Best Practices for Null‑Safe Programming in Java

Calling equals on a string constant Use the literal string to call .equals() (e.g., "string literal".equals(strObject) ) to avoid a NullPointerException when strObject is null . The opposite order ( strObject.equals("string literal") ) can throw the exception.

Use valueOf instead of toString BigDecimal bd = getPrice(); String.valueOf(bd); // good bd.toString(); // not good Calling toString() on a possibly null object can also cause a NullPointerException.

Use null‑safe libraries Apache Commons StringUtils provides methods that never throw NullPointerException, e.g., StringUtils.isEmpty(null) // returns true , StringUtils.isBlank(null) // returns true , StringUtils.isNumeric(null) // returns false , StringUtils.isAllUpperCase(null) // returns false .

Functions should avoid returning null , return empty objects instead Java’s Collections class offers constants such as EMPTY_LIST , EMPTY_SET , and EMPTY_MAP that can be returned from catch blocks or default paths. public List f() { try { // ... return result; } catch (SomeException e) { e.printStackTrace(); return Collections.EMPTY_LIST; } }

Use @NotNull / @Nullable annotations Adding these JSR‑305 annotations helps IDEs warn about unchecked nulls and improves code readability. Java 8 provides @NonNull ; avoid adding heavy dependencies just for the annotation.

@NonNull List strList; // A non‑null list of Strings. List<@NonNull String> strList; // A list of non‑null Strings.

Avoid unnecessary autoboxing // Integer getPrice(); int price = obj.getPrice(); If getPrice() returns Integer and the value is null , assigning it to a primitive int will throw a NullPointerException.

Define reasonable default values and leverage database NOT NULL constraints Initialize class fields with empty objects, e.g., private List intList = new ArrayList<>(); and private String str = ""; . When a database column is declared NOT NULL , you can safely use primitive types like int in Java.

public class A { private List intList = new ArrayList<>(); private String str = ""; }

Implement a class representing null (special case) For specific business logic, a dedicated “null object” can be useful. Jackson’s JSON parser demonstrates this with the path() method, which returns a MissingNode instead of null when a field is absent, allowing safe chained calls without exceptions. JsonNode root = ...; JsonNode child = root.get("lv1").get("lv2"); // may throw NPE Replace with: JsonNode root = ...; JsonNode child = root.path("lv1").path("lv2"); If lv1 is missing, path() returns a MissingNode whose subsequent path() calls also return MissingNode , preventing any NullPointerException. After processing, check with if (child.isMissingNode()) { ... } .

(Source: segmentfault)