在千锋“逆战”学习第25天
每日一句：程序员是值得尊敬的，程序员的双手是魔术师的双手，他们把枯燥无味的代码变成了丰富多彩的软件。
今天学习了Map集合相关内容
下周继续努力。
作业
4.有如下代码：
public class TestListSet {
public static void main(String[] args) {
List<String> list = new ArrayList<String>();
list.add("Hello");
list.add("Learn");
list.add("Hello");
list.add("Welcome");
Set<String> set = new HashSet<String>();
set.addAll(list);
System.out.println(set.size());
}
}
运行结果：
编译运行都正常，输出3
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6.为第5题的worker类，在添加完equals方法的基础上，添加一个hashCode方法
public int hashCode(){
//1
}
(1) return 0;
(2) int result = 0;
if(name!=null) result = name.hashCode();
return result + age;
(3) return super.hashCode();
现在要把Worker类放入HashSet中，并希望HashSet中没有重复元素，则
(1)(2)写法正确,(2)效率更高
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7.代码改错
class Worker {
String name;
int age;
double salary;
public Worker() {
}
public Worker(String name, int age, double salary) {
this.name = name;
this.age = age;
this.salary = salary;
}
int hashCode() {
return name.hashCode() + age + salary;
}
public boolean equals(Worker w) {
if (w.name == name && w.salary == salary && w.age == age) {
return true;
} else {
return false;
}
}
}
public class TestWorker {
public static void main(String[] args) {
Set<Worker> set = new HashSet<Worker>();
set.add(new Worker("tom", 18, 2000));
set.add(new Worker("tom", 18, 2000));
set.add(0,new Worker("jerry", 18, 2000));
System.out.println(set.size());
}
}
//1 重写父类方法，权限不能小于父类；salary是double类型，返回值要强转为int型
public int hashCode() {
return (int) (name.hashCode() + age + salary);
}
//2 HashSet是无序的集合，不能在指定位置添加
set.add(new Worker("jerry", 18, 2000));
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8.在前面Worker类的基础上，为Worker类添加相应的方法，使得Worker放入HashSet中时，Set中没有重复元素。并编写相应的测试代码。
public class Worker {
private String name;
private Integer age;
private Integer salary;
public Worker() {
}
public Worker(String name, int age, Integer salary) {
this.name = name;
this.age = age;
this.salary = salary;
}
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 double getSalary() {
return salary;
}
public void setSalary(Integer salary) {
this.salary = salary;
}
public void work() {
System.out.println(name + " work");
}
public String toString() {
return "Worker [name=" + name + ", age=" + age + ", salary=" + salary + "]";
}
// 为worker类添加equals方法
public boolean equals(Object obj) {
if (this == obj) {
return true;
}
if (obj == null) {
return false;
}
if (this.getClass() != obj.getClass()) {
return true;
}
Worker worker = (Worker) obj;
if (this.name.equals(worker.name) && this.age == worker.age && this.salary == worker.salary) {
return true;
}
return false;
}
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result + ((age == null) ? 0 : age.hashCode());
result = prime * result + ((name == null) ? 0 : name.hashCode());
result = prime * result + ((salary == null) ? 0 : salary.hashCode());
return result;
}
}
public class TestWorker {
public static void main(String[] args) {
HashSet<Worker> set = new HashSet<>();
Worker wor1 = new Worker("zhang3", 18, 3000);
Worker wor2 = new Worker("li4", 25, 3500);
Worker wor3 = new Worker("wang5", 22, 3200);
Worker wor4 = new Worker("zhao6", 24, 3300);
Worker wor5 = new Worker("wang5", 22, 3200);
set.add(wor1);
set.add(wor2);
set.add(wor3);
set.add(wor4);
set.add(wor5);
System.out.println(set.size() + "个元素");
Iterator<Worker> it = set.iterator();
while (it.hasNext()) {
System.out.println(it.next());
}
}
}
运行结果：
5个元素
Worker [name=zhao6, age=24, salary=3300]
Worker [name=wang5, age=22, salary=3200]
Worker [name=wang5, age=22, salary=3200]
Worker [name=zhang3, age=18, salary=3000]
Worker [name=li4, age=25, salary=3500]
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10.利用Map完成以下功能 从命令行读取一个字符串，表示一个年份，输出该年世界杯冠军是哪支球队。如果该年没有举办世界杯，则输出：没有举办世界杯
public class TestWorldCup {
public static void main(String[] args) {
Map<String, String> map = new HashMap<String, String>();
