Modules to Packages - TypeScript to Java | codemirr

Introduction

In this lesson, you'll learn about modules to packages in Java. Coming from TypeScript, you already have a foundation for understanding this concept. We'll build on that knowledge while highlighting the key differences.

Mirror Card

From TypeScript:

In TypeScript, you're familiar with modules to packages.

In Java:

Java has its own approach to modules to packages, which we'll explore step by step.

The Java Way

Let's see how Java handles this concept. Here's a typical example:

Java Example

// File: com/example/utils/MathUtils.java
package com.example.utils;

public class MathUtils {
    public static final double PI = 3.14159;

    public static int add(int a, int b) {
        return a + b;
    }
}

// File: com/example/Main.java
package com.example;

import com.example.utils.MathUtils;
import com.example.utils.*;  // wildcard import

public class Main {
    public static void main(String[] args) {
        System.out.println(MathUtils.add(2, 3));
    }
}

Comparing to TypeScript

Here's how you might have written similar code in TypeScript:

TypeScript (What you know)

// utils/math.ts
export function add(a: number, b: number): number {
  return a + b;
}
export const PI = 3.14159;

// main.ts
import { add, PI } from "./utils/math";
import type { User } from "./types";

Mirror Card

From TypeScript:

You may be used to different syntax or behavior.

In Java:

TypeScript uses file-based ES modules; Java uses package declarations

Mirror Card

From TypeScript:

You may be used to different syntax or behavior.

In Java:

Java package names mirror the directory structure (com.example.utils)

Mirror Card

From TypeScript:

You may be used to different syntax or behavior.

In Java:

No export keyword — public class/method is importable from outside the package

Mirror Card

From TypeScript:

You may be used to different syntax or behavior.

In Java:

Static utility methods live in classes (no standalone functions)

Step-by-Step Breakdown

1. package Declaration

Every Java file starts with a package declaration that mirrors its folder path. com.example.utils → src/com/example/utils/.

TypeScript

// TypeScript: folder structure is implicit
export function add() {}

Java

package com.example.utils;

public class MathUtils {
    public static int add(int a, int b) { return a + b; }
}

Rule of Thumb

Package names are conventionally reverse domain: com.yourcompany.project.module.

2. import Statements

Java import uses fully qualified class names. import com.example.utils.MathUtils imports a single class. import com.example.utils.* imports all public classes in that package.

TypeScript

import { add } from "./utils/math";

Java

import com.example.utils.MathUtils;
// then use: MathUtils.add(2, 3)

3. No Standalone Functions — Use Static Methods

Java has no module-level functions. Utility functions are static methods in utility classes.

TypeScript

export function clamp(n: number, min: number, max: number): number {}

Java

public class NumberUtils {
    public static int clamp(int n, int min, int max) {
        return Math.max(min, Math.min(max, n));
    }
}

4. Build Tools: Maven and Gradle

npm manages packages in TypeScript. Java uses Maven (pom.xml) or Gradle (build.gradle) as both build tools and package managers, pulling from Maven Central.

TypeScript

// package.json
{
  "dependencies": { "axios": "^1.0" }
}

Java

<!-- pom.xml -->
<dependency>
    <groupId>org.apache.httpcomponents</groupId>
    <artifactId>httpclient</artifactId>
    <version>4.5.14</version>
</dependency>

Common Mistakes

When coming from TypeScript, developers often make these mistakes:

TypeScript uses file-based ES modules; Java uses package declarations
Java package names mirror the directory structure (com.example.utils)
No export keyword — public class/method is importable from outside the package

Common Pitfall

Don't assume Java works exactly like TypeScript. While the concepts may be similar, the syntax and behavior can differ significantly.

Key Takeaways

Java uses package declarations that mirror the directory structure
import uses fully qualified class names (com.example.utils.MyClass)
No standalone functions — use public static methods in classes
Maven (pom.xml) or Gradle is the Java equivalent of npm

Rule of Thumb

The best way to learn is by doing. Try rewriting some of your TypeScript code in Java to practice these concepts.

