Counts down from a specified value the number of bytes actually read from the wrapped InputStream. : InputStream « File « Java Tutorial

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import java.io.IOException;
import java.io.InputStream;

/**
 * Counts down from a specified value the number of bytes actually read
 * from the wrapped InputStream. 
 *
 * Returns minus one (-1) early from readXXX methods if the count
 * down reaches zero (0) before the end of the wrapped InputStream
 * is encountered. 
 *
 * This class is especially useful when a fixed number of bytes is to be read
 * from an InputStream that is in turn to be used as the source for an
 * {@link java.io.InputStreamReader InputStreamReader}.
 *
 * @author boucherb@users
 * @version 1.8.x
 * @since 1.8.x
 */
public final class CountdownInputStream extends InputStream {

    private long        count;
    private InputStream input;

    public CountdownInputStream(final InputStream is) {
        this.input = is;
    }

    public int read() throws IOException {

        if (this.count <= 0) {
            return -1;
        }

        final int b = this.input.read();

        if (b >= 0) {
            this.count--;
        }

        return b;
    }

    public int read(final byte[] buf) throws IOException {

        if (this.count <= 0) {
            return -1;
        }

        int len = buf.length;

        if (len > this.count) {
            len = (int) this.count;
        }

        final int r = this.input.read(buf, 0, len);

        if (r > 0) {
            this.count -= r;
        }

        return r;
    }

    public int read(final byte[] buf, final int off,
                    int len) throws IOException {

        if (this.count <= 0) {
            return -1;
        }

        if (len > this.count) {
            len = (int) this.count;
        }

        final int r = this.input.read(buf, 0, len);

        if (r > 0) {
            this.count -= r;
        }

        return r;
    }

    public void close() throws IOException {
        this.input.close();
    }

    public long getCount() {
        return this.count;
    }

    public void setCount(long count) {
        this.count = count;
    }
}