Java集合（例题及代码）
1.ArrayList集合
创建Student类，属性：学号 姓名 年龄
通过上面的字符串获取学号，姓名，年龄创建Student类，并将Student类的对象保存到一个ArrayList集合中，
以下完成每个步骤完成需要定义方法：
1.写出3种方式遍历集合中的数据（for;for each;iterator）
2.查询里面有没有叫田七的学生如果存在则打印学生的具体信息（封装成方法，键盘输入姓名）
3.计算所有学生的平均年龄，最大年龄，最小年龄
4.如果存在名字叫张三的将名字改为张兵
5.删除年龄小于20岁 的学生从集合中。
public class Student {
private int id;
private String name;
private int age;
public Student() {
super();
}
public Student(int id, String name, int age) {
super();
this.id = id;
this.name = name;
this.age = age;
}
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
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;
}
@Override
public String toString() {
return "Student [id=" + id + ", name=" + name + ", age=" + age + "]";
}
}
import java.util.List;
public class Function {
public static Student find(List list, String str) {
for (int i = 1; i < list.size(); i++) {
if (str.equals(((Student) list.get(i)).getName())) {
return (Student) list.get(i);
}
}
return null;
}
}
import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;
import java.util.Scanner;
public class Test {
public static void main(String[] args) {
List<Student> list = new ArrayList<>();
list.add(new Student(1, "张三", 30));
list.add(new Student(1, "张4", 18));
list.add(new Student(1, "张5", 16));
list.add(new Student(1, "张6", 25));
list.add(new Student(1, "张7", 26));
list.add(new Student(1, "张8", 14));
list.add(new Student(1, "田七", 22));
// 遍历方式1
for (int i = 1; i < list.size(); i++) {
}
System.out.println(list);
System.out.println("-------------------------------------------");
// 遍历方式2
for (Student s2 : list) {
System.out.println(s2);
}
System.out.println("-------------------------------------------");
// 遍历方式3
Iterator<Student> it = list.iterator();
while (it.hasNext()) {
Student st = it.next();
System.out.println(st);
}
System.out.println("-------------------------------------------");
// 2.按姓名查找
Scanner sc = new Scanner(System.in);
System.out.println("请输入姓名");
String thename = sc.next();
Student s1 = Function.find(list, thename);
if (s1 != null) {
System.out.println(s1);
} else
System.out.println("没有这个学生");
// 3.计算平均年龄
double sum = 0;
int average = 0;
for (int i = 1; i < list.size(); i++) {
sum += (list.get(i).getAge());
average = (int) (sum / list.size());
}
System.out.println("平均年龄：" + average);
// 计算最大年龄
double max = 0;
for (int i = 0; i < list.size(); i++) {
if (list.get(i).getAge() > max) {
max = list.get(i).getAge();
}
}
System.out.println("最大值：" + max);
// 计算最小年龄
double min = list.get(0).getAge();
for (int i = 0; i < list.size(); i++) {
if (list.get(i).getAge() < min) {
min = list.get(i).getAge();
}
}
System.out.println("最小值：" + min);
// 4.如果存在名字叫张三的将名字改为张兵
for (int i = 0; i < list.size(); i++) {
if (list.get(i).getName().equals("张三")) {
list.get(i).setName("张兵");
}
}
// 5.删除年龄小于20岁 的学生从集合中
for (int i = 0; i < list.size(); i++) {
if (list.get(i).getAge() < 20) {
list.remove(i);
i--;
}
}
for (Student student : list) {
System.out.println(student);
}
}
}
2.Set集合（能去重，需重写HashCode方法和equals方法）
HashSet
创建一个Student类，有成员变量name和cardId。如果两个学生对象的姓名和学号一样视为同一个学生，
在HashSet中添加学生对象 并遍历打印学生信息。
public class Student {
private String name;
private int cardid;
public Student() {
super();
}
public Student(String name, int cardid) {
super();
this.name = name;
this.cardid = cardid;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public int getCardid() {
return cardid;
}
public void setCardid(int cardid) {
this.cardid = cardid;
}
@Override
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result + cardid;
result = prime * result + ((name == null) ? 0 : name.hashCode());
return result;
}
@Override
public boolean equals(Object obj) {
if (this == obj)
return true;
if (obj == null)
return false;
if (getClass() != obj.getClass())
return false;
Student other = (Student) obj;
if (cardid != other.cardid)
return false;
if (name == null) {
if (other.name != null)
return false;
} else if (!name.equals(other.name))
return false;
return true;
}
@Override
public String toString() {
return "Student [name=" + name + ", cardid=" + cardid + "]";
}
}
import java.util.HashSet;
public class Test1 {
public static void main(String[] args) {
HashSet<Student> set = new HashSet<>();
set.add(new Student("张三", 1001));
set.add(new Student("张三", 1001));
set.add(new Student("李四", 1002));
set.add(new Student("王五", 1003));
for (Student s : set) {
System.out.println(s);
}
}
}
3.TreeSet集合实现自然排序，必须要实现Comparable接口（重写compareTo方法）
将学生对象添加到TreeSet集合
import java.util.Set;
import java.util.TreeSet;
public class Treeset {
public static void main(String[] args) {
Set<Student> ts = new TreeSet<>();
ts.add(new Student("张三", 21));
ts.add(new Student("张4", 22));
ts.add(new Student("张5", 19));
ts.add(new Student("张6", 19));
ts.add(new Student("张7", 20));
