Common Java Pitfalls: Equality, Objects.equals, BigDecimal, Streams, Autoboxing, and String Replacement

As a Java developer you may have encountered mysterious bugs caused by seemingly simple code; this article collects six common pitfalls and shows how to avoid them.

1. Using == with Integer

Comparing two Integer objects with == checks reference equality, not value equality, so the result is false even when both objects contain the same value.

Integer orderStatus1 = new Integer(1);
Integer orderStatus2 = new Integer(1);
System.out.println(orderStatus1 == orderStatus2); // false

The Integer cache (‑128 to 127) is used only by valueOf, not by the constructor. Use equals or Integer.valueOf for reliable comparison.

2. Objects.equals Pitfall

When a user ID is stored as Integer but compared with a Long literal (e.g., 888L), Objects.equals returns false because the types differ.

if (Objects.equals(userInfo.getId(), 888L)) {
    sendEmail(userInfo);
}

The underlying Integer.equals method only returns true when the argument is also an Integer. Ensure both sides have the same type or convert explicitly.

3. BigDecimal Precision Issues

Creating a BigDecimal with the double constructor can introduce binary‑floating‑point errors:

BigDecimal amount1 = new BigDecimal(0.02);
BigDecimal amount2 = new BigDecimal(0.03);
System.out.println(amount2.subtract(amount1)); // 0.009999999999999998...

Use BigDecimal.valueOf or the String constructor to avoid loss of precision:

BigDecimal amount1 = BigDecimal.valueOf(0.02);
BigDecimal amount2 = BigDecimal.valueOf(0.03);
System.out.println(amount2.subtract(amount1)); // 0.01

4. Java 8 Stream filter Pitfall

Filtering a list with stream().filter(...) returns a view that holds references to the original objects. Modifying an element in the filtered list also changes the element in the original list because both lists share the same object instances.

List<User> filtered = users.stream()
    .filter(u -> u.getId() > 1000 && u.getAge() > 18)
    .collect(Collectors.toList());
for (User u : filtered) {
    u.setName(u.getName() + "Test");
}
// The names in the original 'users' list are also changed.

5. Autoboxing/Unboxing Pitfalls

Automatic unboxing of a

null

Integer

to a primitive int throws a NullPointerException:

Integer a = null;
int sum = a + 5; // NPE

Similarly, passing a

null

Integer

to a method that expects primitive int parameters causes the same error because the JVM invokes intValue() on a null reference.

6. String.replace vs replaceAll

String.replace

has two overloads: one for char arguments and one for CharSequence. It replaces every occurrence without interpreting the arguments as regular expressions.

String s = "A*B";
System.out.println(s.replace('A', 'C')); // C*B
System.out.println(s.replace("A", "C")); // C*B

replaceAll

treats the first argument as a regex, so special characters must be escaped:

String s = "A*B";
System.out.println(s.replaceAll("\\*", "C")); // ACB

To replace only the first match, use replaceFirst with a regex.

These seemingly simple constructs can easily become sources of bugs if their nuances are not understood.