Java中的属性类

Properties 类表示一组持久的属性。属性可以保存到流中或从流中加载。它属于Java.util包。属性定义以下实例变量。此变量保存与Properties对象关联的默认属性列表。

Properties defaults: This variable holds a default property list associated with a Properties object.
 

属性类的特点：

• 属性是 Hashtable 的子类。
• 它用于维护一个值列表，其中键是字符串，值也是字符串，即；它可用于存储和检索属性文件中的字符串类型数据。
• 属性类可以指定其他属性列表，因为它是默认的。如果原始属性列表中不存在特定的关键属性，则将搜索默认属性。
• Properties 对象不需要外部同步，多个线程可以共享一个 Properties 对象。
• 此外，它还可用于检索系统的属性。

属性文件的优点

如果从属性记录中更改了任何数据，则不必重新编译Java类。它用于存储要习惯性更改的数据。

注意： Properties 类没有从其超类Hashtable继承负载因子的概念。

宣言

public class Properties extends Hashtable

属性的构造函数

1. Properties()：这将创建一个没有默认值的Properties对象。

Properties p = new Properties();

2.Properties(Properties propDefault)：第二个创建一个使用propDefault作为其默认值的对象。

Properties p = new Properties(Properties propDefault);

示例 1：下面的程序展示了如何使用 Properties 类从属性文件中获取信息。

让我们创建一个属性文件并将其命名为db.properties 。

数据库属性

username = coder
password = geeksforgeeks

代码

Java

// Java program to demonstrate Properties class to get
// information from the properties file
  
import java.util.*;
import java.io.*;
public class GFG {
    public static void main(String[] args) throws Exception
    {
        // create a reader object on the properties file
        FileReader reader = new FileReader("db.properties");
  
        // create properties object
        Properties p = new Properties();
  
        // Add a wrapper around reader object
        p.load(reader);
  
        // access properties data
        System.out.println(p.getProperty("username"));
        System.out.println(p.getProperty("password"));
    }
}

输出

示例 2：下面的程序显示了如何使用 Properties 类来获取所有系统属性。使用System.getProperties() 方法，我们可以获得系统的所有属性。

Java

// Java program to demonstrate Properties class to get all
// the system properties
  
import java.util.*;
import java.io.*;
  
public class GFG {
    public static void main(String[] args) throws Exception
    {
        // get all the system properties
        Properties p = System.getProperties();
  
        // stores set of properties information
        Set set = p.entrySet();
  
        // iterate over the set
        Iterator itr = set.iterator();
        while (itr.hasNext()) {
  
            // print each property
            Map.Entry entry = (Map.Entry)itr.next();
            System.out.println(entry.getKey() + " = "
                               + entry.getValue());
        }
    }
}

输出

示例 3：下面的程序展示了如何使用 Properties 类来创建一个属性文件。

Java

// Java program to demonstrate Properties class to create
// the properties file
  
import java.util.*;
import java.io.*;
  
public class GFG {
    public static void main(String[] args) throws Exception
    {
        // create an instance of Properties
        Properties p = new Properties();
  
        // add properties to it
        p.setProperty("name", "Ganesh Chowdhary Sadanala");
        p.setProperty("email",
                      "[e-mail]");
  
        // store the properties to a file
        p.store(new FileWriter("info.properties"),
                "GeeksforGeeks Properties Example");
    }
}

