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Java 12

A comprehensive guide to all Java 12 concepts with practical examples for interview preparation.

Table of Contents

  1. Switch Expressions (Preview)
  2. Shenandoah Garbage Collector
  3. Microbenchmark Suite
  4. CDS Archive Improvements
  5. G1 GC Improvements
  6. JVM Constants API
  7. Compact Number Formatting
  8. String Methods Enhancements
  9. Files.mismatch() Method
  10. Common Interview Questions

1. Switch Expressions (Preview)

Switch expressions allow switch to be used as an expression that returns a value.

Traditional Switch Statement

String day = "MONDAY";
int numLetters;

switch (day) {
    case "MONDAY":
    case "FRIDAY":
    case "SUNDAY":
        numLetters = 6;
        break;
    case "TUESDAY":
        numLetters = 7;
        break;
    case "WEDNESDAY":
    case "THURSDAY":
    case "SATURDAY":
        numLetters = 9;
        break;
    default:
        numLetters = -1;
}

Switch Expression (Preview)

// Switch as expression
int numLetters = switch (day) {
    case "MONDAY", "FRIDAY", "SUNDAY" -> 6;
    case "TUESDAY" -> 7;
    case "WEDNESDAY", "THURSDAY", "SATURDAY" -> 9;
    default -> -1;
};

// With arrow syntax (no break needed)
String type = switch (value) {
    case 1 -> "One";
    case 2 -> "Two";
    case 3 -> "Three";
    default -> "Unknown";
};

// Yield keyword for complex cases
int result = switch (day) {
    case "MONDAY", "FRIDAY" -> {
        System.out.println("Weekday");
        yield 6;  // Use yield to return value
    }
    case "WEDNESDAY" -> 9;
    default -> -1;
};

Benefits

  • No fall-through (eliminates break bugs)
  • Can return values
  • More concise syntax
  • Exhaustive (must handle all cases or have default)

2. Shenandoah Garbage Collector

Low-pause-time garbage collector for large heaps.

Usage

# Enable Shenandoah GC
java -XX:+UnlockExperimentalVMOptions -XX:+UseShenandoahGC MainClass

# Benefits:
# - Low pause times (independent of heap size)
# - Good for large heaps (multi-gigabyte)
# - Concurrent evacuation (no stop-the-world pauses)

3. Microbenchmark Suite

Java Microbenchmark Harness (JMH) now part of JDK.

Overview

JMH provides a framework for writing, running, and analyzing nano/micro/milli/macro benchmarks.

Basic Usage

import org.openjdk.jmh.annotations.*;
import java.util.concurrent.TimeUnit;

@BenchmarkMode(Mode.AverageTime)
@OutputTimeUnit(TimeUnit.MILLISECONDS)
@State(Scope.Thread)
public class MyBenchmark {
    
    @Benchmark
    public void testMethod() {
        // Code to benchmark
    }
}

Benefits

  • Accurate microbenchmarking
  • Handles JVM warmup
  • Statistical analysis
  • Part of JDK (no external dependency)

4. CDS Archive Improvements

Class-Data Sharing improvements for faster startup.

Default CDS Archives

Java 12 automatically generates a default CDS archive during JDK build.

# Use default CDS archive (automatic)
java -Xshare:auto -jar app.jar

# Explicitly use CDS
java -Xshare:on -jar app.jar

# Disable CDS
java -Xshare:off -jar app.jar

Benefits

  • Faster application startup
  • Reduced memory footprint
  • Automatic (no manual setup needed)
  • Shared class metadata across JVM instances

Usage

# Check CDS status
java -Xshare:auto -version

# Create custom CDS archive
java -Xshare:dump -jar app.jar

5. G1 GC Improvements

Improvements to G1 garbage collector.

5.1 Abortable Mixed Collections

G1 can abort mixed collections if they exceed pause time target.

# Enable abortable mixed collections
java -XX:+UseG1GC \
     -XX:G1MixedGCLiveThresholdPercent=85 \
     -XX:G1HeapWastePercent=5 \
     -jar app.jar

5.2 Promptly Return Unused Committed Memory

G1 can automatically return unused heap memory to the OS.

# Enable automatic memory return
java -XX:+UseG1GC \
     -XX:G1PeriodicGCInterval=2000 \
     -XX:MaxGCPauseMillis=200 \
     -jar app.jar

Benefits

  • Better pause-time predictability
  • Reduced memory footprint
  • More efficient memory usage
  • Automatic memory management

6. JVM Constants API

API to model nominal descriptions of key class-file and runtime artifacts, particularly constants loadable from the constant pool.

Overview

The Constants API provides a way to describe constants that can be loaded from the constant pool, useful for tools and frameworks.

import java.lang.constant.*;
import java.lang.invoke.MethodHandles;

// String constant
StringDesc stringDesc = StringDesc.of("Hello, Java 12!");
String resolved = stringDesc.resolveConstantDesc(MethodHandles.lookup());

// Class constant
ClassDesc classDesc = ClassDesc.of("java.lang.String");

Note: For detailed documentation and examples, see JVM Constants API Guide.

Use Cases

  • Code generation tools
  • Bytecode manipulation frameworks
  • Dynamic class creation
  • Reflection improvements

7. Compact Number Formatting

Support for compact number formatting, allowing numbers to be formatted in a more human-readable form.

Basic Usage

import java.text.NumberFormat;
import java.util.Locale;

// Short format
NumberFormat shortFormat = NumberFormat.getCompactNumberInstance(
    Locale.US, NumberFormat.Style.SHORT);
System.out.println(shortFormat.format(1000));  // "1K"
System.out.println(shortFormat.format(1000000));  // "1M"

// Long format
NumberFormat longFormat = NumberFormat.getCompactNumberInstance(
    Locale.US, NumberFormat.Style.LONG);
System.out.println(longFormat.format(1000));  // "1 thousand"
System.out.println(longFormat.format(1000000));  // "1 million"

Benefits

  • Human-readable number formatting
  • Locale-aware
  • Short and long formats
  • Useful for UI displays

8. String Methods Enhancements

New methods added to String class.

indent()

Adds or removes indentation from each line.

String text = "Line 1\nLine 2\nLine 3";
String indented = text.indent(4);  // Add 4 spaces
String unindented = text.indent(-2);  // Remove 2 spaces

transform()

Applies a function to the string and returns the result.

String text = "Hello, World!";
String upper = text.transform(String::toUpperCase);
String reversed = text.transform(s -> new StringBuilder(s).reverse().toString());

Benefits

  • More functional string operations
  • Chainable transformations
  • Convenient indentation control

9. Files.mismatch() Method

New utility method to efficiently compare two files for content differences.

Basic Usage

import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;

Path file1 = Paths.get("file1.txt");
Path file2 = Paths.get("file2.txt");

long mismatch = Files.mismatch(file1, file2);
if (mismatch == -1) {
    System.out.println("Files are identical");
} else {
    System.out.println("Files differ at byte position: " + mismatch);
}

Benefits

  • Efficient file comparison
  • Returns first differing byte position
  • Returns -1 if files are identical
  • Useful for file synchronization and validation

10. Common Interview Questions

Q1: What are Switch Expressions and why were they introduced?

A: Switch expressions allow switch to return values:

  • Eliminates fall-through bugs (no break needed)
  • More concise syntax with arrow notation
  • Can be used as expressions
  • Better readability

Q2: What's the difference between switch statement and switch expression?

A:

  • Statement: Executes code, no return value
  • Expression: Returns a value, can be assigned
  • Expression requires all cases to return values or have default
  • Expression uses arrow syntax (->) or yield keyword

Last Updated: 2024
Version: 1.0