2025 CSA Quick Review | 现代数学启蒙

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Part 1

Learning Java can be effectively structured around the units you've listed. Here's an essential set of code examples for each unit to help you get started or enhance your learning experience:

txt for typing practice

1: Primitive Types public class Main { public static void main(String[] args) { int number = 5; // integer double decimal = 5.99; // floating-point number char letter = 'A'; // character boolean flag = true; // boolean System.out.println("Integer: " + number); System.out.println("Double: " + decimal); System.out.println("Char: " + letter); System.out.println("Boolean: " + flag); } }

2: Using Objects public class Main { public static void main(String[] args) { String greeting = "Hello, world!"; System.out.println(greeting.toUpperCase()); // using the toUpperCase method from the String class } }

3: Boolean Expressions and if Statements public class Main { public static void main(String[] args) { int score = 75; if (score >= 90) { System.out.println("Grade A"); } else if (score >= 80) { System.out.println("Grade B"); } else if (score >= 70) { System.out.println("Grade C"); } else { System.out.println("Grade F"); } } }

4: Iteration public class Main { public static void main(String[] args) { for (int i = 0; i < 5; i++) { System.out.println("i = " + i); }

int j = 0; while (j < 5) { System.out.println("j = " + j); j++; } }

}

5: Writing Classes public class Car { private String model; private int year;

public Car(String model, int year) { this.model = model; this.year = year; }

public void display() { System.out.println("Model: " + model + ", Year: " + year); }

public static void main(String[] args) { Car myCar = new Car("Toyota", 2021); myCar.display(); }

}

6: Array public class Main { public static void main(String[] args) { int[] numbers = {1, 2, 3, 4, 5}; for (int number : numbers) { System.out.println(number); } } }

7: ArrayList import .util.ArrayList; public class Main { public static void main(String[] args) { ArrayList<String> names = new ArrayList<>(); names.add("John"); names.add("Anna"); names.add("Steve"); for (String name : names) { System.out.println(name); } } }

8: 2D Array public class Main { public static void main(String[] args) { int[][] matrix = {{1, 2}, {3, 4}}; for (int i = 0; i < matrix.length; i++) { for (int j = 0; j < matrix[i].length; j++) { System.out.println("Element at " + i + "," + j + ": " + matrix[i][j]); } } } }

9: Inheritance class Animal { void eat() { System.out.println("This animal eats food."); } }

class Dog extends Animal { void bark() { System.out.println("The dog barks."); } }

public class Main { public static void main(String[] args) { Dog myDog = new Dog(); myDog.eat(); myDog.bark(); } }

10: Recursion public class Main { public static int factorial(int n) { if (n == 1) return 1; // base case return n * factorial(n - 1); // recursive case }public static void main(String[] args) { System.out.println("Factorial of 5 is " + factorial(5)); }

}

UNIT 1: Primitive Types

UNIT 2: Using Objects

UNIT 3: Boolean Expressions and if Statements

UNIT 4: Iteration

UNIT 5: Writing Classes

UNIT 6: Array

UNIT 7: ArrayList

UNIT 8: 2D Array

UNIT 9: Inheritance

UNIT 10: Recursion

Part 2

For a student learning AP Computer Science A, it's important to grasp fundamental Java programming concepts and structures. Here's a list of essential Java code snippets that cover the core topics generally included in the AP Computer Science A curriculum:

1. Hello World - Basic Program Structure

2. Variables and Data Types

3. Conditionals - If-Else Statements

4. Loops - For Loop

5. While Loop

6. Arrays

7. Methods

8. Object-Oriented Programming - Classes and Objects

9. Using ArrayList

10. Inheritance

11. Polymorphism

12. Recursion

Part 3 Quiz

Certainly! Here are the 20 multiple-choice questions (MCQs) based on the content of your PDF file without the answers provided:

Primitive Types: What does the following line of Java code initialize?

A) A character B) An integer C) A floating-point number D) A boolean

Using Objects: What does the toUpperCase() method do to the string greeting in the code below?

A) Converts it to lowercase B) Converts it to uppercase C) Reverses the string D) Trims whitespace from the string

Boolean Expressions and if Statements: What is the output of the following code if score is 85?

A) Grade A B) Grade B C) Grade C D) Grade F

Iteration: Which statement best describes the purpose of the for loop in Java? A) Executes a block of statements repeatedly as long as a specified condition is true B) Executes a block of statements for each element in a collection C) Only executes a block of statements if a condition is true D) None of the above

Writing Classes: What is encapsulated within the Car class as defined above? A) Only methods B) Only fields C) Both fields and methods D) Neither fields nor methods

Array: What is the type of elements stored in the array numbers?

A) int B) String C) boolean D) double

ArrayList: What does the add method do in an ArrayList? A) Deletes an element from the list B) Adds an element to the list C) Finds an element in the list D) Updates an element in the list

2D Array: What does the following line of code output?

A) 1 B) 2 C) 3 D) 4

Inheritance: Which class does Dog inherit from? A) Animal B) Main C) System D) Object

Recursion: What is the base case in the recursive factorial method? A) n == 1 B) n == 0 C) n < 1 D) n > 1

Primitive Types: What type of data does the following Java line declare?

A) Integer B) Character C) Floating-point number D) Boolean

Using Objects: Which class does the method toUpperCase() belong to? A) Object B) System C) Main D) String

Boolean Expressions and if Statements: What will the following code snippet print?

A) Greater than 20 B) 20 or less C) No output D) Error

Iteration: What is the output of the following while loop?

A) i = 0, i = 1, i = 2 B) i = 1, i = 2, i = 3 C) i = 0, i = 1, i = 2, i = 3 D) No output

Writing Classes: What will the following code snippet output?

A) Model: Honda, Year: 2020 B) Honda, 2020 C) Display Honda 2020 D) Error

Array: What is the length of the array defined as follows?

A) 4 B) 5 C) 6 D) 10

ArrayList: What will the following code output?

A) Alice B) Bob C) Charlie D) None of the above

2D Array: How many rows does the following 2D array have?

A) 2 B) 3 C) 4 D) 6

Inheritance: What feature of OOP does the following Java code demonstrate?

A) Encapsulation B) Inheritance C) Polymorphism D) Composition

Recursion: How many recursive calls are made when calculating factorial(5) using the following method?

A) Four B) Five C) Six D) Seven

Answers

Encapsulation问题详解

在面向对象编程中，封装（Encapsulation）是指将数据（字段）和操作数据的方法捆绑到一个单独的单元或类中。封装有助于保护数据，防止外部干扰和误用，这是面向对象编程的基本原则之一。

让我们来看一下Car类的定义：

字段（Fields）：

private String color;

private int year;

这些是类的数据成员或字段。它们存储Car对象的状态（即车的颜色和年份）。

方法（Methods）：

public void displayInfo()

这个方法定义了Car类的行为，它操作字段（如color和year），并打印出车的信息。

为什么是“字段和方法”呢？

字段用来存储数据，方法用来定义可以对这些数据进行的操作。

封装将数据和操作这些数据的方法放在同一个类中，确保对象的内部状态只能通过方法来访问和修改（方法可以控制访问规则）。

在Car类中，既有字段（color、year），也有方法（displayInfo()），因此C) Both fields and methods（字段和方法）是正确答案。