buffers and channels

import java.nio.ByteBuffer;
 
static void show(String op, ByteBuffer b) {
    IO.println(op + " pos=" + b.position() + " lim=" + b.limit() + " rem=" + b.remaining());
}
 
void main() {
    ByteBuffer buf = ByteBuffer.allocate(8);
    buf.put("HELLO".getBytes());    show("put  ", buf);
    buf.flip();                     show("flip ", buf);
    buf.get(); buf.get();           show("get×2", buf);
    buf.compact();                  show("cmpct", buf);
    IO.println(new String(buf.array(), 0, 5));
}

put   pos=5 lim=8 rem=3
flip  pos=0 lim=5 rem=5
get×2 pos=2 lim=5 rem=3
cmpct pos=3 lim=8 rem=5
LLOLO

import java.nio.ByteBuffer;
import java.nio.channels.FileChannel;
import java.nio.file.*;
import static java.nio.file.StandardOpenOption.*;
 
void main() throws Exception {
    Path src = Files.createTempFile("src", ".txt");
    Path dst = Files.createTempFile("dst", ".txt");
    Files.writeString(src, "bytes in, bytes out. ".repeat(1000));
 
    ByteBuffer buf = ByteBuffer.allocateDirect(4096);
    int reads = 0;
    try (FileChannel in = FileChannel.open(src, READ);
         FileChannel out = FileChannel.open(dst, WRITE, TRUNCATE_EXISTING)) {
        while (in.read(buf) != -1 || buf.position() > 0) {
            reads++;
            buf.flip();       // switch to draining
            out.write(buf);   // may not write everything
            buf.compact();    // keep the leftovers, switch back to filling
        }
    }
    IO.println(Files.size(dst) + " bytes, " + reads + " loops, identical: " + (Files.mismatch(src, dst) == -1));
    Files.delete(src); Files.delete(dst);
}

21000 bytes, 6 loops, identical: true

this one needs a real JVM (files, the network or a database), so it doesn't run in the browser. copy it into java Main.java to try it.

import java.nio.*;
import java.nio.channels.FileChannel;
import java.nio.file.*;
import java.util.HexFormat;
import static java.nio.file.StandardOpenOption.*;
 
void main() throws Exception {
    Path file = Files.createTempFile("mapped", ".bin");
    Files.write(file, new byte[8]);
 
    try (FileChannel ch = FileChannel.open(file, READ, WRITE)) {
        MappedByteBuffer m = ch.map(FileChannel.MapMode.READ_WRITE, 0, 8);
        m.putInt(0, 0xCAFEBABE);       // a plain memory write
        IO.println(HexFormat.ofDelimiter(" ").formatHex(Files.readAllBytes(file)));
 
        Files.write(file, new byte[] { 1, 2, 3, 4, 5, 6, 7, 8 });   // change the file
        IO.println(m.get(7));          // the mapping sees it
    }
    Files.delete(file);
}

ca fe ba be 00 00 00 00
8

this one needs a real JVM (a library class the browser runtime doesn't have), so it doesn't run in the browser. copy it into java Main.java to try it.

import java.nio.*;
import java.util.HexFormat;
 
void main() {
    var hex = HexFormat.ofDelimiter(" ");
    ByteBuffer b = ByteBuffer.allocate(4);
    b.putInt(1);
    IO.println(b.order() + ": " + hex.formatHex(b.array()));
 
    b.clear().order(ByteOrder.LITTLE_ENDIAN).putInt(1);
    IO.println(b.order() + ": " + hex.formatHex(b.array()));
    IO.println("this CPU: " + ByteOrder.nativeOrder());
}

BIG_ENDIAN: 00 00 00 01
LITTLE_ENDIAN: 01 00 00 00
this CPU: LITTLE_ENDIAN

this one needs a real JVM (a library class the browser runtime doesn't have), so it doesn't run in the browser. copy it into java Main.java to try it.

import java.nio.ByteBuffer;
 
void main() {
    ByteBuffer b = ByteBuffer.wrap("HELLOWORLD".getBytes());
    b.position(5);
    ByteBuffer world = b.slice();          // a window, not a copy
    world.put(0, (byte) 'w');
    IO.println(new String(b.array()) + " " + world.capacity() + " " + world.position());
 
    ByteBuffer ro = b.asReadOnlyBuffer();
    try {
        ro.put(0, (byte) 'h');
    } catch (Exception e) {
        IO.println(e.getClass().getSimpleName());
    }
}

HELLOwORLD 5 0
ReadOnlyBufferException

this one needs a real JVM (catching a runtime exception), so it doesn't run in the browser. copy it into java Main.java to try it.