Main.java

/**
 * Created by alik on 3/31/17.
 */
public class Main {

    // TODO: Support other EndOfLines, like "\r\n".
    // One way is to just replace all "\r\n" with "\n" and then run the @getLastLines method.
    public static List<String> getLastLines(String string, int numLines) {
        List<String> lines = new ArrayList<>();
        int currentEndOfLine = string.length();
        if (string.endsWith("\n")) {
            currentEndOfLine = currentEndOfLine - "\n".length();
        }
        for (int i = 0; i < numLines; ++i) {
            int lastEndOfLine = currentEndOfLine;
            currentEndOfLine = string.lastIndexOf("\n", lastEndOfLine - 1);
            String lastLine = string.substring(currentEndOfLine + 1, lastEndOfLine);
            lines.add(0, lastLine);
        }
        return lines;
    }

    @Test
    public void test1() {
        String text = "111\n" +
                "222\n" +
                "333\n" +
                "444\n" +
                "555\n" +
                "666\n" +
                "777\n";
        List<String> lastLines = getLastLines(text, 4);
        Assert.assertEquals("777", lastLines.get(lastLines.size() - 1));
        Assert.assertEquals(4, lastLines.size());
    }

    @Test
    public void test2() {
        String text = "111\n" +
                "222\n" +
                "333\n" +
                "444\n" +
                "555\n" +
                "666\n" +
                "777";
        List<String> lastLines = getLastLines(text, 4);
        Assert.assertEquals("777", lastLines.get(lastLines.size() - 1));
        Assert.assertEquals(4, lastLines.size());
    }
}