Returning Collections From Methods in Java

Never return null in place of a collection

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Writing software often means fetching and returning data. In an object-oriented language like Java, we generally express data in terms of properties of a class. Unlike many newer OO languages, Java doesn’t offer properties as a first-class construct, so we instead use JavaBean getters and setters, in order to maintain encapsulation. As a result, most of us have grown accustomed to writing code like the following:

public class MyClass {
    private String data;
    public String getData() { return this.data; }
    public void setData(String data) { this.data = data; }
}

In the example above, we see a simple class with a simple property: a String called data. Of course, data can either contain a value, or it can be null.

How do we deal with Collection-type properties? Let’s fabricate a simple class that encapsulates a List:

public class MyClass {
    private List<String> myStrings;
}

We have a field — a List of Strings — called myStrings, which is encapsulated in MyClass. Now, we need to provide accessor methods:

public class MyClass {
    private List<String> myStrings;
    public void setMyStrings(List<String> s) {
        this.myStrings = s;
    }
    public List<String> getMyStrings() {
        return this.myStrings;
    }
}

Here we have a properly-encapsulated — if verbose — class. So we’ve done good, right? Hold that thought.

Optional Lessons

Consider the Optional class, introduced in Java 8. By now, you’ve probably heard the mantra that you should never return null from a method that returns an Optional. Why? Consider the following contrived example:

public class Foo {
    private String bar;
    public Optional<String> getBar() {
        // don't actually ever do this
        return (bar == null) ? null : Optional.of(bar);
    }
}