Variable Scopes in Java

1. Global Scope

Variables that are accessible from anywhere in the program.

public static final HttpClient client = HttpClient.newHttpClient();

client is accessible from any class or method in the program using ClassName.client

2. Class Scope

Variables that are accessible from any method within the class.

3. Method Scope

Variables that are accessible only within the method they are declared.

4. Block Scope

Variables that are accessible only within the block they are declared.

try {
    URI uri = new URI("http://httpbin.org/delay/" + secs);
    HttpRequest request = HttpRequest.newBuilder().GET().uri(uri).build();
} catch (IOException | URISyntaxException exp) {
    throw new RuntimeException(exp);
}

5. Local Scope

Variables declared within a method (including method parameters) and are only accessible within that method.

public void exampleMethod() {
    int localVar = 10; // local variable
}

6. Instance Scope

Variables that are associated with an instance of a class (non-static fields).

private String instanceVar;

instanceVar is accessible by all methods within the instance of the class.

7. ThreadLocal Scope

Variables that are local to the thread, providing a separate variable for each thread that accesses it.

private static final ThreadLocal<SimpleDateFormat> dateFormat = ThreadLocal.withInitial(() -> new SimpleDateFormat("dd-MM-yyyy"));

dateFormat provides a separate SimpleDateFormat instance for each thread that accesses it.

8. Static Scope

Variables that are associated with the class rather than any instance, and are * *shared** among all instances of the class.

public static int staticVar;

Final Example

public class ExampleClass {
    // Global scope (static)
    public static final HttpClient client = HttpClient.newHttpClient();

    // Class scope (instance)
    private final String instanceVar;

    // Static scope
    public static int staticVar = 42;
    
    // ThreadLocal scope, usually static so that it is visible to all threads
    private static final ThreadLocal<SimpleDateFormat> dateFormat 
            = ThreadLocal.withInitial(() -> new SimpleDateFormat("dd-MM-yyyy"));//with Supplier
    //Getter and Setter will be applicable to the thread accessing it.

    private static final ThreadLocal<SimpleDateFormat> dateFormat = new ThreadLocal<>();
    
    // Constructor
    public ExampleClass(String instanceVar) {
        this.instanceVar = instanceVar;
    }

    // Method demonstrating different scopes
    public void exampleMethod(int parameter) { // Method scope
        // Local scope
        int localVar = 10;

        try { // Block scope
            URI uri = new URI("http://example.com/" + localVar + "/" + parameter);
            HttpRequest request = HttpRequest.newBuilder().GET().uri(uri).build();
        } catch (URISyntaxException e) {
            e.printStackTrace();
        }
    }
}