicode escape appears in the input.
      Since:
        1.6

    - #### load

          public void load(InputStream inStream)
                    throws IOException

      Reads a property list (key and element pairs) from the input byte stream. The input stream is in a simple line-oriented format as specified in `load(Reader)` and is assumed to use the ISO 8859-1 character encoding; that is each byte is one Latin1 character. Characters not in Latin1, and certain special characters, are represented in keys and elements using Unicode escapes as defined in section 3.3 of The Java™ Language Specification.

      The specified stream remains open after this method returns.

      Parameters:
        `inStream` - the input stream.
      Throws:
        `IOException` - if an error occurred when reading from the input stream.
        `IllegalArgumentException` - if the input stream contains a malformed Unicode escape sequence.
      Since:
        1.2

    - #### save

          @Deprecated
          public void save(OutputStream out,
                                       String comments)

      Deprecated. This method does not throw an IOException if an I/O error occurs while saving the property list. The preferred way to save a properties list is via the `store(OutputStream out, String comments)` method or the `storeToXML(OutputStream os, String comment)` method.
      Calls the `store(OutputStream out, String comments)` method and suppresses IOExceptions that were thrown.
      Parameters:
        `out` - an output stream.
        `comments` - a description of the property list.
      Throws:
        `ClassCastException` - if this `Properties` object contains any keys or values that are not `Strings`.

    - #### store

          public void store(Writer writer,
                            String comments)
                     throws IOException

      Writes this property list (key and element pairs) in this `Properties` table to the output character stream in a format suitable for using the `load(Reader)` method.

      Properties from the defaults table of this `Properties` table (if any) are not written out by this method.

      If the comments argument is not null, then an ASCII `#` character, the comments string, and a line separator are first written to the output stream. Thus, the `comments` can serve as an identifying comment. Any one of a line feed ('\n'), a carriage return ('\r'), or a carriage return followed immediately by a line feed in comments is replaced by a line separator generated by the `Writer` and if the next character in comments is not character `#` or character `!` then an ASCII `#` is written out after that line separator.

      Next, a comment line is always written, consisting of an ASCII `#` character, the current date and time (as if produced by the `toString` method of `Date` for the current time), and a line separator as generated by the `Writer`.

      Then every entry in this `Properties` table is written out, one per line. For each entry the key string is written, then an ASCII `=`, then the associated element string. For the key, all space characters are written with a preceding `\` character. For the element, leading space characters, but not embedded or trailing space characters, are written with a preceding `\` character. The key and element characters `#`, `!`, `=`, and `:` are written with a preceding backslash to ensure that they are properly loaded.

      After the entries have been written, the output stream is flushed. The output stream remains open after this method returns.

      Parameters:
        `writer` - an output character stream writer.
        `comments` - a description of the property list.
      Throws:
        `IOException` - if writing this property list to the specified output stream throws an `IOException`.
        `ClassCastException` - if this `Properties` object contains any keys or values that are not `Strings`.
        `NullPointerException` - if `writer` is null.
      Since:
        1.6

    - #### store

          public void store(OutputStream out,
                            String comments)
                     throws IOException

      Writes this property list (key and element pairs) in this `Properties` table to the output stream in a format suitable for loading into a `Properties` table using the `load(InputStream)` method.

      Properties from the defaults table of this `Properties` table (if any) are not written out by this method.

      This method outputs the comments, properties keys and values in the same format as specified in `store(Writer)`, with the following differences:

      - The stream is written using the ISO 8859-1 character encoding.
      - Characters not in Latin-1 in the comments are written as `\u` xxxx for their appropriate unicode hexadecimal value xxxx.
      - Characters less than `\u0020` and characters greater than `\u007E` in property keys or values are written as `\u` xxxx for the appropriate hexadecimal value xxxx.

      After the entries have been written, the output stream is flushed. The output stream remains open after this method returns.

      Parameters:
        `out` - an output stream.
        `comments` - a description of the property list.
      Throws:
        `IOException` - if writing this property list to the specified output stream throws an `IOException`.
        `ClassCastException` - if this `Properties` object contains any keys or values that are not `Strings`.
        `NullPointerException` - if `out` is null.
      Since:
        1.2

    - #### loadFromXML

          public void loadFromXML(InputStream in)
                           throws IOException

      Loads all of the properties represented by the XML document on the specified input stream into this properties table.

      The XML document must have the following DOCTYPE declaration:

           <!DOCTYPE properties SYSTEM "http://java.sun.com/dtd/properties.dtd">

      Furthermore, the document must satisfy the properties DTD described above.

