Chapter 94: Files Class in Java

java.nio.file.Files is a static toolbox sitting beside Path. You already used readString, writeString, createTempFile. This chapter widens the drawer: copy/move, tree walk, existence checks, metadata.

Try-with-resources (Chapter 95) matters for walk streams.

1. Topic title

Files: one-stop static helpers for common path operations

2. What it means

• exists / notExists — beware race conditions between check and act—often prefer CREATE options or atomic moves.
• copy / move — take CopyOption / StandardCopyOption flags (REPLACE_EXISTING, ATOMIC_MOVE when supported).
• walk — depth-first stream of paths—must close stream or use try-with-resources.
• readAttributes — grab size, modified time, directory flag in one shot.

3. Why it is used

Deployment scripts, cache eviction, bulk imports, static site generators—any tool that shuffles files.

4. Mental sketch

If Path is the address, Files is the moving company—boxes, tape, insurance forms included.

5. Java code example

import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.StandardCopyOption;
import java.nio.file.attribute.BasicFileAttributes;
 
public class FilesClassDemo {
    public static void main(String[] args) throws IOException {
        Path dir = Files.createTempDirectory("demo-tree");
        Path a = dir.resolve("a.txt");
        Path b = dir.resolve("b.txt");
        Files.writeString(a, "hello");
        Files.copy(a, b, StandardCopyOption.REPLACE_EXISTING);
 
        BasicFileAttributes attr = Files.readAttributes(b, BasicFileAttributes.class);
        System.out.println("size=" + attr.size() + " regular=" + attr.isRegularFile());
 
        try (java.util.stream.Stream<Path> stream = Files.walk(dir)) {
            long txtCount = stream.filter(p -> p.toString().endsWith(".txt")).count();
            System.out.println("txt files=" + txtCount);
        }
 
        Files.walkFileTree(dir, new java.nio.file.SimpleFileVisitor<>() {
            @Override
            public java.nio.file.FileVisitResult visitFile(Path file, BasicFileAttributes attrs) throws IOException {
                Files.deleteIfExists(file);
                return java.nio.file.FileVisitResult.CONTINUE;
            }
 
            @Override
            public java.nio.file.FileVisitResult postVisitDirectory(Path d, IOException exc) throws IOException {
                Files.deleteIfExists(d);
                return java.nio.file.FileVisitResult.CONTINUE;
            }
        });
    }
}

walk stream counts .txt entries. SimpleFileVisitor deletes files then directories—teaching pattern only; production deletes need error policy.

6. Explanation of code

REPLACE_EXISTING avoids FileAlreadyExistsException on copy.

7. Common mistakes

Walking without depth limit on deep trees—pass maxDepth overload when exploring unknown trees.

Following symlinks accidentally—FileVisitOption.FOLLOW_LINKS can loop; default no follow is safer for deletes.

8. Best practices

Prefer Files.isSameFile(a,b) over path.equals when comparing possible symlinks.

Use probeContentType only as a hint—sniffing bytes is safer for security decisions.

9. Small practice task

Create two temp files with the same text, then Files.mismatch(a,b)—print the byte offset of first difference (should be -1 when identical).

Beginner tip

If AccessDeniedException surprises you, run with fewer privileges in tests—catching permission issues early beats production-only discovery.