Methods

Java Methods

The word method in Java is essentially the same thing as the word function used in other languages like C and C++. A method is a function or subroutine. Every method in Java belongs to a class (this is like the C++ member function).

Why Write Methods?

Divide-and-conquer: Break programs into smaller, more manageable pieces

Reusability

Avoid repetition of code in a program

Building Methods

The structure of a method:

  modifier(s) returnType methodName(parameter list)
  {
    // method body (i.e. what it does, how it works)
  }

The pieces:

methodName - identifier chosen by the builder
parameter list - a comma-separated list of the parameters that the method will receive. This is data passed IN to the method by the caller. The parameter list indicates the types, order, and number of parameters
returnType - the data type of the value returned by the method. A method that returns no value should have return type void
modifier(s) - optional labels that can specify certain properties or restrictions on the method

(For now, we will use the modifier static on our methods)

method body - code statements that make up the definition of the method and describe what the method does

To return a value (from the body of a method with a non-void return type), use the keyword return:

  return expression;

A return statement will force immediate exit from the method, and it will return the value of the expression to the caller.
Note: A method with a non-void return type needs to return an appropriate value.

Calling Methods

A call is a command to execute a method, which transfers program execution to the code inside the method body. A call must use the method name and must pass in appropriate arguments, matching the expected number and types of parameters in the parameter list.

Examples:

A method with void return type:

  static void printGreeting(int age)
  {
    System.out.println("Hello, my name is Johnny");
    System.out.println("And I am " + age + " years old");
  }

Sample calls:

  printGreeting(6);	// can pass in a literal number

  int x = 10;
  printGreeting(x);	// can pass in a variable of matching type

Note: For a method with return type void, simply call the method as a single statement, ending with a semicolon.

A method with non-void return type:

  static double Compute(int x, double y)
  {
    double result;
    result = x * 1.5 + y;
    return result;
  }

Sample calls:

  double ans;
  ans = Compute(5, 3.6);   // returned value is assigned to ans

  // in this sample call, the returned value is printed by the 
  //  System.out.print statement
  int a = 16;
  double b = 3.4;
  System.out.print("Answer is " + Compute(a,b));

Note: For a function with non-void return type, use the returned value by placing the function call in the middle of another statement, where that tye of value is valid. For example, a returned value can be assigned to a variable. The returned value replaces the call when evaluating what happens next in a statement with multiple operations.

Scope

Scope - the portion of code in which a variable is valid.

a variable declared inside a block { } of executable code has scope from the point of its delcaration to the end of the block

Note that this means any variable declared inside a method body has scope only in that method. Often called a local variable

A variable declared as a formal parameter of a method also has local scope (only within that method).

Pass By Value

Default mode of passing parameters into methods
Means that the parameter inside the method body is a copy of the original argument that was passed in
Changes to the local parameter only affect the local copy, not the original argument in the call

Example

Method:

  static int myMethod(int x, int y)
  {
    x = x * 2;
    System.out.println("x = " + x);
    y = y * 2;
    System.out.println("y = " + y);
    return x + y;
  }

Sample call:

  int a = 5, b = 8, ans;
  ans = myMethod(a, b);
  System.out.println("ans = " + ans);
  System.out.println("a = " + a);
  System.out.println("b = " + b);

Notice that the output of the code is:

  x = 10
  y = 16
  ans = 26
  a = 5
  b = 8

Try this code yourself, here