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

Actual implementation of concate method is like this . this method need an object of type String as a parameter and if argument is null then it throws Null Pointer Exp.

public String concat(String paramString)
  {
    int i = paramString.length();
    if (i == 0) {
      return this;
    }
    int j = this.value.length;
    char[] arrayOfChar = Arrays.copyOf(this.value, j + i);
    paramString.getChars(arrayOfChar, j);
    return new String(arrayOfChar, true);
  }