Chapter 2 - Using Objects Study Notes

Objects and Functions: Just as a home builder uses parts (e.g., furnace, water heater) that fulfill specific functions, a programmer builds applications from objects, each with specific behaviors.
- In Java, programs are constructed using objects.
- Each object has defined behaviors that can be manipulated to achieve specific outcomes.

Definition of Object: An entity in a program that you can manipulate by calling one or more of its methods.
Definition of Method: A sequence of instructions that can access the data of an object.
- The instructions of a method are often not visible to the user, but its behavior is clearly defined.
- Example: System.out has a println method that performs a specific action without the user needing to understand its internal workings.

Definition of Class: A class describes a set of objects with the same behavior.
- Examples of String Objects include:
- "Hello World"
- "Goodbye"
- "Mississippi"
- All string objects can invoke the same methods.
Example of System.out: It is a member of the PrintStream class, which writes output to the console.
- Other objects of the PrintStream class can write to different destinations.
- All PrintStream objects share the methods println and print.
Behavior of Different Classes:
- Objects of the PrintStream class have different behaviors compared to objects of the String class.
- Responsibilities of Different Classes:
- A string object understands its contents, such as the letters it contains.
- A string does not have the capability to send itself to a console window or file.
- For example, all objects of the Window class share common behaviors

Using Methods: You interact with an object by calling its methods.
- All objects of a specific class share a common set of methods.
Example of PrintStream Methods:
- println
- print

Methods Applicable to All String Objects:
- Example: length method
- String initialization: String greeting = "Hello, World!";
- Method call: int numberOfCharacters = greeting.length();
- Example: toUpperCase method
- String initialization: String river = "Mississippi";
- Method call: String bigRiver = river.toUpperCase();

Most methods require input values to perform their tasks.
- Technical Term for Inputs: Arguments.
- Example using Java: To call println, the string being printed is an argument:
- System.out.println(greeting);
Multiple Arguments: Some methods may require more than one argument, while others do not need any arguments at all.
- Example of a method that requires no arguments: length() method of the String class, as it operates on the object itself and doesn't require additional input.

Method Actions:
- Some methods perform actions and do not return any values, like println.
- Other methods compute and return values.
- Example: The length() method returns the count of characters in a string:
  - int numberOfCharacters = greeting.length();
Using Return Values:
- The return value computed by one method can serve as an argument for another method:
- System.out.println(greeting.length());
  - Here, greeting.length() returns the integer value 13, which is then used as the argument in println.

Example using replace Method:
- Implementation:
- Assume String river = "Mississippi";
- The following statement constructs a new string:
  - river = river.replace("issipp", "our");
- This method:
- Replaces all occurrences of "issipp" with "our"
- Returns the new String object "Missouri"
- Saves the returned value in the variable river
- The result can also be used in another method call:
- System.out.println(river.replace("issipp", "our"));

Detailed Analysis of Method Call:
- The method call river.replace("issipp", "our"); does the following:
- Invokes the method on the String object "Mississippi"
- Accepts two arguments: "issipp" and "our"
- Returns the newly constructed string "Missouri".

Example Method Call: System.out.println(greeting)
- Comments: greeting is the argument for println.
Example Method Call: greeting.replace("e", "3")
- Comments: The replace method has two arguments: "e" and "3".
Example Method Call: greeting.length()
- Comments: The length method does not take arguments.
Assignment Example: int n = greeting.length();
- Comments: length returns an integer value.
Method Call: System.out.println(n);
- Comments: println does not return a value; its return type is void per API documentation.
Example Method Chain: System.out.println(greeting.length());
- Comments: The return value of a method can be passed as an argument to another method.

Self Check 2.3: How can you compute the length of the string "Mississippi"?
- Answer: river.length() or "Mississippi".length()
Self Check 2.4: How can you print out the uppercase version of "Hello, World !"?
- Answer: System.out.println(greeting.toUpperCase()); or System.out.println("Hello, World!".toUpperCase());
Self Check 2.5: Is it legal to call river.println()? Why or why not?
- Answer: Not legal; river is a String and does not have println as a method.
Self Check 2.6: What are the arguments in the method call river.replace("p", "s")?
- Answer: The arguments are the strings "p" and "s".
Self Check 2.7: What is the result of the call river.replace("p", "s"), where river is "Mississippi"?
- Answer: "Missississi"
Self Check 2.8: What is the result of the call greeting.replace("World", "Dave").length(), where greeting is "Hello, World!"?
- Answer: 12