Properties Class in Java

The Properties class in Java is used to store configuration settings as key-value pairs, where both keys and values are strings. It can load settings from a text file with the .properties extension and store them back. Programs use it for things like database URLs, the application name, or a default language, so those values can change without recompiling the code. Properties extends Hashtable<Object, Object>. In this lesson, you will learn its methods and the format of a properties file.

Declaration

public class Properties extends Hashtable<Object, Object>

The .properties File Format

# Lines starting with # are comments
app.name=PoolString
app.version=2.1
db.url=jdbc:mysql://localhost:3306/school
db.user=navika

Each line is key=value (a colon or a space can also separate them). Spaces around = are ignored.

Methods of Properties

Example: Loading and Reading Settings

Let's see an example that loads settings from text. A real program would read a file with Files.newBufferedReader(Path.of("app.properties")); here the text is inline so the example is self-contained.

import java.io.IOException;
import java.io.StringReader;
import java.util.Properties;
 
public class Config {
    public static void main(String[] args) throws IOException {
        String file = """
            # school app settings
            app.name=PoolString
            app.version=2.1
            db.user = navika
            """;
 
        Properties props = new Properties();
        props.load(new StringReader(file));
 
        System.out.println("Name: " + props.getProperty("app.name"));
        System.out.println("User: " + props.getProperty("db.user"));
        System.out.println("Theme: " + props.getProperty("ui.theme", "light"));
 
        int version = (int) Double.parseDouble(
            props.getProperty("app.version"));
        System.out.println("Major version: " + version);
    }
}

Name: PoolString
User: navika
Theme: light
Major version: 2

Explanation: load reads every key=value line and skips comments. getProperty returns strings only, so numbers must be parsed. The two-argument getProperty supplies "light" because ui.theme isn't in the file.

Example: Defaults and Saving

import java.io.IOException;
import java.io.StringWriter;
import java.util.Properties;
import java.util.TreeSet;
 
public class Defaults {
    public static void main(String[] args) throws IOException {
        Properties defaults = new Properties();
        defaults.setProperty("ui.theme", "light");
        defaults.setProperty("ui.lang", "en");
 
        Properties user = new Properties(defaults);
        user.setProperty("ui.theme", "dark");
 
        System.out.println("theme: " + user.getProperty("ui.theme"));
        System.out.println("lang: " + user.getProperty("ui.lang"));
        System.out.println("keys: "
            + new TreeSet<>(user.stringPropertyNames()));
 
        StringWriter out = new StringWriter();
        user.store(out, null);
        System.out.println("Saved lines: " + out.toString().lines().count());
    }
}

theme: dark
lang: en
keys: [ui.lang, ui.theme]
Saved lines: 2

Explanation: new Properties(defaults) creates settings that fall back to defaults for missing keys, so ui.lang comes from the defaults while ui.theme is overridden. store writes only the user's own settings: a timestamp comment line plus ui.theme=dark, which is 2 lines.

Common Mistakes

Using put and get instead of setProperty and getProperty. put accepts non-string values, which store then rejects.

Forgetting that values are strings. "2.1" must be parsed before doing arithmetic.

Expecting store to keep comments and order from the original file. It writes its own format.

Practice

1. Easy: Load a properties text with width and height keys and print the area.
2. Harder: Write a program that reads app.properties from disk (creating it with defaults if it doesn't exist), changes one setting, and saves the file again.

Key Takeaways