map.put("2006", "意大利");
map.put("2002", "巴西");
map.put("1998", "法国");
map.put("1994", "巴西");
map.put("1990", "德国");
map.put("1986", "阿根廷");
map.put("1982", "意大利");
map.put("1978", "阿根廷");
map.put("1974", "德国");
map.put("1970", "巴西");
map.put("1966", "英格兰");
map.put("1962", "巴西");
map.put("1958", "巴西");
map.put("1954", "德国");
map.put("1950", "乌拉圭");
map.put("1938", "意大利");
map.put("1934", "意大利");
map.put("1930", "乌拉圭");
Scanner input = new Scanner(System.in);
System.out.print("输入一个年份：");
String str = input.next();
if (map.containsKey(str)) {
System.out.println("该年世界杯冠军是：" + map.get(str));
} else {
System.out.println("没有举办世界杯");
}
}
}
运行结果：
输入一个年份：1970
该年世界杯冠军是：巴西
输入一个年份：1996
没有举办世界杯
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11.已知某学校课程安排
(1)使用Map，以老师名字作为键，教授的课程名作为值，表示上述课程安排
(2)增加一位新教师Allen教JDBC
(3)Lucy改为教CoreJava
(4)遍历Map，输出所有老师及老师教授的课程
(5)利用Map，输出所有教JSP的老师
public class TestMap {
public static void main(String[] args) {
Map<String,String> map = new HashMap<String, String>();
//1.使用Map，以老师名字作为键，教授的课程名作为值，表示上述课程安排
map.put("Tom","CoreJava");
map.put("John","Oracle");
map.put("Susan","Oracle");
map.put("Jerry","JDBC");
map.put("Jim","Unix");
map.put("Kevin","JSP");
map.put("Lucy","JSP");
//2.增加一位新教师Allen教JDBC
map.put("Allen", "JDBC");
//3.Lucy改为教CoreJava
map.put("Lucy", "CoreJava");
//4.输出所有老师及老师教授的课程
Set<String> keySet = map.keySet();
Iterator<String> iterator = keySet.iterator();
System.out.println("老师"+"\t"+"课程");
while (iterator.hasNext()) {
String key = (String) iterator.next();
System.out.println(key+"\t"+map.get(key));
}
//5.输出所有教JSP的老师
System.out.println("教JSP的老师");
for(String key:keySet) {
if(map.get(key).equals("JSP")) {
System.out.println(key);
}
}
}
}
运行结果：
老师 课程
Kevin JSP
Tom CoreJava
Susan Oracle
John Oracle
Lucy CoreJava
Jerry JDBC
Allen JDBC
Jim Unix
教JSP的老师
Kevin
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12.有如下代码：
class Student {
String name;
int age;
public Student() {
}
public Student(String name, int age) {
this.name = name;
this.age = age;
}
public int hashCode() {
return name.hashCode() + age;
}
public boolean equals(Object o) {
if (o == null)
return false;
if (o == this)
return true;
if (o.getClass() != this.getClass())
return false;
Student stu = (Student) o;
if (stu.name.equals(name) && stu.age == age)
return true;
else
return false;
}
}
public class TestHashSet {
public static void main(String[] args) {
Set<Student> set = new HashSet<Student>();
Student stu1 = new Student();
Student stu2 = new Student("Tom", 18);
Student stu3 = new Student("Tom", 18);
set.add(stu1);
set.add(stu2);
set.add(stu3);
System.out.println(set.size());
}
}
运行结果：
编译正确，运行时异常，空指针异常
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13.在原有世界杯Map的基础上，增加如下功能
读入一支球队的名字，输出该球队夺冠的年份列表
Scanner input = new Scanner(System.in);
System.out.print("输入一个球队：");
String str = input.nextLine();
if (map.containsValue(str)) {
for (String s : map.keySet()) {
if (map.get(s).contains(str)) {
System.out.println(s + "\t");
}
}
} else {
System.out.println("该球队没有获得过世界杯");
}
}
}
运行结果：
输入一个球队：巴西
1962
1994
1970
2002
1958
输入一个球队：中国
该球队没有获得过世界杯
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14.给定一个字符串，输出该字符串由哪些字符组成，每个字符出现多少次
public class Test {
public static void main(String[] args) {
LetterCount test = new LetterCount();
test.countChar("aaabbcd985ABBbb789");
}
}
class LetterCount {
private Map<Character, Integer> countMap = new HashMap<Character, Integer>();
public void countChar(String str) {
char[] chars = str.toCharArray();
for (char ch : chars) {
if (!countMap.containsKey(ch)) {
countMap.put(ch, 0);
}
int count = countMap.get(ch);
countMap.put(ch, count + 1);
}
Set<Character> keys = countMap.keySet();
for (Character ch : keys) {
System.out.println("字符" + ch + "出现次数：" + countMap.get(ch));
}
}
}
运行结果：
字符a出现次数：3
字符A出现次数：1
字符b出现次数：4
字符B出现次数：2
字符c出现次数：1
字符d出现次数：1
字符5出现次数：1
字符7出现次数：1
字符8出现次数：2
字符9出现次数：2
