集合基础知识及练习

import java.util.ArrayList;public class Solution {//将字符串转化为整数public static void main(String[] args) {ArrayList<String> list=new ArrayList();list.add("aaa");list.add("aaa");list.add("bbb");list.add("ccc");list.add("ddd");list.add("eee");String result=list.set(3,"aaa");System.out.println(result);//返回被修改的元素for (int i = 0; i < list.size(); i++) {String str=list.get(i);System.out.println(str);}}
}

import java.util.ArrayList;public class Solution {//将字符串转化为整数public static void main(String[] args) {//1.创建集合ArrayList<Student> list=new ArrayList<>();//2.创建学生对象Student s1=new Student("lajfl",2);Student s2=new Student("fsda",4);//3.添加元素list.add(s1);list.add(s2);//4.循环遍历for (int i = 0; i < list.size(); i++) {Student st=list.get(i);System.out.println(st);}}
}

是自己写的，并没有进行一些特殊的处理，所以直接打印时会打印首地址

import java.util.ArrayList;public class Solution {//将字符串转化为整数public static void main(String[] args) {//1.创建集合ArrayList<Student> list=new ArrayList<>();//2.创建学生对象Student s1=new Student("lajfl",2);Student s2=new Student("fsda",4);//3.添加元素list.add(s1);list.add(s2);//4.循环遍历for (int i = 0; i < list.size(); i++) {Student st=list.get(i);System.out.println(st.getAge()+""+st.getName());}}
}

添加用户对象，并判断是否存在

import java.util.ArrayList;public class Javabean {public static void main(String[] args) {
ArrayList<Phone> list=new ArrayList<>();Phone p1=new Phone("1a","12345a","jf");Phone p2=new Phone("1b","2345a","lajf");Phone p3=new Phone("1c","345a","jlf");list.add(p1);list.add(p2);list.add(p3);int result=User(list,"1c");System.out.println(result);}//调用方法根据id查找对应的用户信息public static int User(ArrayList<Phone> list,String id){for (int i = 0; i < list.size(); i++) {String id1 =list.get(i).getId();if(id1==id){return i;}}return -1;}
}

添加手机对象，并返回要求的数据

import java.util.ArrayList;public class Javabean {public static void main(String[] args) {//1.创建一个集合ArrayList<Phone> list=new ArrayList<>();//2.创建对象Phone p1=new Phone("小米",1000);Phone p2=new Phone("苹果",8000);Phone p3=new Phone("锤子",2900);
//3.添加对象list.add(p1);list.add(p2);list.add(p3);
ArrayList<Phone> lowlist=judge(list);for (int i = 0; i < lowlist.size(); i++) {Phone phone1=lowlist.get(i);System.out.println(phone1.getBrand()+" "+ phone1.getPrice());}}//方法：将《3000的手机返回public static ArrayList<Phone> judge(ArrayList<Phone> list){ArrayList<Phone> lowlist=new ArrayList<>();for (int i = 0; i < list.size(); i++) {Phone phone=list.get(i);int price1=phone.getPrice();if(price1<3000){lowlist.add(phone);}}return lowlist;}

关键点：怎么返回多个对象？

可以创建一个集合储存要返回的元素

后返回这个集合

多态练习

行为:eat

属性:年龄,颜色

1.定义狗类
属性:年龄,颜色

行为:eat(String something)(something表示吃的东西)
看家lookHome方法(无参数)

2.定义猫类
属性:年龄,颜色

行为:
eat(String something)方法(something表示吃的东西)
逮老鼠catchMouse方法(无参数)

3.定义Person类//饲养员
属性:姓名,年龄

行为:keepPet(Dog dog,String something)方法
功能:喂养宠物狗,something表示喂养的东西

行为:keepPet(Cat cat,String something)方法
功能:喂养宠物猫,something表示喂养的东西
生成空参有参构造,set和get方法

4.定义测试类(完成以下打印效果):
keepPet(Dog dog,String somethind)方法打印内容如下:
年龄为30岁的老王养了一只黑颜色的2岁的狗
2岁的黑颜色的狗两只前腿死死的抱住骨头猛吃
keepPet(Cat cat,String somethind)方法打印内容如下:
年龄为25岁的老李养了一只灰颜色的3岁的猫
3岁的灰颜色的猫眯着眼睛侧着头吃鱼

5.思考:1.Dog和Cat都是Animal的子类,以上案例中针对不同的动物,定义了不同的keepPet方法,过于繁琐,能否简化,并体会简化后的好处?
2.Dog和Cat虽然都是Animal的子类,但是都有其特有方法,能否想办法在keepPet中调用特有方法?

Package src

class animals;

public class Animals {private String color;private int age;public Animals(String color, int age) {this.color = color;this.age = age;}public String getColor() {return color;}public void setColor(String color) {this.color = color;}public int getAge() {return age;}public void setAge(int age) {this.age = age;}public void eat(String something){System.out.println("小动物吃"+something);}
}

Package src

class Cat;

public class Cat  extends Animals{public Cat(String color, int age) {super(color, age);}@Overridepublic void eat(String something) {System.out.println(getAge()+"岁的"+getColor()+"的猫眯着眼睛侧着头吃"+something);}public void catchMouse(){System.out.println("捉老鼠");}
}

Package src

class Cat;

public class Dog extends Animals{public Dog(String color, int age) {super(color, age);}@Overridepublic void eat(String something) {System.out.println(getAge()+"岁的"+getColor()+"颜色的狗两只前腿死死的抱住"+something+"猛吃");}
}

Package src

class Cat;

public class Person {/*定义Person类//饲养员属性:姓名,年龄行为:keepPet(Dog dog,String something)方法功能:喂养宠物狗,something表示喂养的东西行为:keepPet(Cat cat,String something)方法功能:喂养宠物猫,something表示喂养的东西生成空参有参构造,set和get方法*/private String name;private int age;public Person(String name, int age) {this.name = name;this.age = age;}public String getName() {return name;}public void setName(String name) {this.name = name;}public int getAge() {return age;}public void setAge(int age) {this.age = age;}

public void keepPet(Dog dog, String something){System.out.println("年龄为"+getAge()+"岁的"+getName()+"养了一只"+dog.getColor()+"颜色的"+dog.getAge()+"的猫");dog.eat(something);}public void keepPet(Cat cat,String something){System.out.println("年龄为"+getAge()+"岁的"+getName()+"养了一只"+cat.getColor()+"颜色的"+cat.getAge()+"岁的猫");cat.eat(something);}

优化：

思考:1.Dog和Cat都是Animal的子类,以上案例中针对不同的动物,定义了不同的keepPet方法,过于繁琐,能否简化,并体会简化后的好处?
2.Dog和Cat虽然都是Animal的子类,但是都有其特有方法,能否想办法在keepPet中调用特有方法?*/

    public void keepPet(Animals animal,String something){if(animal instanceof Dog d){System.out.println("年龄为"+getAge()+"岁的"+getName()+"养了一只"+animal.getColor()+"颜色的"+animal.getAge()+"岁的猫");d.eat(something);}else if(animal instanceof Cat c){System.out.println("年龄为"+getAge()+"岁的"+getName()+"养了一只"+animal.getColor()+"颜色的"+animal.getAge()+"岁的狗");c.eat(something);}else{System.out.println("没有此动物");}}

Package src

class Cat;

public class Test {public static void main(String[] args) {
Person p=new Person("老王",78);
Cat c=new Cat("blue",2);
p.keepPet(c,"胡萝卜");Person p1=new Person("老李",12);
Dog dog=new Dog("white",15);
p.keepPet(dog,"肉");}
}