泛型Practices
1：
定义个泛型类 DAO<T>，在其中定义一个 Map 成员变量，Map 的键 为 String 类型，值为 T 类型。 分别创建以下方法： public void save(String id,T entity)： 保存 T 类型的对象到 Map 成员 变量中
public T get(String id)：从 map 中获取 id 对应的对象
public void update(String id,T entity)：替换 map 中 key 为 id 的内容, 改为 entity 对象
public List<T> list()：返回 map 中存放的所有 T 对象
public void delete(String id)：删除指定 id 对象
定义一个 User 类：
该类包含：private 成员变量（int 类型） id，age；（ String 类型）name。
定义一个测试类：
创建 DAO 类的对象， 分别调用其 save、get、update、list、delete 方 法来操作 User 对象， 使用 Junit 单元测试类进行测试。
Dao类
package June.JuneEight.JuneEightAfternoon; import java.util.*; public class JuneEightAfternoonDao<T> { private HashMap<String,T> hashMap = new HashMap<String, T>(); public void save(String id,T entity){ hashMap.put(id,entity); } public T getT(String id){ return hashMap.get(id); } public void update(String id, T entity){ if(hashMap.containsKey(id)){ hashMap.put(id,entity); } } public List<T>list(){ // Collection listCollection = hashMap.values(); // // List<T> list = new ArrayList<>(); // list.add(listCollection); ArrayList<T> list = new ArrayList(); Collection <T> collection = hashMap.values(); for (T e:collection ) { list.add(e); } return list; } public void deletes(String id){ hashMap.remove(id); } }
User类
package June.JuneEight.JuneEightAfternoon; import java.util.Objects; public class JuneEightAfternoonUser<T> { private int id; private int age; private String name; public JuneEightAfternoonUser(int id, int age, String name) { this.id = id; this.age = age; this.name = name; } public int getId() { return id; } public void setId(int id) { this.id = id; } public int getAge() { return age; } public void setAge(int age) { this.age = age; } public String getName() { return name; } public void setName(String name) { this.name = name; } @Override public String toString() { return "JuneEightAfternoonUser{" + "id=" + id + ", age=" + age + ", name='" + name + '\'' + '}'; } @Override public boolean equals(Object o) { if (this == o) return true; if (o == null || getClass() != o.getClass()) return false; JuneEightAfternoonUser<?> user = (JuneEightAfternoonUser<?>) o; return id == user.id && age == user.age && Objects.equals(name, user.name); } @Override public int hashCode() { return Objects.hash(id, age, name); } }
切记： Map使用的自定义类要重写equals()方法
测试类
@Test public void TestOne(){ JuneEightAfternoonDao<JuneEightAfternoonUser> dao = new JuneEightAfternoonDao<JuneEightAfternoonUser>(); JuneEightAfternoonUser userOne = new JuneEightAfternoonUser(1,18,"老王"); dao.save("1001",userOne); System.out.println(dao.getT("1001")); dao.update("1002",new JuneEightAfternoonUser(1,46,"隔壁老王")); System.out.println(dao.list()); dao.save("1002",new JuneEightAfternoonUser(2,46,"隔壁老王")); dao.save("1003",new JuneEightAfternoonUser(3,23,"隔壁小姐姐")); System.out.println(dao.list()); dao.deletes("1003"); System.out.println(dao.list()); }
2:
定义一个 Employee 类， 该类包含：private 成员变量 name,age,birthday，其中 birthday 为 MyDate 类的 对象； 并为每一个属性定义 getter, setter 方法； 并重写 toString 方法输出 name, age, birthday MyDate 类包含: private 成员变量 month,day,year；并为每一个属性定义 getter, setter 方法； 创建该类的 5 个对象，并把这些对象放入 TreeSet 集合中（TreeSet 需使用泛型 来定义）， 分别按以下两种方式对集合中的元素进行排序，并遍历输出： 1). 使 Employee 继承 Comparable 接口，并按 name 排序
2). 创建 TreeSet 时传入 Comparator 对象，按生日日期的先后排序。
Employee类
package June.JuneNine.JuneNineForenoon; import org.junit.Test; public class JuneNineForenoonEmployee<T> implements Comparable<JuneNineForenoonEmployee> { private String name; private int age; private JuneNineForenoonMyDate birthday; public JuneNineForenoonEmployee(String name, int age, JuneNineForenoonMyDate birthday) { this.name = name; this.age = age; this.birthday = birthday; } // 泛型使用后的方法 @Override public int compareTo(JuneNineForenoonEmployee juneNineForenoonEmployee) { return this.name.compareTo(juneNineForenoonEmployee.name); } /* // 按照姓名排序 ---- 为指明泛型时的使用 @Override public int compareTo(Object object){ if(object instanceof JuneNineForenoonEmployee){ JuneNineForenoonEmployee obj = (JuneNineForenoonEmployee) object; return this.name.compareTo(obj.name); } throw new RuntimeException("输入的有误"); } */ 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 JuneNineForenoonMyDate getBirthday() { return birthday; } public void setBirthday(JuneNineForenoonMyDate birthday) { this.birthday = birthday; } @Override public String toString() { return "JuneNineForenoonEmployee{" + "name='" + name + '\'' + ", age=" + age + ", birthday=" + birthday + '}'; } }
