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static final int BUFFER = 512;
// ...

private String validateFilename(String filename) { ... }

public final void unzip(String filename) throws java.io.IOException{
  FileInputStream fis = new FileInputStream(filename);
  ZipInputStream zis = new ZipInputStream(new BufferedInputStream(fis));
  ZipEntry entry;
  while ((entry = zis.getNextEntry()) != null) {
    System.out.println("Extracting: " + entry);
    int count;
    byte data[] = new byte[BUFFER];
    // write the files to the disk
    String name = validateFilename(entry.getName());
    FileOutputStream fos = new FileOutputStream(name);
    BufferedOutputStream dest = new BufferedOutputStream(fos, BUFFER);
    while ((count = zis.read(data, 0, BUFFER)) != -1) {
      dest.write(data, 0, count);
    }
    dest.flush();
    dest.close();
    zis.closeEntry();
  }
  zis.close();
}

static final int BUFFER = 512;
static final int TOOBIG = 0x6400000; // 100MB
// ...

private String validateFilename(String filename) { ... }

public final void unzip(String filename) throws java.io.IOException{
  FileInputStream fis = new FileInputStream(filename);
  ZipInputStream zis = new ZipInputStream(new BufferedInputStream(fis));
  ZipEntry entry;
  try{
    while ((entry = zis.getNextEntry()) != null) {
      System.out.println("Extracting: " + entry);
      int count;
      byte data[] = new byte[BUFFER];  // write the files to the disk, but ensure that the file is not insanely big
      int total = 0;
      String name = validateFilename(entry.getName());
      FileOutputStream fos = new FileOutputStream(name);
      BufferedOutputStream dest = new BufferedOutputStream(fos, BUFFER);
      while (total <= TOOBIG && (count = zis.read(data, 0, BUFFER)) != -1) {
        dest.write(data, 0, count);
        total += count;
      }
      dest.flush();
      dest.close();
      zis.closeEntry();

      if (total > TOOBIG){
        throw new IllegalStateException("File being unzipped is huge.");  }
    }
  } finally {
    zis.close();
  }
}