---

id: 62

slug: "files-mismatch"

title: "Files.mismatch()"

category: "io"

difficulty: "beginner"

jdkVersion: "12"

oldLabel: "Java 8"

modernLabel: "Java 12+"

oldApproach: "Manual Byte Compare"

modernApproach: "Files.mismatch()"

oldCode: |-

// Compare two files byte by byte

byte[] f1 = Files.readAllBytes(path1);

byte[] f2 = Files.readAllBytes(path2);

boolean equal = Arrays.equals(f1, f2);

// loads both files entirely into memory

modernCode: |-

long pos = Files.mismatch(path1, path2);

// -1 if identical

// otherwise: position of first difference

summary: "Compare two files efficiently without loading them into memory."

explanation: "Files.mismatch() returns the position of the first byte that differs,\

\ or -1 if the files are identical. It reads lazily and short-circuits on the first\

\ difference."

whyModernWins:

- icon: "⚡"

title: "Memory-efficient"

desc: "Doesn't load entire files into byte arrays."

- icon: "🎯"

title: "Pinpoints difference"

desc: "Returns the exact byte position of the first mismatch."

- icon: "📏"

title: "One call"

desc: "No manual byte array comparison logic."

support:

state: "available"

description: "Widely available since JDK 12 (March 2019)"

prev: "io/file-memory-mapping"

next: "io/deserialization-filters"

related:

- "io/inputstream-transferto"

- "io/deserialization-filters"

- "io/writing-files"

docs:

- title: "Files.mismatch()"

href: "https://docs.oracle.com/en/java/javase/25/docs/api/java.base/java/nio/file/Files.html#mismatch(java.nio.file.Path,java.nio.file.Path)"