Defining Methods, Return Types, and Parameters

A method in Java is a named block of code that performs a specific task. Methods are always declared inside a class. The method signature consists of the access modifier, optional modifiers (static, final, synchronized), return type, method name, and parameter list. The return type void indicates the method produces no value; otherwise a return statement must supply a value of the declared type. Parameters are variables that receive values when the method is called. Java uses pass-by-value semantics: primitive arguments are copied, and object arguments pass a copy of the reference. Changing a primitive parameter inside a method does not affect the original variable. However, calling mutating methods on an object through its reference does affect the object, because both the caller and the method reference the same heap object. Methods can be declared static, meaning they belong to the class rather than to an instance. Static methods can be called without creating an object and are commonly used for utility functions. Instance methods have access to instance fields via the implicit this reference. Choosing between static and instance methods is an important design decision that reflects whether the behaviour belongs to the class or to a particular object.

Example

public class MethodBasics {

    // Static utility method: belongs to the class
    public static int add(int a, int b) {
        return a + b;
    }

    // Static method returning a boolean
    public static boolean isPrime(int n) {
        if (n < 2) return false;
        for (int i = 2; i <= Math.sqrt(n); i++) {
            if (n % i == 0) return false;
        }
        return true;
    }

    // Instance method using instance state
    private String name;

    public MethodBasics(String name) {
        this.name = name;
    }

    public String greet(String greeting) {
        return greeting + ", " + this.name + "!";
    }

    // Pass-by-value demo
    static void tryToChange(int x, int[] arr) {
        x = 999;          // changes local copy, not caller's variable
        arr[0] = 999;     // mutates the actual array object
    }

    public static void main(String[] args) {
        // Static method calls
        System.out.println("3 + 4 = " + add(3, 4));

        System.out.print("Primes up to 20: ");
        for (int i = 2; i <= 20; i++) {
            if (isPrime(i)) System.out.print(i + " ");
        }
        System.out.println();

        // Instance method call
        MethodBasics obj = new MethodBasics("Alice");
        System.out.println(obj.greet("Good morning"));

        // Pass-by-value demo
        int num = 10;
        int[] data = {1, 2, 3};
        tryToChange(num, data);
        System.out.println("num after tryToChange: " + num);      // still 10
        System.out.println("data[0] after tryToChange: " + data[0]); // 999
    }
}

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