Originally published at ssojet

Instance Main Methods and Compact Source Files

JEP 512, Compact Source Files and Instance Main Methods, has been integrated into JDK 25 after a comprehensive preview cycle. This feature simplifies the Java programming experience for beginners by allowing streamlined syntax and automatic imports for core libraries. By introducing implicitly declared classes, developers can write simpler programs without the need for explicit class declarations.

Previously, a basic Java program required defining a class explicitly. JEP 512 allows for a new approach:

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

// Using JEP 512 features
void main() {
    IO.println("Hello, World!");
}

This enhancement also includes instance main methods, which allows for program entry points without requiring static methods. If a selected class lacks a suitable static main method, the launcher can look for an instance main method, simplifying the entry point for novice developers.

JDK 25 Features

Image courtesy of InfoWorld

For enhanced console I/O operations, JEP 512 introduces java.lang.IO, a utility class that allows developers to perform basic interactions without the need for explicit import statements. This feature is particularly beneficial for beginners who need to engage in simple console-based programming.

Flexible Constructor Bodies

Another significant addition in JDK 25 is the feature of flexible constructor bodies. This allows developers to include statements before calling super(...) or this(...) within constructors. The flexibility aims to improve the safety and initialization process of objects.

Flexible constructor bodies enable more natural expression of constructors, allowing fields to be initialized before they are used by other methods. This reduces unnecessary restrictions and enhances code readability.

Module Import Declarations

JEP 511 introduces module import declarations, enabling succinct importing of all packages exported by a module. This enhancement simplifies the use of modular libraries and improves code organization. It allows developers to leverage powerful features without the complexity of managing extensive import statements.

Stable Values API

The stable values API, part of JDK 25, allows for immutable data management while providing flexibility in initialization timing. This is designed to improve application startup time and enable better performance optimizations. Stable values can help in scenarios where constants need to be initialized while maintaining thread safety.

Performance Improvements for Strings

In JDK 25, performance enhancements have been made to the String class, improving the speed of String::hashCode. This change allows for better performance when using strings as keys in immutable maps. By marking the internal hash code field with the @Stable annotation, the Java Virtual Machine can optimize operations involving string lookups significantly.

static final Map<String, MethodHandle> SYSTEM_CALLS = Map.of(
        "malloc", linker.downcallHandle(mallocSymbol,…),
        "free", linker.downcallHandle(freeSymbol,…)
);

This optimization results in dramatic performance improvements, particularly when working with immutable maps containing string keys.

OpenJDK Enhancements and Future Directions

The evolution of OpenJDK continues with several features set to enhance the Java programming experience. These include improvements to the Foreign Function and Memory API, structured concurrency, and more. As JDK 25 approaches its September release, these features promise to simplify the development process and improve performance across various applications.

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