Why String concatenate null with + operator and throws NullPointerException with concate() method

Question

Here is my class, where i am concatenating two string. String concatenate with null using + operator execute smoothly but throws NullPointerException

Answer 1

Because inside concat method in String class, length method is getting invoked on passed parameter , due to which NPE is thrown.

public String concat(String str) {
        int otherLen = str.length();<------
        if (otherLen == 0) {
            return this;
        }
        int len = value.length;
        char buf[] = Arrays.copyOf(value, len + otherLen);
        str.getChars(buf, len);
        return new String(buf, true);
    }