I met a quite strange case in a program with jdk1.8,
that I got a null pointer exception in StringBuilder.append() method,

the exact position of the null pointer exception is here,in AbstractStringBuilder class :

public AbstractStringBuilder append(String str) {
        if (str == null)
            return appendNull();
        int len = str.length();  //here 
        ensureCapacityInternal(count + len);
        str.getChars(0, len, value, count);
        count += len;
        return this;
    }

I can’t understand how this can happen, as there is a null check before str.length() called.

And maybe due to the same reason, all the way I tried to print the string content crashed, so I can’t get what string content caused this.

Now, what I want to know is what can cause this, can I build a string content to reproduce this? Thanks for any ideas.