StringBuffer 使用注意点

public class StartApplication
{
    public static void main(String[] args)
    {
        String str=null;
        StringBuffer stringBuffer=new StringBuffer();
        stringBuffer.append(str);
        System.out.println("stringBuffer："+stringBuffer);//"null"
        System.out.println("stringBuffer.length()："+stringBuffer.length());//4

        StringBuffer stringBuffer1=new StringBuffer(str);//运行时报错
        System.out.println("stringBuffer1"+stringBuffer1);

    }
}

结果：
stringBuffer：null
Exception in thread "main" java.lang.NullPointerException
stringBuffer.length()：4
    at java.lang.StringBuffer.<init>(StringBuffer.java:139)
    at com.gsafety.StartApplication.main(StartApplication.java:16)
 

原因：

以下为jdk源码

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;
}

/**
 * Constructs a string buffer initialized to the contents of the
 * specified string. The initial capacity of the string buffer is
 * {@code 16} plus the length of the string argument.
 *
 * @param   str   the initial contents of the buffer.
 */
public StringBuffer(String str) {
    super(str.length() + 16);
    append(str);
}