StringBuffer类的对象append(null) 的问题分析

package java1;

import org.junit.Test;

public class IDEADebug {

    @Test
    public void testStringBuffer(){
        String str = null;//String类型存地址值，此处存null，空指针
        StringBuffer sb = new StringBuffer();
        sb.append(str);
        //此处底层，变成添加四个字符
        //private AbstractStringBuilder appendNull() {
        //        int c = count;
        //        ensureCapacityInternal(c + 4);
        //        final char[] value = this.value;
        //        value[c++] = 'n';
        //        value[c++] = 'u';
        //        value[c++] = 'l';
        //        value[c++] = 'l';
        //        count = c;
        //        return this;
        //    }

        System.out.println(sb.length());//4

        System.out.println(sb);//"null"

        StringBuffer sb1 = new StringBuffer(str);//抛异常NullPointerException

        //此处底层是str.length()等于null.length()调用了空指针了
        // public StringBuffer(String str) {
        //        super(str.length() + 16);
        //        append(str);
        //    }

        System.out.println(sb1);

    }
}