java - StringBuilder 的 indexOf() 不返回任何内容

Question

    StringBuilder builder = new StringBuilder();
    builder.setLength(10);
    builder.append("d");        
    System.out.println(builder.length() + "\t" + builder.toString() + "\t" + builder.indexOf("d"));

输出:

11 

问题:

Why indexOf() doesn't return anything.

我的理解:

 As per my understanding, it should return 10, since StringBuilder is counting the "d" as part of its length in length().
 But in case if "d" is not part of string hold by StringBuilder then, it should return -1 and length should be 10.

Answer 1

如果您查看 StringBuilder#setLength(int newLength) 的文档

If the newLength argument is greater than or equal to the current length, sufficient null characters ('\u0000') are appended so that length becomes the newLength argument.

这就是为什么当您在设置长度后附加“d”时，它被放置在 10 个空字符之后。

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Q. Why indexOf() doesn't return anything.

它确实返回了一个值，即 10，因为索引是基于 0 的。这是您的代码的输出。

11           d  10         // the 10 represents the index of d
^-length     ^-10 null chars followed by d

您没有得到输出的原因可能是因为您的控制台不支持 null 字符。这就是为什么当它遇到空字符 \u0000 时，它会停止在控制台中打印值。尝试使用支持打印 Unicode 字符的 eclipse。

示例快照: