[Chapter 17] Properties

#

| Chapter 17 The java.util Package |

# Properties

## Name

Properties

## Synopsis

Class Name:

  `java.util.Properties`

Superclass:

  `java.util.Hashtable`

Immediate Subclasses:

  None

Interfaces Implemented:

  None

Availability:

  JDK 1.0 or later

## Description

The `Properties` class is a subclass of `Hashtable` that deals exclusively with string keys and string values. Furthermore, a `Properties` object can be written to an `OutputStream` and read from an `InputStream`. Note that the `load()` and `save()` correctly convert Unicode strings to and from byte streams, using the `getLocalizedInputStream()` and `getLocalizedOutputStream()` methods of `Runtime`.

## Class Summary

    public class java.util.Properties extends java.util.Hashtable {
      // Variables
      protected Properties defaults;
      // Constructors
      public Properties();
      public Properties(Properties defaults);
      // Instance Methods
      public String getProperty(String key);
      public String getProperty(String key, String defaultValue);
      public void list(PrintStream out);
      public void list(PrintWriter out);                     // New in 1.1
      public synchronized void load(InputStream in);
      public Enumeration propertyNames();
      public synchronized void save(OutputStream out, String header);
    }

## Variables

### defaults

#### protected Properties defaults

Description

  A collection of default property values. If a key/value pair is not found in this Properties object, the `defaults` object is searched.

## Constructors

### Properties

#### public Properties()

Description

  This constructor creates an empty `Properties` object.

#### public Properties(Properties defaults)

Parameters

  `defaults`

    A set of default key/value pairs.

Description

  This constructor creates an empty `Properties` object that gets default values for keys that it does not contain from the given `Properties` object.

## Instance Methods

### getProperty

#### public String getProperty(String key)

Parameters

  `key`

    The key of the value to retrieve.

Returns

  The value of the given property or `null` if the key is not associated with any value.

Description

  This method returns the value that is associated with the given key. If the key is not found, a default value is returned if this object was created with a default `Properties` table that contains a value for the key. If neither a value nor a default value can be found, this method returns `null`.

#### public String getProperty(String key, String defaultValue)

Parameters

  `key`

    The key of the value to retrieve.

  `defaultValue`

    The value to return if `key` cannot be found.

Returns

  The value of the given property or `defaultValue` if the key is not associated with any value.

Description

  This method returns the value that is associated with the given key. If the key is not found, a default value is returned if this object was created with a default `Properties` table that contains a value for the key. If neither a value nor a default value can be found, this method returns `defaultValue`.

### list

#### public void list(PrintStream out)

Parameters

  `out`

    The output stream to use.

Description

  This method writes a listing of the contents of this object, in a format suitable for debugging, to the given `PrintStream`. As of JDK 1.1, use `list(PrintWriter)` instead.

#### public void list(PrintWriter out)

Availability

  New as of JDK 1.1

Parameters

  `out`

    The output stream to use.

Description

  This method writes a listing of the contents of this object, in a format suitable for debugging, to the given `PrintWriter`.

### load

#### public synchronized void load(InputStream in) throws IOException

Parameters

  `in`

    The input stream to use.

Throws

  `IOException`

    If any kind of I/O error occurs.

Description

  This method reads key/value pairs from the given `InputStream`. Here is the format the method expects:

  - Lines that begin with `#` or `!` are comments and are ignored.

  - Blank lines are ignored.

  - All other lines should specify a key/value pair and be of the form:

        key = value

    or

        key : value

    or

        key value

  All of these forms are equivalent. The method also recognizes the following escape characters and treats them as described:

Character | Treatment
--- | ---
`\`newline | An escaped newline character is ignored, along with the spaces or tabs that follow it
`\n` | Expands to a newline character
`\r` | Expands to a return character
`\t` | Expands to a tab character
`\u`xxxx`` | Expands to the Unicode character code specified by the hexadecimal digits

### propertyNames

#### public Enumeration propertyNames()

Returns

  The keys in this `Properties` object as an `Enumeration`.

Description

  This method returns an `Enumeration` that iterates through the keys in this `Properties` object.

### save

#### public synchronized void save(OutputStream out, String header)

Parameters

  `out`

    The output stream to use.

  `header`

    A header string.

Description

  This method writes key/value pairs to the given `OutputStream`. The format of the output is such that it can be read by the `load()` method.

  If `header` is not `null`, a `#` followed by `header` is written to the `OutputStream` first, thereby making the content of the string a comment that precedes the key/value pairs.

## Inherited Methods

Method | Inherited From | Method | Inherited From
--- | --- | --- | ---
`clear()` | `Hashtable` | `clone()` | `Hashtable`
`contains(Object)` | `Hashtable` | `containsKey(Object)` | `Hashtable`
`elements()` | `Hashtable` | `equals(Object)` | `Object`
`finalize()` | `Object` | `get(Object)` | `Hashtable`
`getClass()` | `Object` | `hashCode()` | `Object`
`isEmpty()` | `Hashtable` | `keys()` | `Hashtable`
`notify()` | `Object` | `notifyAll()` | `Object`
`put(Object, Object)` | `Hashtable` | `remove(Object)` | `Hashtable`
`size()` | `Hashtable` | `toString()` | `Hashtable`
`wait()` | `Object` | `wait(long)` | `Object`
`wait(long, int)` | `Object` |  |

## See Also

`Enumeration`, `Hashtable`, `InputStream`, `IOException`, `OutputStream`, `PrintStream`, `PrintWriter`, `Runtime`

Observer |  | PropertyResourceBundle