Main.java

package com.company;

import java.io.*;
import java.nio.charset.Charset;

public class Main {

    public static byte[] to1251Bytes(final String str) {
        try {
            final ByteArrayOutputStream os = new ByteArrayOutputStream(str.length());
            final Writer out = new BufferedWriter(new OutputStreamWriter(os, Charset.forName("Cp1251")));
            out.write(str);
            out.flush();
            return os.toByteArray();
        } catch (IOException ex) {
            return null;
        }
    }

    public static void main(String[] args) {
        to1251Bytes("hello");
    }
}

test.java

package com.company;

import static com.company.Main.to1251Bytes;
import static org.junit.Assert.assertArrayEquals;

public class MainTest {

    @org.junit.Test
    public void testTo1251Bytes() {
        assertArrayEquals(
                new byte[]{97, 97, 97},
                to1251Bytes("aaa")
        );
    }

    @org.junit.Test
    public void testCyrillics() {
        // See https://ru.wikipedia.org/wiki/Windows-1251
        // -23 = (byte) 0xE9
        assertArrayEquals(
                new byte[]{-23, -23, -23},
                to1251Bytes("ййй")
        );
    }

    @org.junit.Test
    public void testNonEncodable() {
        // 63 is the ascii code for '?'
        assertArrayEquals(
                new byte[]{63, 63, 63},
                to1251Bytes("ééé")
        );
    }
}