      The specified stream is closed after this method returns.

      Parameters:
        `in` - the input stream from which to read the XML document.
      Throws:
        `IOException` - if reading from the specified input stream results in an `IOException`.
        `InvalidPropertiesFormatException` - Data on input stream does not constitute a valid XML document with the mandated document type.
        `NullPointerException` - if `in` is null.
      Since:
        1.5
      See Also:
        `storeToXML(OutputStream, String, String)`

    - #### storeToXML

          public void storeToXML(OutputStream os,
                                 String comment)
                          throws IOException

      Emits an XML document representing all of the properties contained in this table.

      An invocation of this method of the form `props.storeToXML(os, comment)` behaves in exactly the same way as the invocation `props.storeToXML(os, comment, "UTF-8");`.

      Parameters:
        `os` - the output stream on which to emit the XML document.
        `comment` - a description of the property list, or `null` if no comment is desired.
      Throws:
        `IOException` - if writing to the specified output stream results in an `IOException`.
        `NullPointerException` - if `os` is null.
        `ClassCastException` - if this `Properties` object contains any keys or values that are not `Strings`.
      Since:
        1.5
      See Also:
        `loadFromXML(InputStream)`

    - #### storeToXML

          public void storeToXML(OutputStream os,
                                 String comment,
                                 String encoding)
                          throws IOException

      Emits an XML document representing all of the properties contained in this table, using the specified encoding.

      The XML document will have the following DOCTYPE declaration:

           <!DOCTYPE properties SYSTEM "http://java.sun.com/dtd/properties.dtd">

      If the specified comment is `null` then no comment will be stored in the document.

      The specified stream remains open after this method returns.

      Parameters:
        `os` - the output stream on which to emit the XML document.
        `comment` - a description of the property list, or `null` if no comment is desired.
        `encoding` - the name of a supported character encoding
      Throws:
        `IOException` - if writing to the specified output stream results in an `IOException`.
        `NullPointerException` - if `os` is `null`, or if `encoding` is `null`.
        `ClassCastException` - if this `Properties` object contains any keys or values that are not `Strings`.
      Since:
        1.5
      See Also:
        `loadFromXML(InputStream)`

    - #### getProperty

          public String getProperty(String key)

      Searches for the property with the specified key in this property list. If the key is not found in this property list, the default property list, and its defaults, recursively, are then checked. The method returns `null` if the property is not found.
      Parameters:
        `key` - the property key.
      Returns:
        the value in this property list with the specified key value.
      See Also:
        `setProperty(java.lang.String, java.lang.String)`, `defaults`

    - #### getProperty

          public String getProperty(String key,
                                    String defaultValue)

      Searches for the property with the specified key in this property list. If the key is not found in this property list, the default property list, and its defaults, recursively, are then checked. The method returns the default value argument if the property is not found.
      Parameters:
        `key` - the hashtable key.
        `defaultValue` - a default value.
      Returns:
        the value in this property list with the specified key value.
      See Also:
        `setProperty(java.lang.String, java.lang.String)`, `defaults`

    - #### propertyNames

          public Enumeration<?> propertyNames()

      Returns an enumeration of all the keys in this property list, including distinct keys in the default property list if a key of the same name has not already been found from the main properties list.
      Returns:
        an enumeration of all the keys in this property list, including the keys in the default property list.
      Throws:
        `ClassCastException` - if any key in this property list is not a string.
      See Also:
        `Enumeration`, `defaults`, `stringPropertyNames()`

    - #### stringPropertyNames

          public Set<String> stringPropertyNames()

      Returns a set of keys in this property list where the key and its corresponding value are strings, including distinct keys in the default property list if a key of the same name has not already been found from the main properties list. Properties whose key or value is not of type `String` are omitted.

      The returned set is not backed by the `Properties` object. Changes to this `Properties` are not reflected in the set, or vice versa.

      Returns:
        a set of keys in this property list where the key and its corresponding value are strings, including the keys in the default property list.
      Since:
        1.6
      See Also:
        `defaults`

    - #### list

          public void list(PrintStream out)

      Prints this property list out to the specified output stream. This method is useful for debugging.
      Parameters:
        `out` - an output stream.
      Throws:
        `ClassCastException` - if any key in this property list is not a string.

    - #### list

          public void list(PrintWriter out)

      Prints this property list out to the specified output stream. This method is useful for debugging.
      Parameters:
        `out` - an output stream.
      Throws:
        `ClassCastException` - if any key in this property list is not a string.
      Since:
        JDK1.1

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