Java集合架構淺談-List（三）

大多數時我們要將自訂的對象存入到集合中，在操作自訂對象時常會遇到的問題。

1. 首先是使用普通for迴圈遍曆對象時，將滿足條件的對象刪除等操作。

if(26 == list.get(i).getAge())
    list.remove(i);

刪除後發現結果用仍有年齡為26的對象被保留下來，這是為什麼呢？參見。是因為在遍曆時有的對象沒有被判斷到。

package com.test.list;import java.util.ArrayList;import java.util.Iterator;import java.util.List;public class AddCustomElements{public static void main(String[] args){List<Student> list = new ArrayList<Student>();list.add(new Student("zhangsan", 26));list.add(new Student("lisi", 26));list.add(new Student("wangwu",30));list.add(new Student("niuqi", 26));//如果要將年齡為26的元素刪除使用普通for迴圈和Iterator迭代器有點區別：for(int i = 0; i < list.size(); i++){if(26 == list.get(i).getAge())//list.remove(i);//為了避免有漏掉的對象list.remove(i--);}System.out.println("For: "+list);//使用迭代器就可以將所有滿足條件的對象刪除for(Iterator<Student> it = list.iterator(); it.hasNext();){if(26 == it.next().getAge())it.remove();}System.out.println("Iterator: "+list);}}class Student{private String name;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;}private int age;public Student(){}public Student(String name, int age){this.name = name;this.age = age;}public String toString(){return name.toString()+", "+ age;}}

2. 如果是同年齡，同名字的Student就看成是同一個對象，要在集合中刪除相同元素時，就要複寫Student類中的equals()方法。

package com.test.list;import java.util.ArrayList;import java.util.List;public class CustomElements{public static void main(String[] args){List<Student> list = new ArrayList<Student>();list.add(new Student("zhangsan", 24));list.add(new Student("lisi", 29));list.add(new Student("zhangsan", 24));list.add(new Student("zhaoliu", 30));List<Student> tempList = singleElements(list);for(Student stu: tempList){System.out.println(stu.getName()+", "+stu.getAge());}}//定義一個方法，將List集合中的重複元素去掉public static <T> List<T> singleElements(List<T> list) {         List<T> tlist = new ArrayList<T>();        if (list == null || list.isEmpty())         {                 return tlist;        }        for (T t : list)         {                 if (!tlist.contains(t))                  {                         tlist.add(t);                 }        }        return tlist;}}class Student{private String name;private int age;public Student(){}public Student(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;}//如果不複寫equals()方法，就不知道什麼樣的元素是相同的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 (age != other.age)            return false;        if (name == null)        {            if (other.name != null)               return false;        }        else if (!name.equals(other.name))            return false;        return true;     }}

 因為複寫了equals()方法，可以使用indexOf(), lastIndexOf()方法等

int index = list.lastIndexOf(new Student("zhangsan", 24));System.out.println("index = "+ index);