When Does a.equals(b) Throw? Understanding Java Null Handling and Objects.equals

1. Null values

When a is null, calling a.equals(b) throws a NullPointerException. If a is non‑null and b is null, a.equals(b) returns false. Objects.equals(a, b) returns true only when both arguments are null, returns false when exactly one is null, and never throws 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. Empty string values

If both a and b are empty strings ( ""), a.equals(b) returns true. If only one of them is empty, the result is false. Objects.equals exhibits the same behavior in these cases.

"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

1. Source

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

2. Explanation

The method first checks reference equality with a == b. If that is true, it returns true immediately. If not, it ensures a is not null before invoking a.equals(b), which prevents a NullPointerException when only one argument is null.

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

a==b

compares the two object references; it yields true only when both references point to the exact same object in memory. a.equals(b) performs logical equality, which can be overridden by a class to compare the internal state of objects, and returns true when the objects are considered equal according to that logic.