When Does a.equals(b) Throw NullPointerException? Understanding Java Equality

1. Cases When the Value Is null

When a is null and you call a.equals(b), a NullPointerException is thrown.

If a is not null but b is null, a.equals(b) simply returns false. Objects.equals(a, b) safely handles null: it returns true when both arguments are null, false when exactly one is null, and otherwise delegates to a.equals(b) without throwing an exception.

null.equals("abc")            → throws NullPointerException
"abc".equals(null)            → returns false
null.equals(null)             → throws NullPointerException
Objects.equals(null, "abc")  → returns false
Objects.equals("abc", null) → returns false
Objects.equals(null, null)   → returns true

2. Cases When the Value Is an Empty String

If both a and b are empty strings ( ""), a.equals(b) returns true. If only one side is empty, the result is false. Objects.equals behaves identically in these scenarios.

"abc".equals("")   → returns false
"".equals("abc")   → returns false
"".equals("")      → returns true
Objects.equals("abc", "") → returns false
Objects.equals("", "abc") → returns false
Objects.equals("", "")    → returns true

3. Source Code Analysis of Objects.equals

public final class Objects {
    private Objects() {
        throw new AssertionError("No java.util.Objects instances for you!");
    }

    /**
     * Returns {@code true} if the arguments are equal to each other and
     * {@code false} otherwise.
     * If both arguments are {@code null}, {@code true} is returned.
     * If exactly one argument is {@code null}, {@code false} is returned.
     * Otherwise equality is determined by using {@link Object#equals(Object)}.
     */
    public static boolean equals(Object a, Object b) {
        return (a == b) || (a != null && a.equals(b));
    }
}

The method first checks reference equality ( a == b). If that fails, it ensures a is not null before delegating to a.equals(b), thereby avoiding a NullPointerException.

4. Difference Between a == b and a.equals(b)

a == b

compares object references; it returns true only when both references point to the exact same object in memory. a.equals(b) performs a logical comparison based on the object's content. For meaningful equality checks, classes typically override equals to define what constitutes logical equivalence.