List of Integers into byte array

问题

I have a LinkedList < Integer > with big size (3912984 or more) and I wanna copy these elements in a byte array. The integers are 0 or 1, so I don't need any change of size for array, I want just to copy elements one by one, exactly how they are. Of course, I know the simplest way is :

 for(int i = 0; i < list.size(); i++)      
 array[i] = (byte)(int) list.get(i);

But this method is too slow and my program doesn't end before hours ! Can you know another way (faster, something like Buffer.BlockCopy() of .NET) or I have to change data structures?

回答1:

There is byteValue() method available in Number class. Number is extended by Integer, Double, Float etc.

List<Integer> list = getNumbers();

Iterator<Integer> iterator = list.iterator();

while(iterator.hasNext())
{
   Integer i = iterator.next()
   byteArray[index] = i.byteValue();
}

You can also use java.nio.MappedByteBuffer class for block copy. see http://docs.oracle.com/javase/7/docs/api/java/nio/MappedByteBuffer.html.

MappedByteBuffer is equivalent to Buffer.BlockCopy() in .NET

回答2:

Another way to do it with Java 8:

List<Integer> enteros = Stream.of(1, 2, 3, 4, 5).collect(Collectors.toList());
Byte[] bytes = enteros.stream().map(entero -> entero.byteValue()).toArray(Byte[]::new);

Here another complex example to test the conversion time:

List<Integer> enteros = new LinkedList<>();
DateTimeFormatter formatter = DateTimeFormatter.ofPattern("HH:mm:ss");

System.out.println("Init list: " + LocalTime.now().format(formatter));
// Fill array with 0 and 1
for(int i = 0; i < 9999999; i++) enteros.add(ThreadLocalRandom.current().nextInt(0, 2));
System.out.println("List complete: " + LocalTime.now().format(formatter));

System.out.println("Init list convert: " + LocalTime.now().format(formatter));
Byte[] bytes = enteros.stream().map(entero -> entero.byteValue()).toArray(Byte[]::new);
System.out.println("End convert! " + LocalTime.now().format(formatter));

回答3:

// example input list
List<Integer> list = new LinkedList<Integer>();
list.add(1);
list.add(0);
list.add(1);
list.add(0);
list.add(1);
list.add(1);

// write to byte array
ByteArrayOutputStream baos = new ByteArrayOutputStream();
DataOutputStream out = new DataOutputStream(baos);
for (int element : list) {
    out.writeUTF(Integer.toString(element));
}
byte[] bytes = baos.toByteArray();

// read from byte array
ByteArrayInputStream bais = new ByteArrayInputStream(bytes);
DataInputStream in = new DataInputStream(bais);
while (in.available() > 0) {
    String element = in.readUTF();
    System.out.println(element);
}

来源：https://stackoverflow.com/questions/25460810/list-of-integers-into-byte-array

标签

java

arrays

list

integer

bytearray