Why Write Methods?

1. Break a problem into small manageable pieces.
2. Code reuse
   • Benefits: Saves time, money, security, simplification, organization

void Methods and Value-Returning Methods

• void method: Performs a task and terminates
• Value-returning method: Performs a task and sends a value back to the code that called it.

Two Parts of Method Declaration

• To create a method you must write a definition
  • Consist of a header and body
    • e.g.,
      • header: public static void displayMessage()
      • body: { system.out.println(Hello"); }”

Parts of a method header:

• Method modifiers:
  • public: Method is available to code outside the class
  • static: Method belong to a class, not a specific object.
• Return type
  • Can be void or a data-type.
• Method name
• Parenthesis
  • Can contain nothing or a list of parameters

Calling a Method

• A method executes when it is called.
• The main method is automatically called on program start
  • Other methods are executed by method call statements.
• Method modifiers and the void return type aren’t written in the method call, they are only written in the method header.

Documenting Methods

• A method should always be documented by writing comments that appear before the method’s definition
  • The comments should provide a brief explanation of the method’s purpose.
  • Documentation comments begin with /** and end with */.
    • These comments can be read and processed by javadoc to produce HTML documentation.

Passing Arguments to a Method

Arguments: Values that are sent into a method.

• Data type of argument in a method call must match the variable declaration in the method header.
  • Java will automatically perform widening conversions, but narrowing conversions will cause a compiler error (e.g., converting double to int).
• You can pass multiple arguments to a method (parameter list).

Parameter: Variable that holds the value being passed into a method.

• You may pass literals, the contents of variables, and the values of expressions as arguments
• Scope is the method in which it is declared. No statement outside the method can access the parameter variable by its name.

Arguments Passed by Value

Passed by Value: Only a copy of an argument’s value is passed into a parameter variable.

• All arguments of the primitive data types are passed by values.

Passing Object References to a Method

When an object such as a String is passed as an argument, it is actually a reference to the object that is passed.

• Recall that a class type variable doesn’t hold the actual data item, but the memory address of the object.
  • Reference Variable: A variable associated with an object.

Strings are Immutable Objects

Immutable Objects: Objects that cannot be changed.

• Strings are immutable objects.

@param Tag and @return Tag in Documentation Comments

@param tag: Lets you provide a description of each parameter in your documentation comments.

• All @param tags must appear after the general description, which can span several lines.

@return tag: Lets you provide a description of the return value.

• All @return tags must appear after the general description, which can span several lines.

General Format:

@param parameterNameDescription

More About Local Variables

Local Variable: Variable declared inside a method.

• Is not accessible to statements outside the method.
  • This is why different methods can have local variables with the same names.
• Exist only while the method is executing. Destroyed when method ends.
• Aren’t automatically initialized with a default value, must be given a value before they can be used.

Returning a Value from a Method

Data can be returned from a method to the statement that called it.

Example:

// Returns int value of 700 and assigned it to num
int num = Integer.parseInt("700");

return statement: Causes method to end execution, returning a value to the statement hat called the method.

• Returned value must match the return type in the method header.

Examples:

1. Returning a boolean value

public static boolean isValid(int number)
{
    if (number > 100)
        return false;
    return true;
}

2. Returning a Reference to a String Object

public static String fullname(String first, String last)
{
    return first + " " + last;
}