From procedures to objects - A practical example using JDK Properties

John is a newcomer to Java and has faced the need to save key-value pairs to a file. He reaches stackoverflow for help and finds the JDK Properties class…

Old IBM computer with two scientist reading its printed output.
© National Museum of Computing

John has not seen it before, so as he opens it, he expects to see code like this.

public interface Props {
    Props with(String key, String value);
    String property(String key);

To John’s surprise, when he finds out the Oracle’s solution to his problem, he sees this:

public void load(InputStream out, String comments)
public void store(OutputStream out, String comments)

Nevertheless, he accepts the solution and moves on with his project.

After a couple of months, John’s code starts to look like this:

Properties props = new Properties();
// In another file (debugging)
Properties props = new Properties(file1);
if (!"v1".equals(props.getProperty("k1")) {
    // expection thrown

He notices that he spends more time on debugging than on anything else. John decides that he cannot continue working on the project, before he resolves the problems introduced by the usage of the Properties class:

  1. Temporal coupling
    • Whenever John changed the order of the lines of code in the methods which dealt with properties, something would always brake. He would become more and more cautious about changing something, especially legacy code.
  2. Lack of self-sufficiency
    • John was not able to integrate the properties objects anywhere, such as in constructors, because these objects were not self-sufficient. “Do they know where to save themselves? How to save? How can I be sure that the contents of the file to which the properties are saved is up-to-date with object content?” These are the questions that John could not answer himself. He decided to use controllers for that, however, at some point John lost control of them, failing to remember all states they can be in.

At last, John creates an abstraction for the Properties class:

(using yegor256/cactoos library)

public final class PropsFile implements Props {
    public PropsFile(
        final File file
    ) {
            new Constant<>(
                () -> file
    private PropsFile(
        final Scalar<Scalar<File>> scalar
    ) {
        this.origin = new UncheckedScalar<>(
            new StickyScalar<>(
                () -> {
                    final File file = scalar.value().value();
                    final Properties props = new Properties();
                    if (file.exists()) {
                        try (FileInputStream fis = new FileInputStream(file)) {
                    } else {
                        try (FileOutputStream fos = new FileOutputStream(file)) {
                  , "");
                    return props;
        this.output = new UncheckedScalar<>(
            new StickyScalar<>(
                () -> new FileOutputStream(
    public String property(final String key) {
        return this.origin.value().getProperty(key);
    public PropsFile with(
        final String key,
        final String value
    ) throws IOException {
        final Properties props = this.origin.value();
        return this;

Now, he is able to use the same Properties in a much cleaner way:

final PropsFile props = new PropsFile(file);
props.with("k1", "v1"); // property added to file.
new Something(props);

The object-oriented abstraction PropsFile, thus, gives the following benefits over the standard Properties class:

  1. No temporal coupling
    • Because properties are auto-saved, there is no need to manually update the contents of the file. Also, the properties are auto-loaded meaning that the manual call of the load() is also not needed.
  2. Destination file initial state abstraction
    • John does not care whether the destination file exists initially or not. He wants to save the properties. And the interface does that for him, creating an empty file if it missing.
  3. Self-sufficiency over third-party control
    • After the PropsFile object is created, it can be used by anyone, without any unintended consequences. As a result, there is no need for any controllers who would manage the properties as now each properties object is self-sufficient and self-manageable.