Java Packages

1// File: com/example/myapp/util/StringUtils.java
2package com.example.myapp.util;
3
4public class StringUtils {
5    public static String reverse(String input) {
6        return new StringBuilder(input).reverse().toString();
7    }
8    
9    public static boolean isPalindrome(String input) {
10        String reversed = reverse(input);
11        return input.equalsIgnoreCase(reversed);
12    }
13}

1public class Example {
2    public void method() {
3        // Using fully qualified name without import
4        java.util.ArrayList<String> list = new java.util.ArrayList<>();
5        
6        // Useful when dealing with naming conflicts
7        java.sql.Date sqlDate = new java.sql.Date(System.currentTimeMillis());
8        java.util.Date utilDate = new java.util.Date();
9    }
10}

1// File: com/example/myapp/internal/Helper.java
2package com.example.myapp.internal;
3
4// No access modifier = package-private class
5class Helper {
6    // This method is only accessible within the same package
7    void helperMethod() {
8        System.out.println("This is a helper method");
9    }
10}
11
12// File: com/example/myapp/internal/InternalService.java
13package com.example.myapp.internal;
14
15public class InternalService {
16    public void doSomething() {
17        // Can access Helper because it's in the same package
18        Helper helper = new Helper();
19        helper.helperMethod(); // This works
20    }
21}
22
23// File: com/example/myapp/service/ExternalService.java
24package com.example.myapp.service;
25
26import com.example.myapp.internal.InternalService;
27// Cannot import Helper because it's package-private
28
29public class ExternalService {
30    public void process() {
31        InternalService service = new InternalService();
32        service.doSomething(); // This works because InternalService is public
33        
34        // The following would not compile:
35        // Helper helper = new Helper(); // Error: Helper is not visible
36    }
37}

1# Compile Java files
2javac -d bin src/com/example/myapp/**/*.java
3
4# Create JAR file
5jar cvf myapp.jar -C bin .

1# Create a manifest file
2echo "Main-Class: com.example.myapp.Main" > manifest.txt
3
4# Create an executable JAR
5jar cvfm myapp.jar manifest.txt -C bin .
6
7# Run the JAR
8java -jar myapp.jar

1/**
2 * This package provides utility classes for string manipulation.
3 * 
4 * <p>It includes methods for reversing strings, checking palindromes,
5 * and other common string operations that are not available in
6 * the standard {@link java.lang.String} class.</p>
7 * 
8 * @author Your Name
9 * @version 1.0
10 * @since 1.0
11 */
12package com.example.myapp.util;
13
14/**
15 * Provides utility methods for string manipulation.
16 */
17public class StringUtils {
18    /**
19     * Reverses the characters in a string.
20     * 
21     * @param input The string to reverse
22     * @return The reversed string
23     */
24    public static String reverse(String input) {
25        return new StringBuilder(input).reverse().toString();
26    }
27    
28    // Other methods...
29}

1# Generate Javadoc for a package
2javadoc -d docs -sourcepath src -subpackages com.example.myapp.util

1// File: module-info.java
2module com.example.myapp {
3    // Export packages to make them accessible to other modules
4    exports com.example.myapp.api;
5    exports com.example.myapp.util;
6    
7    // Internal packages are not exported and remain encapsulated
8    // com.example.myapp.internal is not visible outside this module
9    
10    // Declare dependencies on other modules
11    requires java.base; // Implicit, but can be explicit
12    requires java.sql;
13    requires org.json;
14}