How to change file extension at runtime in Java

Question

I am trying to implement program to zip and unzip a file. All I want to do is to zip a file (fileName.fileExtension) with name as fileName.zip

Answer 1

I would check, if the file has an extension before changing. The solution below works also with files without extension or multiple extensions

public File changeExtension(File file, String extension) {
    String filename = file.getName();

    if (filename.contains(".")) {
        filename = filename.substring(0, filename.lastIndexOf('.'));
    }
    filename += "." + extension;

    file.renameTo(new File(file.getParentFile(), filename));
    return file;
}

@Test
public void test() {
    assertThat(changeExtension(new File("C:/a/aaa.bbb.ccc"), "txt"), 
                            is(new File("C:/a/aaa.bbb.txt")));

    assertThat(changeExtension(new File("C:/a/test"), "txt"), 
                            is(new File("C:/a/test.txt")));
}