Introduction

Mirror Card

From TypeScript:

In TypeScript, you're familiar with modules to packages.

In Java:

Java has its own approach to modules to packages, which we'll explore step by step.

The Java Way

Let's see how Java handles this concept. Here's a typical example:

Java Example

// File: com/example/utils/MathUtils.java
package com.example.utils;

public class MathUtils {
    public static final double PI = 3.14159;

    public static int add(int a, int b) {
        return a + b;
    }
}

// File: com/example/Main.java
package com.example;

import com.example.utils.MathUtils;
import com.example.utils.*;  // wildcard import

public class Main {
    public static void main(String[] args) {
        System.out.println(MathUtils.add(2, 3));
    }
}

Comparing to TypeScript

Here's how you might have written similar code in TypeScript:

TypeScript (What you know)

// utils/math.ts
export function add(a: number, b: number): number {
  return a + b;
}
export const PI = 3.14159;

// main.ts
import { add, PI } from "./utils/math";
import type { User } from "./types";

Mirror Card

From TypeScript:

You may be used to different syntax or behavior.

In Java:

TypeScript uses file-based ES modules; Java uses package declarations

Mirror Card

From TypeScript:

You may be used to different syntax or behavior.

In Java:

Java package names mirror the directory structure (com.example.utils)

Mirror Card

From TypeScript:

You may be used to different syntax or behavior.

In Java:

No export keyword — public class/method is importable from outside the package

Mirror Card

From TypeScript:

You may be used to different syntax or behavior.

In Java:

Static utility methods live in classes (no standalone functions)

Step-by-Step Breakdown

1. package Declaration

Every Java file starts with a package declaration that mirrors its folder path. com.example.utils → src/com/example/utils/.

TypeScript

// TypeScript: folder structure is implicit
export function add() {}

Java

package com.example.utils;

public class MathUtils {
    public static int add(int a, int b) { return a + b; }
}

Rule of Thumb

Package names are conventionally reverse domain: com.yourcompany.project.module.

2. import Statements

Java import uses fully qualified class names. import com.example.utils.MathUtils imports a single class. import com.example.utils.* imports all public classes in that package.

TypeScript

import { add } from "./utils/math";

Java

import com.example.utils.MathUtils;
// then use: MathUtils.add(2, 3)

3. No Standalone Functions — Use Static Methods

Java has no module-level functions. Utility functions are static methods in utility classes.

TypeScript

export function clamp(n: number, min: number, max: number): number {}

Java

public class NumberUtils {
    public static int clamp(int n, int min, int max) {
        return Math.max(min, Math.min(max, n));
    }
}

4. Build Tools: Maven and Gradle

npm manages packages in TypeScript. Java uses Maven (pom.xml) or Gradle (build.gradle) as both build tools and package managers, pulling from Maven Central.

TypeScript

// package.json
{
  "dependencies": { "axios": "^1.0" }
}

Java

<!-- pom.xml -->
<dependency>
    <groupId>org.apache.httpcomponents</groupId>
    <artifactId>httpclient</artifactId>
    <version>4.5.14</version>
</dependency>

Common Mistakes

When coming from TypeScript, developers often make these mistakes:

TypeScript uses file-based ES modules; Java uses package declarations
Java package names mirror the directory structure (com.example.utils)
No export keyword — public class/method is importable from outside the package

Common Pitfall

Don't assume Java works exactly like TypeScript. While the concepts may be similar, the syntax and behavior can differ significantly.

Key Takeaways

Java uses package declarations that mirror the directory structure
import uses fully qualified class names (com.example.utils.MyClass)
No standalone functions — use public static methods in classes
Maven (pom.xml) or Gradle is the Java equivalent of npm

Rule of Thumb

The best way to learn is by doing. Try rewriting some of your TypeScript code in Java to practice these concepts.