Week 19 - 《ARTS打卡》

遍历数组中每个元素，以下两种表达方式是一样的

for (int i = 0; i <= myArray.length; i++) {
  // Do something
}
for (String s : myArray) {
  // Do something
}

return的使用，在主函数中赋值带return的函数

// Method will return an int value
public static int findProduct(int num1, int num2) {
  return num1 * num2;
}
public static void main(String[] args) {
  int product = findProduct(3,4);
  System.out.println(product); // Prints: 12
}

String的方法

.length() 长度
.concat() 连接
.equals() 等于返回true/false
.indexOf() 位置
.charAt() 返回String某个位置的值
.substring() 返回的String的一部分
.toUpperCase() / .toLowerCase() 大小写 ```java //.concat() String name = “Code”;

//一定要重新赋一下值 name = name.concat(“cademy”); System.out.println(name); // Prints: Codecademy

//indexOf() letters.indexOf(“EFG”)

//.charAt() 
String currency = "Yen";

System.out.println(currency.charAt(2)); // Prints: n

//.substring() String line = “It was the best of times, it was the worst of times.”; line.substring(26) line.substring(7, 24)

//大小写变化
String input = "Cricket!";

input.toUpperCase() input.toLowerCase()

数组
```java
//定义
int[] lottoNumbers;
String[] clothingItems;

//声明
int[] lottoNumbers = {12, 29, 4, 38, 3};
String[] clothingItems = {"Huipil", "Beanie", "Kimono", "Sari"};

//创建一个空数组
String[] menuItems = new String[5];

//改变其中一个元素
clothingItems[1] = "Sweater";

//长度
lottoNumbers.length

//遍历数组
int[] lottoNumbers = {12, 29, 4, 38, 3};

for (int num: lottoNumbers) {
    System.out.println(num);
}

二维数组

//int[][] nums;
//int[][] nums = {{10, 9, 8}, {7, 6, 5}, {4, 3, 2}}; 

nums[0][1]

//更新数组
char[][] letters = {{'A', 'a'}, {'B', 'x'}, {'C', 'c'}};
letters[1][1] = 'b';

//创建空数组，加入元素
int[][] intArray = new int[2][3];

intArray[0][0] = 1;
intArray[0][1] = 2;
intArray[0][2] = 4;
intArray[1][0] = 1;
intArray[1][1] = 3;
intArray[1][2] = 6;

//遍历AaBbCc
char[][] letters = {{'A', 'a'}, {'B', 'b'}, {'C', 'c'}};

for (int i = 0; i < letters.length; i++){
  for (int j = 0; j < letters[0].length; j++){
    System.out.print(letters[i][j]);
  }
}

//遍历AaBbCc

char[][] letters = {{'A', 'a'}, {'B', 'b'}, {'C', 'c'}};

for (int i = 0; i < letters[0].length; i++){
  for (int j = 0; j < letters.length; j++){
    System.out.print(letters[j][i]);
  }
}

四类访问修饰符

构造函数

public class Car {
  // Constructor method:
  public Car() {
    System.out.println("I'm a constructor!");
  }  

  public static void main(String[] args) {
    //唤醒构造函数并打印，Prints: I'm a constructor! 
    Car ferrari = new Car(); 
  }
}

创建实例字段

class Cat {
  // instance fields:
  String noise = "meow";
  int numLives = 9;

  public Cat(){
    // Instructions for creating an instance of Cat goes here:
  }

  public static void main(String[] args) {
    Cat myCat = new Cat();           // Access instance variable of an object:
    System.out.println(myCat.noise); // Prints: meow
  }
}

创建实例字段（传递参数）

class Cat {
  // Instance fields:
  String noise;
  int numLives = 9;

  // Constructor takes in one String parameter
  public Cat(String animalNoise){
    // Assign instance variable to parameter value:
    noise = animalNoise;
  }

  public static void main(String[] args) {
    // Send argument to constructor when creating an object:
    Cat firstCat = new Cat("mew"); 
    // Send argument to constructor when creating another object:
    Cat secondCat = new Cat( "mow");

    System.out.println(firstCat.noise); // Prints: mew
    System.out.println(secondCat.noise); // Prints: mow
  }
}

this的使用，在一个类里面传递参数

class Cat {
  String noise;
  int numLives = 9;

