Java 将 long 转换为字节数组并将其添加到另一个数组中

Convert long to byte array and add it to another array

提问by codeObserver

I want to change a values in byte array to put a long timestamp value in in the MSBs. Can someone tell me whats the best way to do it. I do not want to insert values bit-by-bit which I believe is very inefficient.

我想更改字节数组中的值以在 MSB 中放入一个长时间戳值。有人可以告诉我什么是最好的方法。我不想一点一点地插入值，我认为这非常低效。

long time = System.currentTimeMillis();
Long timeStamp = new Long(time);
byte[] bArray = new byte[128];

What I want is something like:

我想要的是这样的：

byte[0-63] = timeStamp.byteValue(); 

Is something like this possible . What is the best way to edit/insert values in this byte array. since byte is a primitive I dont think there are some direct implementations I can make use of?

这样的事情可能吗。在此字节数组中编辑/插入值的最佳方法是什么。由于字节是一个原语，我不认为有一些我可以使用的直接实现？

Edit:
It seems that System.currentTimeMillis()is faster than Calendar.getTimeInMillis(), so replacing the above code by it.Please correct me if wrong.

编辑：
似乎System.currentTimeMillis()比 快Calendar.getTimeInMillis()，所以用它替换上面的代码。如果错误，请纠正我。

采纳答案by Bozho

There are multiple ways to do it:

有多种方法可以做到：

• Use a ByteBuffer(best option - concise and easy to read):

  byte[] bytes = ByteBuffer.allocate(Long.SIZE / Byte.SIZE).putLong(someLong).array();
  

• You can also use DataOutputStream(more verbose):

  ByteArrayOutputStream baos = new ByteArrayOutputStream();
  DataOutputStream dos = new DataOutputStream(baos);
  dos.writeLong(someLong);
  dos.close();
  byte[] longBytes = baos.toByteArray();
  

• Finally, you can do this manually (taken from the LongSerializerin Hector's code) (harder to read):

  byte[] b = new byte[8];
  for (int i = 0; i < size; ++i) {
    b[i] = (byte) (l >> (size - i - 1 << 3));
  }
  

• 使用ByteBuffer（最佳选择 - 简洁易读）：

  byte[] bytes = ByteBuffer.allocate(Long.SIZE / Byte.SIZE).putLong(someLong).array();
  

• 您还可以使用DataOutputStream（更详细）：

  ByteArrayOutputStream baos = new ByteArrayOutputStream();
  DataOutputStream dos = new DataOutputStream(baos);
  dos.writeLong(someLong);
  dos.close();
  byte[] longBytes = baos.toByteArray();
  

• 最后，您可以手动执行此操作（取自LongSerializerHector 中的代码）（更难阅读）：

  byte[] b = new byte[8];
  for (int i = 0; i < size; ++i) {
    b[i] = (byte) (l >> (size - i - 1 << 3));
  }
  

Then you can append these bytes to your existing array by a simple loop:

然后您可以通过一个简单的循环将这些字节附加到您现有的数组中：

// change this, if you want your long to start from 
// a different position in the array
int start = 0; 
for (int i = 0; i < longBytes.length; i ++) {
   bytes[start + i] = longBytes[i];
}

回答by Matt Phillips

It doesn't look like you can slice a byte array to insert something into a subset without doing it byte by byte. Look at Grab a segment of an array in Java without creating a new array on heap. Basically what I would do is set create a 64 byte array and set the time to it then append a blank 64 byte array to it. Or just do it byte by byte.

看起来您不能对字节数组进行切片以将某些内容插入子集，而无需逐字节进行操作。看看在Java中抓取一个数组的片段而不在 heap 上创建一个新数组。基本上我会做的是设置创建一个 64 字节的数组并为其设置时间，然后将一个空白的 64 字节数组附加到它。或者只是一个字节一个字节地做。

回答by Coder Roadie

I am updating this post because I have just announced a pre-release version of a library that will convert longs to byte arrays (and back again). The library is very small and will convert any java primitive to a byte array.

我正在更新这篇文章，因为我刚刚宣布了一个库的预发布版本，它将把 long 转换为字节数组（然后再转换回来）。该库非常小，可以将任何 java 原语转换为字节数组。

http://rschilling.wordpress.com/2013/09/26/pre-release-announcement-pend-oreille/http://code.google.com/p/pend-oreille/

http://rschilling.wordpress.com/2013/09/26/pre-release-announcement-pend-oreille/ http://code.google.com/p/pend-oreille/

If you use it you can do things like convert long arrays to byte arrays:

如果您使用它，您可以执行诸如将长数组转换为字节数组之类的操作：

Double[] doubles = new Double[1000];
for (int i = 2; i < 1002; i++) {
    doubles[i - 2] = (double) i;
}

byte[] resultBytes1 = (byte[]) new PrimitiveHelper(PrimitiveUtil.unbox(doubles))
        .asType(byte[].class);

You can also convert a single long value as well.

您也可以转换单个 long 值。

byte[] resultBytes1 = (byte[]) new PrimitiveHelper(1000l)
        .asType(byte[].class);

Feel free to provide some feedback.

随意提供一些反馈。

Update on October 4, 2013: I've now released the production of the library http://rschilling.wordpress.com/2013/10/04/pend-oreille-official-1-0-release/

2013 年 10 月 4 日更新：我现在已经发布了库http://rschilling.wordpress.com/2013/10/04/pend-oreille-official-1-0-release/

回答by cr0ck3t

If you want to really get under the hood...

如果你真的想深入了解...

public byte[] longToByteArray(long value) {
    return new byte[] {
        (byte) (value >> 56),
        (byte) (value >> 48),
        (byte) (value >> 40),
        (byte) (value >> 32),
        (byte) (value >> 24),
        (byte) (value >> 16),
        (byte) (value >> 8),
        (byte) value
    };
}

回答by Colateral

For me ByteBuffer and other utils are expensive from time perspective. Here are 2 methods that you can use:

对我来说，从时间的角度来看，ByteBuffer 和其他实用程序是昂贵的。以下是您可以使用的 2 种方法：

// first method (that is using the second method), it return the array allocated and fulfilled

// 第一个方法（即使用第二个方法），它返回分配和完成的数组

public byte[] longToByteArray(long value) 
{
        byte[] array = new byte[8];

        longToByteArray(value,array,0);
        return array;
}

// this method is useful if you have already allocated the buffer and you want to write the long a specific location in the array.

// 如果您已经分配了缓冲区并且想要将 long 写入数组中的特定位置，则此方法很有用。

public void longToByteArray(long value, byte[] array, int startFrom) 
{
    for (int i=7; i>=0; i--)
    {
        array[startFrom+7-i] = (byte) (value >> i*8);
    }
}