Java Methods and Method Overloading

Rules for Overloading

For two methods with the same name to count as valid overloads, their parameter lists must differ in at least one of: the number of parameters, the types of the parameters, or the order of parameter types. The compiler resolves which overload to call by matching the arguments at the call site, using exact type matches first, then widening conversions, then autoboxing, and finally varargs, in that priority order.

• void print(int a) and void print(int a, int b) — valid, different parameter count.
• void print(int a) and void print(double a) — valid, different parameter type.
• void print(int a, String b) and void print(String a, int b) — valid, different parameter order.

What Does NOT Count as Overloading

Changing only the return type while keeping the exact same name and parameter list is not valid overloading — it causes a compile-time error, because the compiler determines which method to call based on the arguments passed, not on what the caller does with the return value. Two methods with an identical signature (name + parameter types) cannot coexist no matter what their return types are.

How the Compiler Picks an Overload

When a call could match more than one overload, the compiler searches in phases and stops at the first phase that finds a match. Within a phase, it chooses the most specific method.

• 1. Exact match or widening — a parameter of the same type, or a wider primitive (byte → short → int → long → float → double).
• 2. Autoboxing — only if phase 1 found nothing, the compiler tries boxing an int to Integer (or unboxing the reverse).
• 3. Varargs — a ... parameter is the last resort, used only when no fixed-parameter method fits.

Overloading You Already Use

The JDK relies on overloading everywhere. System.out.println has separate versions for int, double, char, boolean, String and Object, which is why one method name prints anything. Math.max and Math.abs have int, long, float and double versions so the result keeps the type you passed in. Overload when the operations really do the same job for different inputs; if they do different jobs, give them different names.

Examples

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

    double add(double a, double b) {
        return a + b;
    }

    int add(int a, int b, int c) {
        return a + b + c;
    }

    public static void main(String[] args) {
        Calculator calc = new Calculator();
        System.out.println(calc.add(2, 3));
        System.out.println(calc.add(2.5, 3.5));
        System.out.println(calc.add(1, 2, 3));
    }
}

5
6.0
6

public class OverloadResolution {
    static void show(long value) {
        System.out.println("long version: " + value);
    }

    static void show(Integer value) {
        System.out.println("Integer version: " + value);
    }

    static void show(int... values) {
        System.out.println("varargs version: " + values.length + " values");
    }

    public static void main(String[] args) {
        show(5);                  // no show(int): widening int -> long beats boxing to Integer
        byte small = 3;
        show(small);              // byte also widens to long
        show(Integer.valueOf(7)); // exact match for Integer
        show();                   // only varargs accepts zero arguments
        show(1, 2, 3);            // only varargs accepts three arguments
    }
}

long version: 5
long version: 3
Integer version: 7
varargs version: 0 values
varargs version: 3 values

Exercise

Write three overloaded area methods: one taking a single int (a square), one taking two ints (a rectangle), and one taking a double (a circle of that radius).

Expected output: 25, 12, then 3.141592653589793

public class Area {

    // TODO: area(int side)
    // TODO: area(int width, int height)
    // TODO: area(double radius) using Math.PI

    public static void main(String[] args) {
        System.out.println(area(5));
        System.out.println(area(3, 4));
        System.out.println(area(1.0));
    }
}

public class Area {

    static int area(int side) {
        return side * side;
    }

    static int area(int width, int height) {
        return width * height;
    }

    static double area(double radius) {
        return Math.PI * radius * radius;
    }

    public static void main(String[] args) {
        System.out.println(area(5));    // 25
        System.out.println(area(3, 4)); // 12
        System.out.println(area(1.0));  // 3.141592653589793
    }
}

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