package June.JuneNine.JuneNineForenoon; public class JuneNineForenoonMyDate implements Comparable<JuneNineForenoonMyDate> { private int month; private int day; private int year; public JuneNineForenoonMyDate(int month, int day, int year) { this.month = month; this.day = day; this.year = year; } public int getMonth() { return month; } public void setMonth(int month) { this.month = month; } public int getDay() { return day; } public void setDay(int day) { this.day = day; } public int getYear() { return year; } public void setYear(int year) { this.year = year; } @Override public int compareTo(JuneNineForenoonMyDate myDat) { // 比较年 int minusYear = this.getYear()- myDat.getYear(); if(minusYear != 0 ){ return minusYear; } //比较月 int minusMonth = this.getMonth() - myDat.getMonth(); if(minusMonth != 0){ return minusMonth; } // 比较日 return this.getDay()- myDat.getDay(); } // 未使用泛型分方法 // @Override // public int compareTo(Object object){ // if(object instanceof JuneNineForenoonMyDate){ // JuneNineForenoonMyDate obj = (JuneNineForenoonMyDate) object; // // // 比较年 // int minusYear = this.getYear()- obj.getYear(); // if(minusYear != 0 ){ // return minusYear; // } // // //比较月 // // int minusMonth = this.getMonth() - obj.getMonth(); // if(minusMonth != 0){ // return minusMonth; // } // // // 比较日 // int minusDay = this.getDay()- obj.getDay(); // if(minusDay != 0){ // return minusDay; // } // } // throw new RuntimeException("输入有误"); // } }
Test类
package June.JuneNine.JuneNineForenoon; import org.junit.Test; import java.util.Comparator; import java.util.Iterator; import java.util.TreeSet; public class JuneNineForenoonPracticeTestOne { @Test public void TestOne(){ TreeSet<JuneNineForenoonEmployee>set = new TreeSet<>(); set.add(new JuneNineForenoonEmployee("老王",12,new JuneNineForenoonMyDate(1,23,3004))); set.add(new JuneNineForenoonEmployee("老张",18,new JuneNineForenoonMyDate(5,25,1995))); Iterator<JuneNineForenoonEmployee> iterator = set.iterator(); while (iterator.hasNext()){ JuneNineForenoonEmployee employee = iterator.next(); System.out.println(employee); } } @Test public void TestTwo(){ TreeSet<JuneNineForenoonEmployee> treeSet = new TreeSet<>(new Comparator<JuneNineForenoonEmployee>() { @Override public int compare(JuneNineForenoonEmployee e1, JuneNineForenoonEmployee e2) { JuneNineForenoonMyDate b1 = e1.getBirthday(); JuneNineForenoonMyDate b2 = e2.getBirthday(); return b1.compareTo(b2); } }); treeSet.add(new JuneNineForenoonEmployee("老王头",18,new JuneNineForenoonMyDate(3,23,1869))); treeSet.add(new JuneNineForenoonEmployee("隔壁小姐姐",26,new JuneNineForenoonMyDate(6,23,1996))); for (JuneNineForenoonEmployee employee : treeSet) { System.out.println(employee); } } }
3： 写出使用Iterator 和 增强for 循环遍历List<String>的代码,使用上泛型
@Test public void TestTwo() { ArrayList<String> list = new ArrayList<>(); Iterator<String> iterator = list.iterator(); while (iterator.hasNext()){ iterator.next(); } for (String str:list ) { System.out.println(str); } }
4： 如何遍历Map的key集，value集,key-value集，使用上泛型
@Test public void TestOne() { HashMap<String, Integer> hashMap = new HashMap<>(); hashMap.put("老王", 23); hashMap.put("隔壁老王", 43); hashMap.put("老张头", 57); hashMap.put("老李", 18); // 取key Set<String> str = hashMap.keySet(); for (String string : str ) { System.out.println(string); } // 取value Collection<Integer> va = hashMap.values(); Iterator<Integer> integerIterator = va.iterator(); while (integerIterator.hasNext()) { System.out.println(integerIterator.next()); } }
5： 提供一个方法，用于遍历获取HashMap<String,String>中的所有value，并存放在List中返回。考虑上集合中泛型的使用。
@Test public List<String> getValue(HashMap<String,String> hashMap){ ArrayList<String> arrayList = new ArrayList<>(); Collection<String> collection = hashMap.values(); // Iterator<String> stringIterator = collection.iterator(); for (String s: collection ) { arrayList.add(s); } return arrayList; }
6： Map接口中的常用方法有哪些
添加： put（key, value）
删除: remove(key)
改;put（key，value）
查 get(key)
匹配 :containsKey(key)
