Stringbuffer添加null会怎样

	public void test(){
        String str = null;
        StringBuffer sb = new StringBuffer();
        sb.append(str);//append了四个字母 n u l l
        System.out.println(sb.length());//4
        System.out.println(sb);//"null"
        
        StringBuffer sb1 = new StringBuffer(str);//抛异常NullPointerException  直接使用了
        System.out.println(sb1);//
    }