Input and output are often the performance bottleneck in a program. Reading from or writing to disk can be hundreds of times slower than reading from or writing to memory. Network connections and user input are even slower.
While disk capacities and speeds have increased over time, they have never kept pace with CPU speeds. Therefore, it's important to minimize the number of reads and writes a program actually does.
Most input stream classes have efficient polymorphic read() methods that read chunks of contiguous data into a byte array. This is faster than reading each element of the array separately.
To attempt to read 10 bytes from System.in, you could use the following code:

try {
  byte[] b = new byte[10];
  System.in.read(b);
}
catch (IOException e) {
  System.err.println("Couldn't read from System.in!");
}