# Java Notes

# Properties

Note: The function of the `java.util.Properties` class, to save and restore the properties for a program, might be easier to do using `java.util.Preferences`. See Preferences.

The `java.util.Properties` class stores and loads key/value pairs from a file, and manages them in memory. This is the class to use to implement persistent variables (values which are saved in a file).

### To create and put values in a Properties table

This example creates a new Properties table and assigns a string value to four different keys.

    Properties props = new Properties();
    props.setProperty("recursiveSearch", "true");
    props.setProperty("noCopyPattern", "*.$$$");
    props.setProperty("maxLevel", "7");
    props.setProperty("fileName", "C:\temp\work.html");

### To store a Properties table in a file

Use the Properties `.store(OutputStream, String)` method. It will write the properties to the output stream, with a header line from the String parameter. Assuming you have variables and an OutputStream like:

    boolean recursiveSearch;
    String noCopyPattern;
    int maxLevel;
    . . .
    try {
      OutputStream propOut = new FileOutputStream(
                new File("props.stat"));
    }catch (. . .

To change values from their natural types to String, the usual idiom is to use concatenation of the empty string..

    props.setProperty("recursiveSearch", ""+recursiveSearch);
    props.setProperty("noCopyPattern"  , noCopyPattern");
    props.setProperty("maxLevel"       , ""+maxLevel);

    props.store(propOut, "Macro Processor Properties");

### To load Properties from a file

Use the Properties `.load(InputStream);` method.

You will probably want to get each property value and assign it to an internal variable and perhaps show it in the user interface. For example,

    boolean recursiveSearch;
    String noCopyPattern;
    int maxLevel;
    . . .
    props.load(propsIn);

    // Change the properties back into their natural types.
    recursiveSearch = Boolean.getBoolean(props.getProperty("recursiveSearch"));
    noCopyPattern = props.getProperty("noCopyPattern");
    maxLevel = Integer.parseInt(props.getProperty("maxLevel");

### To use default properties

1. Create a Properties table with all of the default key/value pairs. For example,

       Properties defaultProps = new Properties();
       defaultProps.setProperty("sourceFile", "");
       defaultProps.serProperty("enableScrolling", "false");

2. Use this default table in the constructor for your regular properties table:

       Properties props = new Properties(defaultProps);

### Constructor and Method Summary

    Properties()
    Properties(Properties defaultValues)

    Object setProperty(String key, String value)
    String getProperty(String key)
    String getProperty(String key, String defaultValue)
    Enumeration propertyNames()

    void   load(InputStream in)
    void   store(OutputStream out, String header)

## Related Pages

- System Properties
- Taming Tiger: Loading Properties from XML