Methods and Fields

Field Declarations

Instance Fields

public class Person {
    private String name;          // Instance field
    private int age = 0;          // With initializer
    private final String id;      // Must be initialized in constructor
}

Static Fields

public class Counter {
    private static int count = 0;           // Class variable
    public static final double PI = 3.14159; // Constant
}

Field Modifiers

Method Declarations

Basic Method Syntax

public class Calculator {
    // Instance method
    public int add(int a, int b) {
        return a + b;
    }

    // Static method
    public static int multiply(int a, int b) {
        return a * b;
    }

    // Void method
    public void printResult(int result) {
        System.out.println("Result: " + result);
    }
}

Method with Multiple Return Statements

public String classify(int n) {
    if (n < 0) return "negative";
    if (n == 0) return "zero";
    return "positive";
}

Varargs (Variable Arguments)

public int sum(int... numbers) {
    int total = 0;
    for (int n : numbers) {
        total += n;
    }
    return total;
}

// Usage
sum(1, 2, 3);
sum(1, 2, 3, 4, 5);

Generic Methods

public <T> T firstOrNull(T[] array) {
    if (array.length == 0) return null;
    return array[0];
}

public <T extends Comparable<T>> T max(T a, T b) {
    return a.compareTo(b) > 0 ? a : b;
}

Constructors

Basic Constructor

public class Person {
    private String name;
    private int age;

    public Person(String name, int age) {
        this.name = name;
        this.age = age;
    }
}

Constructor Overloading

public class Person {
    private String name;
    private int age;

    public Person() {
        this("Unknown", 0);
    }

    public Person(String name) {
        this(name, 0);
    }

    public Person(String name, int age) {
        this.name = name;
        this.age = age;
    }
}

Constructor Chaining with super

public class Employee extends Person {
    private String department;

    public Employee(String name, int age, String department) {
        super(name, age);  // Must be first statement
        this.department = department;
    }
}

Method Overloading

public class Printer {
    public void print(int n) {
        System.out.println("int: " + n);
    }

    public void print(double d) {
        System.out.println("double: " + d);
    }

    public void print(String s) {
        System.out.println("String: " + s);
    }

    public void print(int a, int b) {
        System.out.println("two ints: " + a + ", " + b);
    }
}

Method Overriding

public class Animal {
    public void speak() {
        System.out.println("Some sound");
    }
}

public class Dog extends Animal {
    @Override
    public void speak() {
        System.out.println("Woof!");
    }

    public void fetch() {
        super.speak();  // Call parent method
        System.out.println("Fetching...");
    }
}

Static Methods and Fields

Static Members

public class MathUtils {
    public static final double PI = 3.14159265359;

    public static int square(int n) {
        return n * n;
    }

    public static double circleArea(double radius) {
        return PI * radius * radius;
    }
}

// Usage - no instance needed
int sq = MathUtils.square(5);
double area = MathUtils.circleArea(2.0);

Static Initializers

public class Config {
    private static Map<String, String> settings;

    static {
        settings = new HashMap<>();
        settings.put("debug", "false");
        settings.put("version", "1.0");
    }
}

Instance Initializers

public class Person {
    private List<String> hobbies;

    {
        // Instance initializer - runs before every constructor
        hobbies = new ArrayList<>();
        hobbies.add("reading");
    }

    public Person() {
        // Constructor body runs after initializer
    }
}

Access Control

The this Keyword

public class Person {
    private String name;

    public Person(String name) {
        this.name = name;  // Distinguish field from parameter
    }

    public Person withName(String name) {
        this.name = name;
        return this;  // Return current instance for chaining
    }
}

The super Keyword

public class Child extends Parent {
    public Child() {
        super();  // Call parent constructor
    }

    @Override
    public void method() {
        super.method();  // Call parent method
    }

    public void accessParent() {
        int value = super.field;  // Access parent field
    }
}