# Java Core Language

> Java language fundamentals, JVM basics, modern features, and idiomatic patterns.

- **Type:** Skill
- **Install:** `agentstack add skill-ngxtm-devkit-java-core-language`
- **Verified:** Yes — security-reviewed for prompt injection and unsafe behavior
- **Seller:** [ngxtm](https://agentstack.voostack.com/s/ngxtm)
- **Installs:** 0
- **Category:** [Agent Skills](https://agentstack.voostack.com/c/agent-skills)
- **Latest version:** 0.1.0
- **License:** MIT
- **Upstream author:** [ngxtm](https://github.com/ngxtm)
- **Source:** https://github.com/ngxtm/devkit/tree/main/rules/java/java-core-language

## Install

```sh
agentstack add skill-ngxtm-devkit-java-core-language
```

Requires the [AgentStack CLI](https://agentstack.voostack.com/docs/cli). Works with Claude Code, Cursor, and any MCP-compatible agent.

## About

# Java Core Language Standards

## Modern Java Features (17+)

```java
// Records - immutable data carriers
public record User(String id, String name, String email) {
    // Compact constructor for validation
    public User {
        Objects.requireNonNull(id);
        Objects.requireNonNull(name);
    }
}

// Sealed classes - restricted inheritance
public sealed interface Shape permits Circle, Rectangle, Triangle {}
public record Circle(double radius) implements Shape {}
public record Rectangle(double width, double height) implements Shape {}
public final class Triangle implements Shape { /* ... */ }

// Pattern matching for switch (21+)
String describe(Shape shape) {
    return switch (shape) {
        case Circle(var r) -> "Circle with radius " + r;
        case Rectangle(var w, var h) -> "Rectangle " + w + "x" + h;
        case Triangle t -> "Triangle";
    };
}

// Pattern matching for instanceof
if (obj instanceof String s && !s.isBlank()) {
    return s.toUpperCase();
}
```

## Exception Handling

```java
// Custom exception hierarchy
public class DomainException extends RuntimeException {
    private final ErrorCode code;

    public DomainException(ErrorCode code, String message) {
        super(message);
        this.code = code;
    }

    public DomainException(ErrorCode code, String message, Throwable cause) {
        super(message, cause);
        this.code = code;
    }
}

// Try-with-resources
try (var reader = new BufferedReader(new FileReader(path));
     var writer = new BufferedWriter(new FileWriter(output))) {
    reader.lines().map(String::toUpperCase).forEach(writer::write);
} catch (IOException e) {
    throw new DomainException(ErrorCode.IO_ERROR, "File processing failed", e);
}
```

## Generics

```java
// Bounded type parameters
public > T max(T a, T b) {
    return a.compareTo(b) > 0 ? a : b;
}

// Wildcards
void processItems(List items) { /* read-only */ }
void addItems(List items) { /* write-only */ }

// Generic method with multiple bounds
public > void process(T item) {}
```

## Null Safety

```java
// Use Optional for return types
public Optional findById(String id) {
    return Optional.ofNullable(repository.get(id));
}

// Never use Optional as parameter or field
// Use @Nullable annotation for nullable parameters
public void process(@Nullable String input) {
    if (input == null) return;
    // ...
}

// Objects utility methods
Objects.requireNonNull(param, "param must not be null");
Objects.requireNonNullElse(value, defaultValue);
```

## Best Practices

1. **Prefer records** for data transfer objects and value objects
2. **Use sealed classes** when you have a fixed set of subtypes
3. **Pattern matching** over instanceof chains and visitor pattern
4. **Immutability by default** - use final fields, unmodifiable collections
5. **Fail fast** - validate at boundaries, throw early
6. **Meaningful exceptions** - include context, use exception chaining

## References

- [Modern Java Features](references/modern-java-features.md) - Records, Sealed Classes, Pattern Matching
- [JVM Memory Model](references/jvm-memory-model.md) - Memory, GC, Performance

## Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

- **Author:** [ngxtm](https://github.com/ngxtm)
- **Source:** [ngxtm/devkit](https://github.com/ngxtm/devkit)
- **License:** MIT

Install and usage instructions live in the source repository linked above.

## Pricing

- **Free** — Free

## Security capabilities

Automated source analysis of v0.1.0 — what this tool can access:

- **Network access:** no
- **Filesystem access:** no
- **Shell / process execution:** no
- **Environment & secrets:** no
- **Dynamic code execution:** no

*"Yes" means the capability is present in the source — more access means more to trust, not that it is unsafe.*


## Versions

- **0.1.0** — security scan: passed — Imported from the upstream source.

## Links

- Listing page: https://agentstack.voostack.com/l/skill-ngxtm-devkit-java-core-language
- Seller: https://agentstack.voostack.com/s/ngxtm
- Browse the marketplace: https://agentstack.voostack.com/browse

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Listed on AgentStack — the marketplace for AI agent skills and MCP servers. Every listing is security-reviewed. Creators keep 70%.