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public class ForgetfulCritter extends Critter XxX private String event; public void addHistory (String event) { event = "?"; super.addHistory(); }

public class ForgetfulCritter extends Critter XxX private String event; public void addHistory (String event) { event = "?"; super.addHistory(); }

Database System Concepts
Database System Concepts
7th Edition
ISBN:9780078022159
Author:Abraham Silberschatz Professor, Henry F. Korth, S. Sudarshan
Publisher:Abraham Silberschatz Professor, Henry F. Korth, S. Sudarshan
Chapter1: Introduction
Section: Chapter Questions
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### ForgetfulCritter Class Code Explanation The displayed code is a Java class named `ForgetfulCritter`, which extends another class named `Critter`. This means that `ForgetfulCritter` is a subclass and inherits properties and methods from the `Critter` class. #### Key Elements of the Code: 1. **Class Declaration**: ```java public class ForgetfulCritter extends Critter ``` - `public class ForgetfulCritter`: Declares the class `ForgetfulCritter`. - `extends Critter`: Indicates inheritance from the `Critter` class. 2. **Private String Field**: ```java private String event; ``` - Declares a private field `event` of type `String`. Being private means it can only be accessed within this class. 3. **Method: addHistory**: ```java public void addHistory(String event){ event = "?"; super.addHistory(); } ``` - **Method Declaration**: - `public void addHistory(String event)`: A public method with no return value (`void`) that takes a `String` parameter named `event`. - **Parameter Shadowing**: - `event = "?";`: Assigns the string `"?"` to the local parameter `event`, which shadows the class field `event`. The class field `event` remains unaffected due to shadowing. - **Superclass Method Call**: - `super.addHistory();`: Calls the `addHistory` method from the superclass `Critter`. This suggests that `Critter` also has an `addHistory` method. ### Educational Insights: - **Inheritance in Java**: Demonstrates basic inheritance with the `extends` keyword. - **Method Overriding and Use of `super`**: Shows how to override a method and use `super` to call the superclass's version of the method. - **Variable Shadowing**: Illustrates how parameter shadowing occurs when a method's parameter has the same name as a class's field.
Transcribed Image Text:### ForgetfulCritter Class Code Explanation The displayed code is a Java class named `ForgetfulCritter`, which extends another class named `Critter`. This means that `ForgetfulCritter` is a subclass and inherits properties and methods from the `Critter` class. #### Key Elements of the Code: 1. **Class Declaration**: ```java public class ForgetfulCritter extends Critter ``` - `public class ForgetfulCritter`: Declares the class `ForgetfulCritter`. - `extends Critter`: Indicates inheritance from the `Critter` class. 2. **Private String Field**: ```java private String event; ``` - Declares a private field `event` of type `String`. Being private means it can only be accessed within this class. 3. **Method: addHistory**: ```java public void addHistory(String event){ event = "?"; super.addHistory(); } ``` - **Method Declaration**: - `public void addHistory(String event)`: A public method with no return value (`void`) that takes a `String` parameter named `event`. - **Parameter Shadowing**: - `event = "?";`: Assigns the string `"?"` to the local parameter `event`, which shadows the class field `event`. The class field `event` remains unaffected due to shadowing. - **Superclass Method Call**: - `super.addHistory();`: Calls the `addHistory` method from the superclass `Critter`. This suggests that `Critter` also has an `addHistory` method. ### Educational Insights: - **Inheritance in Java**: Demonstrates basic inheritance with the `extends` keyword. - **Method Overriding and Use of `super`**: Shows how to override a method and use `super` to call the superclass's version of the method. - **Variable Shadowing**: Illustrates how parameter shadowing occurs when a method's parameter has the same name as a class's field.
```java public class ForgetfulCritterTester { public static void main(String[] args) { Critter dopey = new ForgetfulCritter(); dopey.addHistory("eat"); System.out.println(dopey.getHistory()); System.out.println("Expected: [?]"); dopey.move(10); System.out.println(dopey.getHistory()); System.out.println("Expected: [?, ?]"); } } ``` ### Explanation: - **Class Definition**: The code defines a class called `ForgetfulCritterTester`. - **Main Method**: The `main` method is the entry point of the application. - **Object Instantiation**: A `Critter` object named `dopey` is created as an instance of `ForgetfulCritter`. - **addHistory Method**: The method `addHistory("eat")` is called on `dopey`, presumably to log or record an activity. - **Output**: The program prints the current history of `dopey` using `System.out.println(dopey.getHistory())`, followed by an expected output statement `Expected: [?]`. - **move Method**: The `move(10)` method is called on `dopey`, likely indicating a movement action. - **Final Output**: The history is printed again, expected to show two entries, indicated by `Expected: [?, ?]`.
Transcribed Image Text:```java public class ForgetfulCritterTester { public static void main(String[] args) { Critter dopey = new ForgetfulCritter(); dopey.addHistory("eat"); System.out.println(dopey.getHistory()); System.out.println("Expected: [?]"); dopey.move(10); System.out.println(dopey.getHistory()); System.out.println("Expected: [?, ?]"); } } ``` ### Explanation: - **Class Definition**: The code defines a class called `ForgetfulCritterTester`. - **Main Method**: The `main` method is the entry point of the application. - **Object Instantiation**: A `Critter` object named `dopey` is created as an instance of `ForgetfulCritter`. - **addHistory Method**: The method `addHistory("eat")` is called on `dopey`, presumably to log or record an activity. - **Output**: The program prints the current history of `dopey` using `System.out.println(dopey.getHistory())`, followed by an expected output statement `Expected: [?]`. - **move Method**: The `move(10)` method is called on `dopey`, likely indicating a movement action. - **Final Output**: The history is printed again, expected to show two entries, indicated by `Expected: [?, ?]`.
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