Topic: Arrays and ArrayList

Part 1 基础训练 Code 1：

1. Declaring and Initializing Arrays

• int[] numbers: This declares an integer array named numbers and initializes it with the values {1, 2, 3, 4, 5}.

• String[] names: This declares a string array named names and initializes it with the values {"Alice", "Bob", "Charlie"}.

2. Modifying Array Elements

• numbers[0] = 10;: This changes the first element (index 0) of the numbers array from 1 to 10.

• names[1] = "Barbara";: This changes the second element (index 1) of the names array from "Bob" to "Barbara".

3. Accessing and Printing Array Elements Using Traditional For Loop

• for(int i = 0; i < numbers.length; i++): This loop iterates over the numbers array, using i as the index. It starts at 0 and continues while i is less than numbers.length (the length of the array).

• System.out.println(numbers[i]);: This prints each element of the numbers array. The output will be:

4. Accessing and Printing Array Elements Using Enhanced For Loop

• for(String name : names): This is an enhanced for loop (also known as a "for-each" loop) that iterates over each element of the names array. name is a temporary variable that holds the value of each element in the array during each iteration.

• System.out.println(name);: This prints each element of the names array. The output will be:

5. Declaring and Initializing a Multidimensional Array

• int[][] matrix: This declares a two-dimensional integer array named matrix. It represents a matrix with two rows and three columns. The first row contains {1, 2, 3}, and the second row contains {4, 5, 6}.

6. Printing a Multidimensional Array Using Nested Loops

• Outer Loop (for(int i = 0; i < matrix.length; i++)): This iterates over each row of the matrix. matrix.length gives the number of rows in the matrix.

• Inner Loop (for(int j = 0; j < matrix[i].length; j++)): This iterates over each element in the current row i. matrix[i].length gives the number of columns in the current row.

• System.out.print(matrix[i][j] + " ");: This prints each element of the matrix in a row, separated by a space.

• System.out.println();: This moves to the next line after printing all elements of the current row.

Summary

• The program illustrates basic operations on single-dimensional and multi-dimensional arrays, including initialization, element modification, and traversal using different loop constructs.

• It uses both traditional for loops and enhanced for loops to demonstrate different methods of iterating through array elements.

• It also shows how to work with multidimensional arrays by using nested loops for element access and printing.

Code 2:

Explanation of the Code:

1. Import Statement: import java.util.ArrayList; is used to import the ArrayList class from the java.util package.

2. Creating an ArrayList: ArrayList<String> fruits = new ArrayList<>(); creates an ArrayList of type String.

3. Adding Elements: fruits.add("Apple"); adds the string "Apple" to the list.

4. Accessing Elements: fruits.get(0); accesses the element at index 0.

5. Modifying Elements: fruits.set(1, "Blueberry"); modifies the element at index 1 to "Blueberry".

6. Removing Elements: fruits.remove(2); removes the element at index 2. fruits.remove("Date"); removes the element "Date" from the list.

7. Checking for an Element: fruits.contains("Apple"); checks if "Apple" is in the list.

8. Size of the ArrayList: fruits.size(); returns the number of elements in the list.

9. Iteration:
• A for loop iterates using index.
• A for-each loop iterates through each element directly.

10. Clearing the ArrayList: fruits.clear(); removes all elements from the list.

Part 2 提升训练

Code 3:

1. Single-Dimensional Array Declaration and Initialization

• int[] numbers: Declares a single-dimensional array of integers.

• {1, 2, 3, 4, 5}: Initializes the array with five elements: 1, 2, 3, 4, and 5.

2. Accessing Array Elements

• numbers[0]: Accesses the first element of the numbers array, which is 1.

3. Iterating Over Array using for loop

• This loop iterates through the array using an index i.

• numbers.length: Returns the size of the array (5 in this case).

• numbers[i]: Accesses the element at the index i of the array and prints it.

4. Iterating Over Array using for-each loop

• The enhanced for loop (for-each) iterates through each element in the numbers array directly without using an index.

• number: Represents the current element in the iteration, and it prints each element sequentially.

5. Multidimensional Array Declaration and Initialization

• int[][] matrix: Declares a two-dimensional array of integers.

• { {1, 2, 3}, {4, 5, 6}, {7, 8, 9} }: Initializes the array as a 3x3 matrix with elements arranged in rows and columns.

6. Accessing Elements of Multidimensional Array

• matrix[1][2]: Accesses the element at row 1, column 2 of the matrix array. This element is 6 (arrays are zero-indexed).

7. Iterating Over Multidimensional Array

• This nested for loop iterates through each element of the two-dimensional array matrix.

• matrix.length: Returns the number of rows in the matrix (3 in this case).

• matrix[i].length: Returns the number of columns in the ith row (3 in this case).

• System.out.print(matrix[i][j] + " "): Prints each element in the current row followed by a space.

• System.out.println(): Moves to the next line after each row is printed.

8. Using Length Property

• numbers.length: Returns the size of the numbers array, which is 5.

• matrix.length: Returns the number of rows in the matrix array, which is 3.

• matrix[0].length: Returns the number of columns in the first row of the matrix, which is 3.

code 4:

1. Import Statement:

• This imports the ArrayList class from the java.util package, allowing the use of the ArrayList data structure, which is a resizable array that can hold elements of any type (in this case, Integer).

2. Class Declaration:

• This declares a public class named PrintArrayListExample. The class contains the main method, which is the entry point for the program.

3. Main Method:

• This is the main method where the execution of the program begins.

4. Creating and Initializing an ArrayList:

• Line 1: An ArrayList named numbers is created to store Integer objects. The <> (diamond) operator indicates that this ArrayList will hold elements of type Integer.

• Lines 2-4: Elements 1, 2, and 3 are added to the numbers ArrayList using the add() method.

5. Method 1: Printing Using a For-Each Loop:

• Line 1: A message is printed to indicate that the ArrayList will be printed using a for-each loop.

• Line 2: A for-each loop is used to iterate over each element (number) in the numbers ArrayList.

• Line 3: Each element (number) is printed to the console. This loop iterates through all elements in the ArrayList and prints each one on a new line.

6. Method 2: Printing the ArrayList Directly:

• Line 1: A message is printed to indicate that the ArrayList will be printed directly.

• Line 2: The entire numbers ArrayList is printed directly using System.out.println(numbers);. The ArrayList's toString() method is called implicitly, which returns a string representation of the list. The output will look like [1, 2, 3].

Output of the Program:

• For-Each Loop Output: Each element of the ArrayList is printed on a new line.

• Direct Print Output: The entire ArrayList is printed in a single line in a bracketed format.

Summary

1. Using a for-each loop to iterate through each element.

2. Printing the ArrayList directly, which utilizes the toString() method of the ArrayList class to format the output as a list of elements enclosed in square brackets.

Part 3 Quiz

1. Array Basics

2. Array Length

3. ArrayList Basic Operations

4. Array vs. ArrayList

• A) Arrays can dynamically change their size.

• B) ArrayLists require you to specify the size at the time of creation.

• C) Arrays have a fixed size once created.

• D) ArrayLists can only store primitive data types.

5. Traversing an Array

• A) int sum = numbers[0] + numbers[1] + numbers[2] + numbers[3];

• B) int sum = 0; for(int i = 0; i < numbers.length; i++) sum += numbers[i];

• C) int sum = 0; for(int num : numbers) sum += num;

• D) All of the above.

6. ArrayList Manipulation

7. Array Sorting

8. Removing Elements from ArrayList