输出

属性方法

METHOD	DESCRIPTION
getProperty​(String key)	Searches for the property with the specified key in this property list.
getProperty​(String key, String defaultValue)	Searches for the property with the specified key in this property list.
list​(PrintStream out)	Prints this property list out to the specified output stream.
list​(PrintWriter out)	Prints this property list out to the specified output stream.
load​(InputStream inStream)	Reads a property list (key and element pairs) from the input byte stream.
load​(Reader reader)	Reads a property list (key and element pairs) from the input character stream in a simple line-oriented format.
loadFromXML​(InputStream in)	Loads all of the properties represented by the XML document on the specified input stream into this properties table.
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.
save​(OutputStream out, String comments)	Deprecated. This method does not throw an IOException if an I/O error occurs while saving the property list.
setProperty​(String key, String value)	Calls the Hashtable method put.
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.
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.
storeToXML​(OutputStream os, String comment)	Emits an XML document representing all of the properties contained in this table.
storeToXML​(OutputStream os, String comment, String encoding)	Emits an XML document representing all of the properties contained in this table, using the specified encoding.
storeToXML​(OutputStream os, String comment, Charset charset)	Emits an XML document representing all of the properties contained in this table, using the specified encoding.
stringPropertyNames()	Returns an unmodifiable set of keys from 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.

在类Java.util.Hashtable 中声明的方法

METHOD	DESCRIPTION
clear()	Clears this hashtable so that it contains no keys.
clone()	Creates a shallow copy of this hashtable.
compute​(K key, BiFunction remappingFunction)	Attempts to compute a mapping for the specified key and its current mapped value (or null if there is no current mapping).
computeIfAbsent​(K key, Function mappingFunction)	If the specified key is not already associated with a value (or is mapped to null), attempts to compute its value using the given mapping function and enters it into this map unless null.
computeIfPresent​(K key, BiFunction remappingFunction)	If the value for the specified key is present and non-null, attempts to compute a new mapping given the key and its current mapped value.
contains​(Object value)	Tests if some key maps into the specified value in this hashtable.
containsKey​(Object key)	Tests if the specified object is a key in this hashtable.
containsValue​(Object value)	Returns true if this hashtable maps one or more keys to this value.
elements()	Returns an enumeration of the values in this hashtable.
entrySet()	Returns a Set view of the mappings contained in this map.
equals​(Object o)	Compares the specified Object with this Map for equality, as per the definition in the Map interface.
get​(Object key)	Returns the value to which the specified key is mapped, or null if this map contains no mapping for the key.
hashCode()	Returns the hash code value for this Map as per the definition in the Map interface.
isEmpty()	Tests if this hashtable maps no keys to values.
keys()	Returns an enumeration of the keys in this hashtable.
keySet()	Returns a Set view of the keys contained in this map.
merge​(K key, V value, BiFunction remappingFunction)	If the specified key is not already associated with a value or is associated with null, associates it with the given non-null value.
put​(K key, V value)	Maps the specified key to the specified value in this hashtable.
putAll​(Map t)	Copies all of the mappings from the specified map to this hashtable.
rehash()	Increases the capacity of and internally reorganizes this hashtable, in order to accommodate and access its entries more efficiently.
remove​(Object key)	Removes the key (and its corresponding value) from this hashtable.
size()	Returns the number of keys in this hashtable.
toString()	Returns a string representation of this Hashtable object in the form of a set of entries, enclosed in braces and separated by the ASCII characters ” , ” (comma and space).
values()	Returns a Collection view of the values contained in this map.

在接口Java.util.Map 中声明的方法

METHOD	DESCRIPTION
forEach​(BiConsumer action)	Performs the given action for each entry in this map until all entries have been processed or the action throws an exception.
getOrDefault​(Object key, V defaultValue)	Returns the value to which the specified key is mapped, or defaultValue if this map contains no mapping for the key.
putIfAbsent​(K key, V value)	If the specified key is not already associated with a value (or is mapped to null) associates it with the given value and returns null, else returns the current value.
remove​(Object key, Object value)	Removes the entry for the specified key only if it is currently mapped to the specified value.
replace​(K key, V value)	Replaces the entry for the specified key only if it is currently mapped to some value.
replace​(K key, V oldValue, V newValue)	Replaces the entry for the specified key only if currently mapped to the specified value.
replaceAll​(BiFunction function)	Replaces each entry’s value with the result of invoking the given function on that entry until all entries have been processed or the function throws an exception.