  // Parameter has same name as the instance variable
  public Cat(String noise){
    // Assign instance variable to parameter value:
    this.noise = noise;
  }
}

给对象赋予行为，就要使用动态（非静态）方法

class Cat {
  String noise;
  int numLives = 9;

  // Non-static method:
  public void speak() {
    System.out.println(noise);
  }

  public Cat(String animalNoise){
    noise = animalNoise;
  }

  public static void main(String[] args) {
    Cat myCat = new Cat("mew");
     //在对象上调用非静态方法
    myCat.speak(); // Prints: mew
  }
}

class Whale {
  String whaleSpecies;
  int whaleWeight;

  public Whale(String name, int weight) {
    whaleSpecies = name;
    whaleWeight = weight;
  }

  public String toString(){
    return "This is a " + whaleSpecies + " whale which weighs about " + whaleWeight + " pounds.";
  }

  public static void main(String[] args){
    Whale whale1 = new Whale("narwhal", 2100);
    Whale whale2 = new Whale("beluga", 3000);
    System.out.println(whale1);
    System.out.println(whale2);
  }
}


/*Output:
This is a narwhal whale which weighs about 2100 pounds.
This is a beluga whale which weighs about 3000 pounds.
*/

使用private变量需要使用get和set

public class Dog {
  private String name;

  // Other methods and constructors

  public String getName() {
    return name;
  }

  public static void main(String[] args) {
    Dog myDog = new Dog("Lassie");
    System.out.println(myDog.getName()); // Prints: Lassie
  }
}

public class Dog {
  private String name;

  // Other methods and constructors

  public void setName(String newName) {
    name = newName;
  }

  public static void main(String[] args){
    Dog myDog = new Dog("Cujo");
    myDog.setName("Lassie");
  }
}

ublic class Dog {
  private String name;

  public Dog(String name) {
    this.name = name;
  }

  public String getName() {
    return name;
  }

  public void setName(String newName) {
    name = newName;
  }

  public static void main(String[] args) {
    Dog myDog = new Dog("Lassie");
    myDog.setName("Cujo");
    System.out.println(myDog.getName());
  }

}

范围

public class MinHeap {
  public ArrayList<Integer> heap;
  public int size;

  public MinHeap() {
    // Body of constructor
  }
  public void add(int value) {
    heap.add(value);
    size++;
    bubbleUp();
  }

  //private的这个函数不能在外使用  
  private void bubbleUp() {
    // Body of .bubbleUp() method
  }
}

public class TrackAges {
  public static void main(String[]args) {
    MinHeap ages = new MinHeap();
    // Can call public MinHeap .add() method here:
    ages.add(42);
    ages.add(15);
    ages.add(27);
    // Can’t call private MinHeap .bubbleUp() method here
    // Don’t need to because .add() calls .bubbleUp()!    
  }
}

继承的两种情况

class Shape {
  int numSides;
  Shape(int numSides) {
    this.numSides = numSides;
  }
}

class Triangle extends Shape {
  Triangle() {
    // Use super() to call the Shape constructor:
    super(3);
  }

  public static void main(String[] args) {
    Shape sq = new Shape(4);
    Triangle tri = new Triangle();
    System.out.println(sq.numSides); // Prints: 4
    System.out.println(tri.numSides); // Prints: 3
  }
}

class Shape {
  Shape() {
    System.out.println("I am a shape!");
  }
}
// Make Triangle a subclass of Shape:
class Triangle extends Shape {
  Triangle() {
    System.out.print("I am a triangle!");
  }
  public static void main(String[] argos) {
    Shape sq = new Shape(); // Prints: I am a shape!
    Triangle tri = new Triangle();
    /* Prints:
    I am a shape!
    I am a triangle!
    */
  }
}