3.1 Class Declarations, Field/Method Signatures, and Access Modifiers

Class Declarations, Field/Method Signatures, and Access Modifiers

Object-oriented programming in Java begins with the precise structural declaration of classes, their packages, and their accessible members. The Oracle Java SE 17 Developer certification exam tests subtle rules regarding top-level declarations, import resolution, cross-package accessibility, and method dispatch resolution.

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1. Class Declarations and Class Modifiers

A top-level class declaration introduces a reference type to the Java compiler. At the class level, access and behavior are governed by a specific subset of access modifiers and non-access specifiers.

Top-Level Access Modifiers

A top-level class can only use two access levels:

1. public: Accessible by any other class in any package across modular boundaries (assuming the package is exported if inside a Java module).
2. Package-Private (Default): Declared without an explicit access modifier. Accessible only by classes residing in the identical package.

Source File Structure Rules

• A single .java compilation unit can declare at most one public top-level class.
• If a public class is present, the filename must match the name of the public class exactly, including case (e.g., public class UserAccount must reside in UserAccount.java).
• A source file may contain multiple top-level classes provided that none or only one of them is public. The non-public classes have package-private accessibility.

Non-Access Class Specifiers

• final: Prevents the class from being extended by any subclass. Common examples in the Java standard library include java.lang.String, java.lang.Integer, and java.lang.Math.
• abstract: Denotes an incomplete class that cannot be directly instantiated via new. An abstract class serves as a supertype for concrete subclasses.
• strictfp: In Java 17, strictfp is effectively obsolete. Under JEP 306, Java SE 17 restored consistently strict floating-point semantics (strict conformance to IEEE 754) for all floating-point expressions across all platforms. While the strictfp keyword remains valid syntax for backward compatibility, specifying it has no operational effect.

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2. Package Structure and Import Mechanics

Package Declarations

A package declaration groups related classes into a shared namespace. If present, the package statement must be the first non-comment, non-whitespace token in the .java file. If omitted, the compilation unit belongs to the unnamed default package.

package com.exam.model; // Must be line 1 (ignoring comments)

import java.util.List;

public class Inventory {}

Import Types and Shadowing

Java provides three distinct import styles:

1. Single-Type Import: import java.util.List; imports a specific type.
2. Type-Import-on-Demand (Wildcard): import java.util.*; imports all public classes and interfaces within java.util. Wildcards do not import subpackages. For example, import java.util.*; does not import java.util.concurrent.ConcurrentHashMap.
3. Static Import: Imports static members (fields or methods) directly into scope without qualifying them with the class name.

import static java.lang.Math.PI;
import static java.lang.Math.sqrt;
import static java.util.Collections.*;

Naming Collisions and Precedence

When two packages contain a class with the same simple name, precedence determines how the name resolves:

1. Single-type imports take precedence over wildcard imports. If import java.util.Date; and import java.sql.*; are both declared, Date unambiguously resolves to java.util.Date.
2. Types in the same package take precedence over wildcard imports. A class named Date in the current package shadows import java.util.*;.
3. Two conflicting wildcard imports cause ambiguity. If import java.util.*; and import java.sql.*; are present, referencing Date directly causes a compile-time error: reference to Date is ambiguous. The developer must use the fully qualified class name (FQCN), such as java.sql.Date d = new java.sql.Date(...);.

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3. Member Access Modifiers

Class members (fields, methods, constructors, and nested types) can declare one of four access modifiers.

The protected Cross-Package Nuance

The protected access modifier permits access to any class in the same package, but across package boundaries, access is granted only to subclasses through inheritance.

A subclass in another package can access a protected member on an instance of itself (or its subclasses) or through super. It cannot access that protected member through a reference to the superclass.

// File: pkgA/SuperClass.java
package pkgA;
public class SuperClass {
    protected int rating = 10;
}

// File: pkgB/SubClass.java
package pkgB;
import pkgA.SuperClass;

public class SubClass extends SuperClass {
    public void inspect() {
        System.out.println(this.rating);   // COMPILES: accessed via inheritance
        System.out.println(super.rating);  // COMPILES: accessed via super
        
        SubClass sub = new SubClass();
        System.out.println(sub.rating);    // COMPILES: accessed through SubClass reference
        
        SuperClass sup = new SuperClass();
        // System.out.println(sup.rating); // COMPILE ERROR: rating has protected access in SuperClass
    }
}

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4. Method Signatures and Declarations

A method signature is strictly composed of two elements: $\text{Method Signature} = \text{Method Name} + \text{Formal Parameter Types (in order)}$

Return types, access modifiers, optional specifiers (static, final, synchronized), and throws clauses are part of the method declaration, but not part of the signature.

Valid Method Declaration Format

[access_modifier] [specifiers] <return_type> <methodName>([parameters]) [throws_clause] { ... }

• A method must declare a return type (or void). A constructor does not declare any return type. If a method shares the class name but declares a return type (public void User()), it is a regular method, not a constructor.
• Specifiers (static, final, synchronized, native) can appear in any order relative to each other, but must precede the return type.

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5. Method Overloading Mechanics

Method overloading occurs when two or more methods in the same class share the exact same method name but possess different parameter lists (differing in count, types, or order).

public void compute(int a, double b) {}   // Method 1
public void compute(double a, int b) {}   // Method 2: Overloaded (different parameter order)
public int compute(int a) { return a; }   // Method 3: Overloaded (different parameter count)
// public int compute(int a, double b) {} // COMPILE ERROR: Duplicate signature with Method 1

Overload Resolution Precedence

When resolving which overloaded method to invoke, the Java compiler processes arguments through four distinct phases in strict hierarchical order:

Tier 1: Exact Match / Subtyping (widening reference types without boxing)
   ↓
Tier 2: Primitive Widening (byte -> short -> int -> long -> float -> double)
   ↓
Tier 3: Autoboxing / Unboxing (int <-> Integer)
   ↓
Tier 4: Variable-Length Arguments (Varargs: ...)

public class OverloadDemo {
    static void process(long n)    { System.out.print("long "); }
    static void process(Integer n) { System.out.print("Integer "); }
    static void process(int... n)  { System.out.print("varargs "); }

    public static void main(String[] args) {
        int x = 42;
        process(x); // Outputs: long (Primitive widening beats boxing & varargs)
    }
}

Key Rule on Widening and Boxing: Java will widen and then box? No. The compiler will widen a primitive to another primitive, or box a primitive to its wrapper. It will never widen a primitive and then box it to a different wrapper (e.g., an int cannot become a Long). However, once boxed, the wrapper can be widened to an interface or superclass (e.g., int -> Integer -> Number -> Object).

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6. Variable-Length Arguments (Varargs)

Varargs (Type... name) allows a method to accept zero, one, or multiple arguments of a specified type.

Constraints on Varargs

1. Position: A varargs parameter must be the last parameter in the formal parameter list.
2. Count: There can be at most one varargs parameter per method.
3. Representation: Inside the method body, the varargs parameter is treated as a standard array (Type[]).

public void log(String prefix, int... values) { // Legal
    System.out.println(prefix + ": Length = " + values.length);
}

// public void bad1(int... a, String... b) {} // COMPILE ERROR: Only one varargs permitted
// public void bad2(int... a, String prefix) {} // COMPILE ERROR: Varargs must be last

When passing zero arguments to log("Data"), values is an empty array (new int[0]), not null. If a method is overloaded with both a fixed-parameter version and a varargs version (e.g., test(int a) and test(int... a)), calling test(10) invokes the exact-match fixed version.