Practice Questions — Introduction to Java

public class SystemInfo {
    public static void main(String[] args) {
        System.out.println("Java Version: " + System.getProperty("java.version"));
        System.out.println("Java Vendor: " + System.getProperty("java.vendor"));
        System.out.println("OS Name: " + System.getProperty("os.name"));
        System.out.println("OS Version: " + System.getProperty("os.version"));
        System.out.println("User Home: " + System.getProperty("user.home"));
    }
}

public class JavaFeatures {
    public static void main(String[] args) {
        System.out.println("1. Platform Independent (Write Once, Run Anywhere)");
        System.out.println("2. Object-Oriented");
        System.out.println("3. Automatic Garbage Collection");
        System.out.println("4. Strongly Typed");
        System.out.println("5. Multithreaded");
    }
}

public class JdkHierarchy {
    public static void main(String[] args) {
        System.out.println("+-----------------------------------+");
        System.out.println("|  JDK (Java Development Kit)       |");
        System.out.println("|  Tools: javac, jdb, javadoc, jar  |");
        System.out.println("|  +-----------------------------+  |");
        System.out.println("|  |  JRE (Java Runtime Env)     |  |");
        System.out.println("|  |  Libraries: java.lang, etc  |  |");
        System.out.println("|  |  +------------------------+ |  |");
        System.out.println("|  |  |  JVM (Virtual Machine) | |  |");
        System.out.println("|  |  |  Executes bytecode     | |  |");
        System.out.println("|  |  +------------------------+ |  |");
        System.out.println("|  +-----------------------------+  |");
        System.out.println("+-----------------------------------+");
    }
}

public class JavaTimeline {
    public static void main(String[] args) {
        System.out.println("=== Java Version Timeline ===");
        System.out.println("1996 | Java 1.0   | Initial release, applets");
        System.out.println("1998 | Java 2     | Collections framework, Swing");
        System.out.println("2004 | Java 5     | Generics, enhanced for-loop, enums");
        System.out.println("2014 | Java 8     | Lambda expressions, Stream API");
        System.out.println("2018 | Java 11    | LTS, HTTP client, var in lambdas");
        System.out.println("2021 | Java 17    | LTS, sealed classes, pattern matching");
        System.out.println("2023 | Java 21    | LTS, virtual threads, record patterns");
    }
}

public class LanguageComparison {
    public static void main(String[] args) {
        System.out.println("====================================================");
        System.out.println("Feature          | Java     | Python   | C++");
        System.out.println("====================================================");
        System.out.println("Typing           | Static   | Dynamic  | Static");
        System.out.println("Speed            | Fast     | Slower   | Fastest");
        System.out.println("Memory Mgmt      | Auto GC  | Auto GC  | Manual");
        System.out.println("Platform Indep.  | Yes(JVM) | Yes(Int) | No");
        System.out.println("Primary Use      | Enterpr. | Data Sci | Systems");
        System.out.println("====================================================");
    }
}

public class MemoryInfo {
    public static void main(String[] args) {
        Runtime runtime = Runtime.getRuntime();
        long totalMemory = runtime.totalMemory() / (1024 * 1024);
        long freeMemory = runtime.freeMemory() / (1024 * 1024);
        long usedMemory = totalMemory - freeMemory;
        long maxMemory = runtime.maxMemory() / (1024 * 1024);

        System.out.println("=== JVM Memory Information ===");
        System.out.println("Total Memory: " + totalMemory + " MB");
        System.out.println("Free Memory: " + freeMemory + " MB");
        System.out.println("Used Memory: " + usedMemory + " MB");
        System.out.println("Max Memory: " + maxMemory + " MB");
    }
}

public class CompilationProcess {
    public static void main(String[] args) {
        System.out.println("=== Java Program Lifecycle ===");
        System.out.println("Step 1: WRITE   - Developer writes .java source file");
        System.out.println("Step 2: COMPILE - javac converts .java to .class (bytecode)");
        System.out.println("Step 3: LOAD    - Class Loader loads .class into JVM memory");
        System.out.println("Step 4: VERIFY  - Bytecode Verifier checks code safety");
        System.out.println("Step 5: EXECUTE - Execution Engine runs bytecode (JIT + Interpreter)");
        System.out.println("Step 6: GC      - Garbage Collector manages memory during execution");
        System.out.println();
        System.out.println("This very program went through all 6 steps to produce this output!");
        System.out.println("Compiled from: CompilationProcess.java");
        System.out.println("Running as: CompilationProcess.class");
        System.out.println("On JVM: " + System.getProperty("java.vm.name"));
    }
}

public class MultithreadingDemo {
    public static void main(String[] args) {
        Runtime runtime = Runtime.getRuntime();
        int processors = runtime.availableProcessors();
        String threadName = Thread.currentThread().getName();

        System.out.println("=== Java Multithreading & Memory Demo ===");
        System.out.println("Available Processors: " + processors);
        System.out.println("Current Thread: " + threadName);
        System.out.println("Java supports running up to " + processors + " threads truly in parallel on this machine.");
        System.out.println();

        System.out.println("--- Memory Demo ---");
        long freeBefore = runtime.freeMemory() / (1024 * 1024);
        System.out.println("Free memory before: " + freeBefore + " MB");

        System.out.println("Creating 10000 objects...");
        String[] temp = new String[10000];
        for (int i = 0; i < 10000; i++) {
            temp[i] = "Object number " + i;
        }
        long freeAfter = runtime.freeMemory() / (1024 * 1024);
        System.out.println("Free memory after: " + freeAfter + " MB");

        temp = null;
        System.gc();
        System.out.println("Requesting garbage collection...");
        long freeAfterGC = runtime.freeMemory() / (1024 * 1024);
        System.out.println("Free memory after GC: " + freeAfterGC + " MB");
    }
}