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Language Intermediate

Private interface methods

Extract shared logic in interfaces using private methods.

Interfaces

Code Comparison

✕ Java 8

interface Logger {
    default void logInfo(String msg) {
        System.out.println(
            "[INFO] " + timestamp() + msg);
    }
    default void logWarn(String msg) {
        System.out.println(
            "[WARN] " + timestamp() + msg);
    }
}

✓ Java 9+

interface Logger {
    private String format(String lvl, String msg) {
        return "[" + lvl + "] " + timestamp() + msg;
    }
    default void logInfo(String msg) {
        IO.println(format("INFO", msg));
    }
    default void logWarn(String msg) {
        IO.println(format("WARN", msg));
    }
}

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Why the modern way wins

🧩

Code reuse

Share logic between default methods without duplication.

🔐

Encapsulation

Implementation details stay hidden from implementing classes.

🧹

DRY interfaces

No more copy-paste between default methods.

Old Approach

Duplicated Logic

Modern Approach

Private Methods

Since JDK

Difficulty

Intermediate

JDK Support

Private interface methods

Available

Widely available since JDK 9 (Sept 2017)

How it works

Java 9 allows private methods in interfaces, enabling you to share code between default methods without exposing implementation details to implementing classes.

Compact source files

Java 8

public class HelloWorld {
    public static void main(String[] args) {
        System.out.println(
            "Hello, World!");
    }
}

Java 25

void main() {
    IO.println("Hello, World!");
}

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JDK 25+ →

Language Intermediate

Flexible constructor bodies

Java 8

class Square extends Shape {
    Square(double side) {
        super(side, side);
        // can't validate BEFORE super!
        if (side <= 0)
            throw new IAE("bad");
    }
}

Java 25+

class Square extends Shape {
    Square(double side) {
        if (side <= 0)
            throw new IAE("bad");
        super(side, side);
    }
}

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JDK 25+ →

Language Beginner

Type inference with var

Java 8

Map<String, List<Integer>> map =
    new HashMap<String, List<Integer>>();
for (Map.Entry<String, List<Integer>> e
    : map.entrySet()) {
    // verbose type noise
}

Java 10+

var map = new HashMap<String, List<Integer>>();
for (var entry : map.entrySet()) {
    // clean and readable
}

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JDK 10+ →