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

If you see in java.lang.String source, and use null as parameter. NPE is thrown in first line in length() method.

public String concat(String str) {
    int otherLen = str.length();//This is where NullPointerException is thrown
    if (otherLen == 0) {
        return this;
    }
    getChars(0, count, buf, 0);
    str.getChars(0, otherLen, buf, count);
    return new String(0, count + otherLen, buf);
}