for (Student student : ts) {
System.out.println(student);
}
}
}
class Student implements Comparable<Student> {
private String name;
private int age;
public Student() {
super();
}
public Student(String name, int age) {
super();
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;
}
@Override
public String toString() {
return "Student [name=" + name + ", age=" + age + "]";
}
@Override
public int compareTo(Student other) {
int i=this.age - other.age;
if(i==0){
i=this.name.compareTo(other.name);
}
return i;
}
}
4.TreeSet集合实现定制排序，（1）创建一个比较器Comparetor，抽象方法compare （2）在创建TreeSet时，通过有参构造把比较器对象传进去
键盘录入5个学生信息(姓名,语文成绩,数学成绩,英语成绩),按照总分从高到低输出到控制台
public class Student {
private String name;
private double score1;
private double score2;
private double score3;
private double sum = score1 + score2 + score3;
public Student() {
super();
}
public Student(String name, double sum) {
super();
this.name = name;
this.sum = sum;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public double getScore1() {
return score1;
}
public void setScore1(double score1) {
this.score1 = score1;
}
public double getScore2() {
return score2;
}
public void setScore2(double score2) {
this.score2 = score2;
}
public double getScore3() {
return score3;
}
public void setScore3(double score3) {
this.score3 = score3;
}
public double getSum() {
return sum;
}
public void setSum(double sum) {
this.sum = sum;
}
@Override
public String toString() {
return "Student [name=" + name + ", sum=" + sum + "]";
}
@Override
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result + ((name == null) ? 0 : name.hashCode());
long temp;
temp = Double.doubleToLongBits(score1);
result = prime * result + (int) (temp ^ (temp >>> 32));
temp = Double.doubleToLongBits(score2);
result = prime * result + (int) (temp ^ (temp >>> 32));
temp = Double.doubleToLongBits(score3);
result = prime * result + (int) (temp ^ (temp >>> 32));
return result;
}
@Override
public boolean equals(Object obj) {
if (this == obj)
return true;
if (obj == null)
return false;
if (getClass() != obj.getClass())
return false;
Student other = (Student) obj;
if (name == null) {
if (other.name != null)
return false;
} else if (!name.equals(other.name))
return false;
if (Double.doubleToLongBits(score1) != Double.doubleToLongBits(other.score1))
return false;
if (Double.doubleToLongBits(score2) != Double.doubleToLongBits(other.score2))
return false;
if (Double.doubleToLongBits(score3) != Double.doubleToLongBits(other.score3))
return false;
return true;
}
}
import java.util.Comparator;
import java.util.Scanner;
import java.util.TreeSet;
public class Test {
public static void main(String[] args) {
Scanner sc = new Scanner(System.in);
Comparator<Student> com = new Comparator<Student>() {
@Override
public int compare(Student o1, Student o2) {
double i = -(o1.getSum() - o2.getSum());
if (i == 0) {
i = o1.getName().compareTo(o2.getName());
}
return (int) i;
}
};
TreeSet<Student> set = new TreeSet<>(com);
for (int i = 1; i < 6; i++) {
System.out.println("输入第" + i + "个同学姓名");
String name = sc.next();
System.out.println("输入第" + i + "个同学语文成绩");
double score1 = sc.nextDouble();
System.out.println("输入第" + i + "个同学数学成绩");
double score2 = sc.nextDouble();
System.out.println("输入第" + i + "个同学英语成绩");
double score3 = sc.nextDouble();
double sum = score1 + score2 + score3;
set.add(new Student(name, sum));
}
for (Student s : set) {
System.out.println(s);
}
}
}
5.Map集合（键值对）
在map集合中存储5个学生对象，用id作为键 学生对象作为值
public class Student {
private String name;
private String sex;
public Student() {
super();
}
private int age;
public Student(String name, String sex, int age) {
super();
this.name = name;
this.sex = sex;
this.age = age;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public String getSex() {
return sex;
}
public void setSex(String sex) {
this.sex = sex;
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
@Override
public String toString() {
return "Student [name=" + name + ", sex=" + sex + ", age=" + age + "]";
}
}
import java.util.HashMap;
import java.util.LinkedHashSet;
import java.util.Map.Entry;
import java.util.Set;
public class Test {
public static void main(String[] args) {
HashMap<Integer, Set<Student>> map = new HashMap<>();
LinkedHashSet<Student> set1 = new LinkedHashSet<>();
LinkedHashSet<Student> set2 = new LinkedHashSet<>();
LinkedHashSet<Student> set3 = new LinkedHashSet<>();
set1.add(new Student("张三", "男", 18));
set2.add(new Student("李四", "女", 18));
set3.add(new Student("王五", "男", 18));
map.put(1, set1);
map.put(2, set2);
map.put(3, set3);
Set<Entry<Integer, Set<Student>>> entrySet = map.entrySet();
for (Entry<Integer, Set<Student>> entry : entrySet) {
System.out.println(entry.getKey() + "\n" + entry.getValue());
}
}
}