Lesson 1.8 · Unit 1 · CED topic 1.9

Method signatures

A method is a named block of code that only runs when it is called. You have been calling methods since the first lesson (System.out.println is one), and in this lesson you learn to read the line that describes a method, its signature, closely enough to know exactly how to call it and exactly what you get back.

Why methods exist

Naming a block of code lets you use it by name, as many times as you want, without rewriting it. That is procedural abstraction: you can use a method knowing what it does even if you do not know how it was written. A block of code is any section of code enclosed in braces.

Parts of a method header

public static double average(int sum, int count)

• public static are modifiers; Lesson 1.10 explains static.
• double is the return type: the type of value the method gives back. A method that gives back nothing says void.
• average is the name.
• (int sum, int count) is the parameter list. A parameter is a variable declared in the header and usable inside the body. It receives a value when the method is called.

The method signature is the name together with the ordered list of parameter types. For the method above it is average(int, int). A method without parameters has an empty parameter list, such as reset(). The return type is not part of the signature.

Calling a method

An argument is the value passed into a method when it is called. The arguments must be compatible in number and order with the parameter types in the signature. Arguments are passed using call by value: each parameter is initialized with a copy of the argument, so changing the parameter inside the method never changes the caller's variable.

Methods live inside a class. The examples in this lesson leave out the public class Main { ... } wrapper to keep them short.

public static double average(int sum, int count) {
    return (double) sum / count;
}

public static void main(String[] args) {
    int total = 17;
    int n = 4;
    double result = average(total, n);
    System.out.println(result);
}

4.25

### void and non-void

• A void method has no return value, so it is called as a statement by itself and is never part of an expression.
• A non-void method returns a value of the type named in its header. To use that value, store it in a variable or use it as part of an expression. Calling it as a statement and ignoring the result is legal but wastes the result.

### Overloading

Methods are overloaded when there are several methods with the same name but different signatures. Java picks the version whose parameter types match the arguments. Two methods that differ only in return type are not allowed, because the signature does not include the return type.

### The flow of control

A method call interrupts the sequential execution of statements: the program first runs the statements in the method, then continues at the point immediately after the call. The method ends when its last statement runs or when a return statement runs.

public static int square(int n)
public static double square(double x)
public static void greet(String name)
public static int max(int a, int b)

public static void addTen(int n) {
    n = n + 10;
    System.out.println("inside: " + n);
}

public static void main(String[] args) {
    int score = 5;
    addTen(score);
    System.out.println("outside: " + score);
}

inside: 15
outside: 5

public static int twice(int x) {
    System.out.println("in twice");
    return x * 2;
    // nothing after return runs
}

public static void main(String[] args) {
    System.out.println("start");
    int r = twice(4) + twice(1);
    System.out.println("r = " + r);
}

start
in twice
in twice
r = 10

public static String describe(int n) {
    return "int " + n;
}

public static String describe(double x) {
    return "double " + x;
}

public static String describe(String s) {
    return "String " + s;
}

public static void main(String[] args) {
    System.out.println(describe(7));
    System.out.println(describe(7.0));
    System.out.println(describe("seven"));
}

int 7
double 7.0
String seven