How to Read a Variable-Length Integer from an InputStream in Java

What is the process for reading a variable-length integer from an InputStream in Java?

import java.io.*;

public class VariableLengthIntegerReader {
    public static int readVarInt(InputStream input) throws IOException {
        int value = 0;
        int bytesRead = 0;
        int currentByte;

        do {
            currentByte = input.read();
            value |= (currentByte & 0x7F) << (7 * bytesRead);
            bytesRead++;
            if (bytesRead > 5) {
                throw new IOException("VarInt is too long");
            }
        } while ((currentByte & 0x80) != 0);

        return value;
    }
}

Reading a variable-length integer from an InputStream requires understanding how such integers are encoded. These integers are commonly represented using a byte format where the most significant bit indicates whether the next byte should be read as part of the integer value.

import java.io.*;

public class VariableLengthIntegerReader {
    public static int readVarInt(InputStream input) throws IOException {
        int value = 0;
        int bytesRead = 0;
        int currentByte;

        do {
            currentByte = input.read();
            value |= (currentByte & 0x7F) << (7 * bytesRead);
            bytesRead++;
            if (bytesRead > 5) {
                throw new IOException("VarInt is too long");
            }
        } while ((currentByte & 0x80) != 0);

        return value;
    }
}

Causes

• Variable-length integers can take up to 5 bytes to represent. Bytes are encoded such that the highest bit (bit 7) indicates there are more bytes to read.
• Improper handling or assumption of fixed-length integers might lead to incorrect values being interpreted.

Solutions

• Implement a reading loop that continues until a byte with the highest bit set to 0 is encountered.
• Use bitwise operations to construct the integer from its byte components.
• Ensure to throw an exception if the integer exceeds the expected length.

Mistake: Assuming the integer will always fit into 4 bytes and not handling longer variable-length integers.

Solution: Implement proper checks and potentially throw exceptions if the length exceeds expected values.

Mistake: Not handling input stream closure correctly can lead to resource leaks.

Solution: Ensure to close InputStreams appropriately in a finally block or use try-with-resources.